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Quark Matter 2022 is the XXIXth International Conference on Ultra-relativistic Nucleus-Nucleus Collisions. This conference brings together theoretical and experimental physicists from around the world to discuss new developments in high energy heavy ion physics. The focus of the discussions is on the fundamental understanding of strongly-interacting matter at extreme conditions, as formed in ultra-relativistic nucleus-nucleus collisions, as well as on emergent QCD phenomena in high-multiplicity proton-proton and proton-nucleus collisions.
Honorary patronage: Jacek Majchrowski, Mayor of the City of Kraków
I review the basics and current status of machine learning in particle/nuclear physics, with emphasis on understanding how it works.
The Age of Computation is yet to Come
The theory of classical universal computation was laid down in 1936, was implemented within a decade, became commercial within another decade, and dominated the world’s economy half a century later. This success story relied on the progress in technology. As computers become faster they must become smaller. The history of computer technology has involved a sequence of changes from one type of physical realisation to another, with smaller and smaller components. The unavoidable step to the quantum level will be one in this sequence; but it promises something more exciting as well. It can support entirely new modes of computation that do not have classical analogues. At present it is not clear when, how and even whether fully-fledged quantum computers will eventually be built; but notwithstanding this, the quantum theory of computation already plays a much more fundamental role in the scheme of things than its classical predecessor did. I believe that anyone who seeks a fundamental understanding of either physics, computation or logic must incorporate its new insights into their world view. There is so much potential in this fundamentally new way of harnessing nature that it appears as though the age of computation has not yet even begun!
Axions arose in theoretical attempts to understand the observed symmetry of QCD. Remarkably, they have the right properties to provide the "dark matter" that astronomers need. There are promising new ideas to test that hypothesis, especially including the ALPHA plasma haloscope that I will describe. I will also discuss how better understanding and data in QCD can help us refine our ideas about axions, and add a few words about emergent axions.
Isolated photons and dijets measurements in small collision systems probe the initial state of the collision, providing the opportunity to constrain PDFs, test pQCD predictions, and probe cold nuclear matter effects. In addition, dijet measurements are sensitive to interactions of partons with the medium produced in Pb-Pb collisions that induce modifications in jet properties. Measurements in small collision systems therefore also offer a baseline for Pb-Pb collision measurements.
We present the measurement of isolated photons and dijets in small collisions systems, pp and p-Pb by ALICE. Isolated photons are measured in pp collisions at $\sqrt s = 5.02$, 8, and 13 TeV and in p-Pb collisions at $\sqrt{s_{\mathrm{NN}}} = 5.02$ and 8.16 TeV, down to $p_{\mathrm{T}} = 10$ GeV/$c$, extending previous measurements at these centre-of-mass energies down to small $x\sim10^{-3}$. Dijets are measured in pp and p-Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV with $R=0.4$ and the anti-$k_\mathrm{T}$ algorithm, and with azimuthal angle of at least $\pi/2$ between the two jets. The dijet invariant mass is measured in the range from $80$ to $150$ GeV$/c^2$, probing a region where medium effects are expected to be strong.
Beams of relativistic heavy ions are accompanied by a large flux of equivalent photons, and thus photon-induced reactions are the dominant interaction mechanism in heavy-ion collisions when the colliding nuclei have transverse separation larger than the nuclear diameter. In these ultra-peripheral collisions (UPCs) the photon can provide a clean probe of the partonic structure of the nucleus analogous with deep inelastic scattering. This talk presents a new measurement of dijet production in ultra-peripheral Pb+Pb collisions performed with the ATLAS detector using high-statistics 2018 Pb+Pb data. Events are selected using requirements on rapidity gaps and forward neutron production to identify the photo-nuclear processes. The relatively clean environment of these events allows for precision measurements in a phase-space regions where significant nuclear PDF modifications are expected to be present, and which are not strongly constrained by previous measurements.
Over the last decades, many of the available theoretical jet quenching formalisms have been extended to account for the medium’s finite longitudinal extension and expansion. However, only recently a first-principle approach has been developed that allows to study jet evolution in anisotropic media in the dilute limit. In this talk, we show how to extend some of the previous results to the dense regime, where the resummation of multiple in-medium scatterings is necessary. We consider, in particular, a non-flowing background with finite matter gradients and compute the single particle momentum broadening distribution and the single gluon production rate, two crucial observables for jet quenching phenomenology. The resumation is performed by either computing the opacity series or starting from the all order BDMPS-Z formalism. The (novel) resulting modifications to jets’ substructure are discussed.
Angular correlations present in dijet photoproduction are studied, for the first time, using ultraperipheral lead-lead collisions at a nucleon-nucleon center-of-mass energy of 5.02 TeV. The second moment of the angular distribution, $\langle \cos(2\Phi) \rangle$, where $\Phi$ is the angle between the vector sum $\vec{Q}_\mathrm{T}$ and the vector difference $\vec{P}_\mathrm{T}$ of the transverse momentum vectors of the jets, is measured as a function of $\vec{Q}_\mathrm{T}$. This analysis amounts to the first, yet essential, step towards the extraction of the Wigner or Husimi gluon distributions, which are believed to be the most fundamental gluon distributions. It also introduces new techniques for the analysis of jet angular correlations in exclusive dijet events at colliders.
Within the framework of a 3+1D Boltzmann transport approach at fixed $\eta/s$ with the full collision integral, we investigate the existence of far-from equilibrium attractors in momentum moments of the one particle distribution function. We first compare our results for a conformal and non conformal gas for different values of $\eta/s$ and $m$, in order to investigate the role of the equation of state in approaching the universal attractor. We then extend our study by employing a quasi-particle model of quarks and gluons with thermal masses tuned to reproduce lattice QCD thermodynamics. We finally examine the possible existence of attractors in the anisotropic flow coefficients, under the influence of initial momentum anisotropy.
We establish the existence of a far-from-equilibrium attractor in weakly-coupled gauge theory undergoing one-dimensional Bjorken expansion. We demonstrate that the resulting far-from-equilibrium evolution is insensitive to certain features of the initial condition, including both the initial momentum-space anisotropy and initial occupancy. We find that this insensitivity extends beyond the energy-momentum tensor to the detailed form of the one-particle distribution function. Based on our results, we assess different procedures for reconstructing the full one-particle distribution function from the energy-momentum tensor along the attractor and discuss implications for the freeze-out procedure used in the phenomenological analysis of ultra-relativistic nuclear collisions.
The relativistic generalization of the relaxation time approximation proposed by Anderson and Witting [1] is widely used in several fields of physics and, in particular, in the study of the hydrodynamization of the matter produced in ultrarelativistic heavy ion collisions. We demonstrate that the approximation proposed by Anderson and Witting contains basic flaws, not being consistent with fundamental properties of the Boltzmann collision operator [2]. This makes it impossible to consistently model relativistic gases using energy dependent relaxation times or more general choices of the local equilibrium state. We propose a new relaxation time approximation which fixes these fundamental flaws [2]. In this contribution, this new formulation is used to consistently calculate the bulk and shear viscosity coefficients using QCD-inspired energy-dependent relaxation times [3] and phenomenological thermal masses obtained from fits to lattice QCD thermodynamics [4].
[1] J. L. Anderson and H. Witting, Physica 74, 466 (1974).
[2] G. S. Rocha, G. S. Denicol and J. Noronha, Phys. Rev. Lett. 127, no. 4, 042301 (2021).
[3] K. Dusling, G. D. Moore and D. Teaney, Phys. Rev. C 81, 034907 (2010).
[4] M. Alqahtani, M. Nopoush and M. Strickland, Phys. Rev. C 92, no.5, 054910 (2015).
In non-central heavy-ion collisions (HIC), the large initial angular momentum can induce a non-vanishing polarization for hadrons with non-zero spin. The global spin alignment of vector mesons, quantified by the $00^{th}$ element of spin density matrix ($\rho_{00}$), can offer information on the spin-orbital interactions of the QCD medium. Surprisingly large signal of vector meson $\rho_{00}$ compared to hyperon spin polarization poses challenges to the conventional theoretical understanding of polarization in HIC. Preliminary observations from Beam Energy Scan (BES-I) of large deviations of $\rho_{00}$ from 1/3 for $\phi$ mesons can only be explained by introducing the vector meson strong force fields.
In this talk, we will present transverse momentum and collision centrality dependence of $\phi$, $K^{*0}$, $\overline{K^{*0}}$, $K^{*+}$, and $K^{*-}$ vector mesons using recent high statistics Beam Energy Scan (BES-II) data of Au+Au collisions at $\sqrt{s_{\rm NN}}$ = 7.7 - 27 GeV, and isobar collisions (Zr+Zr and Ru+Ru) at $\sqrt{s_{\rm NN}}$ = 200 GeV. The BES-II data will provide unprecedented precision in $\rho_{00}$ at these energies. Comparison of $\rho_{00}$ between Au+Au and isobar species can provide information on the system size dependence of $\rho_{00}$. Moreover, since the magnetic moment of charged and neutral $K^{*}$ differ by a factor of seven, the comparison of their $\rho_{00}$ may serve as a new probe for the initial strong magnetic field in HIC.
Polarization and spin-alignment measurements represent an important tool for the understanding of the particle production mechanisms occurring in proton–proton collisions. When considering heavy-ion collisions, quarkonium polarization could also be used to investigate the characteristics of the hot and dense medium (quark-gluon plasma) created at LHC energies. In ALICE, this observable was extracted for the first time in Pb-Pb collisions and a significant difference with respect to a corresponding pp measurement of LHCb was found. This discrepancy could be related to the modification of the J/$\psi$ feed down fractions, due to the suppression of the excited states in the QGP, but also to the contribution of the regenerated J/$\psi$ in the low $p_{\rm{T}}$ region. Moreover, it has been hypothesized that quarkonium states could be polarized by the strong magnetic field, generated in the early phase of the evolution of the system, and by the large angular momentum of the medium in non-central heavy-ion collisions. This kind of information can be assessed by defining an ad hoc reference frame where the quantization axis is orthogonal to the event plane of the collision.
In this contribution, the new result of J/$\psi$ polarization with respect to the event-plane in Pb-Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV will be presented. The $p_{\rm T}$-differential measurement is performed at forward rapidity (2.5 $<$ y $<$ 4) and the results will be shown for different centrality classes. The preliminary measurement of the $\Upsilon$ polarization in pp collisions at $\sqrt{s} = 13$ TeV as a function of the transverse momentum will also be discussed.
The physics interpretation of the recent measurements of the spin polarization of Λ hyperons produced in relativistic heavy-ion collisions is discussed. It is suggested that the polarization measured in the Λ rest frame should be projected along the direction of the total angular momentum that is first transformed to the same frame, and only then averaged over Λ’s with different momenta in the center-of-mass frame. While the improved procedure is not expected to significantly change the present results regarding the global spin polarization, it may affect the estimates of the magnitude of the polarization and its energy dependence. Such a treatment is also generally more appropriate whenever directions in the Λ rest frame and in the center-of-mass frame are compared.
Recent observations for the spin polarization and alignments in RHIC and LHC have triggered intensive studies for vorticity-induced polarization and spin dynamics in relativistic fluids. We study the important, yet widely overlooked, role of gluons for spin transport with a connection to local parity violation in quark gluon plasmas. We extend the newly developed quantum kinetic theory for relativistic fermions coupled with background electromagnetic fields to the case coupled with non-Abelian chromo-electromagnetic fields and employ this formalism to derive the source terms in the axial-vector Wigner function and kinetic equation for spin polarization of quarks. These source terms, which may dominate over collisional effects at weak coupling, involve parity-odd correlators of dynamically generated color fields in near-equilibrium quark gluon plasmas and give rise to locally fluctuating axial charge currents. Our results provide a possible explanation for the spin alignment of vector mesons measured in high-energy nuclear collisions and allude to its connection with local parity violation.
References :
[1] Berndt Müller, Di-Lun Yang, Anomalous spin polarization from turbulent color fields, arXiv:2110.15630.
[2] Di-Lun Yang, Koichi Hattori, Yoshimasa Hidaka, Effective quantum kinetic theory for spin transport of fermions with collisional effects, JHEP 07 (2020) 070.
[3] Axial Kinetic Theory and Spin Transport for Fermions with Arbitrary Mass, Koichi Hattori, Yoshimasa Hidaka, Di-Lun Yang, Phys. Rev. D 100 (2019) 9, 096011.
By including the recently introduced thermal shear term of the spin polarization vector at local equilibrium we determine longitudinal polarization of Λ hyperons emitted from a hot and rotating hadronic medium using the thermal model with single freeze-out. In our analysis we consider top RHIC energy and use the model parameters which were determined in the previous analyses of particle spectra and elliptic flow. We confirm that unlike the previous calculations done by using only the thermal vorticity, the thermal shear term alone leads to the correct sign of the quadrupole structure of the longitudinal component of the polarization three vector as measured in experiments. However, we find almost complete cancellation between thermal and shear vorticity terms, which leads to the disagreement with the data. To clarify the role played by velocity and temperature gradient terms, we present a systematic analysis of different contributions to the longitudinal polarization.
The measurements of spin polarization of particles emitted in heavy-ion collisions has opened the possibility for new phenomenological investigations of spin physics in relativistic fluids. The theoretical predictions of global polarization are in agreement with the data, but consistent discrepancies stand out for the local polarization. In this talk, I will show that the covariant theory of quantum relativistic fluids at local equilibrium implies an additional, non-dissipative, contribution to the spin polarization vector which is proportional to the thermal shear which has been previously overlooked. This additional contribution together with an improved approximation in the expansion of the density operator at local equilibrium is able to restore the quantitative agreement between the theoretical predictions and the experimental data.
[F. Becattini, M. Buzzegoli and A. Palermo, Phys.Lett.B 820 (2021) 136519]
[F. Becattini, M. Buzzegoli, A. Palermo, G. Inghirami and I. Karpenko, arXiv:2103.14621]
The local Lambda polarization puzzle associated with the model calculations by thermal vorticity has attracted lots of attention in heavy ion community [1].
In addition to the widely studied thermal vorticity effect, we identify an undiscovered contribution from the fluid shear [2]. We obtain the explicit expression for shear-induced polarization (SIP) from quantum kinetic equation and linear response theory. Using hydrodynamic simulations, we find SIP effect competes with the thermal vorticity effect and shows right sign (trend) to the azimuthal dependent local spin polarization $P_y(\phi)$ and $P_z(\phi)$. Especially, in the scenario that Lambda inherits and memorizes the spin polarization of strange quark, SIP wins the competition and the obtained local $P_y(\phi)$ and $P_z(\phi)$ qualitatively agree with the data measured at top RHIC energy [2].
Furthermore, we extend the calculation to event-by-event simulations for various collision systems at RHIC and LHC. We calculate the 2nd order Fourier coefficient of $P_z$, which qualitatively agrees with the ALICE measurements in 5.02 A TeV Pb+Pb collisions [3, 4]. We also predict the 3rd order Fourier coefficients, which provide more details for spin polarization and can be measured in the RHIC isobar run with high statics [4].
Refs.
[1] B. Fu, K.Xu, X. G. Huang and H. Song, Phys. Rev. C103, no.2, 024903 (2021) ; and many related papers from other groups before.
[2] B. Fu, S.Y.F. Liu, L. -G. Pang, H. Song, Y. Yin, Phys.Rev.Lett. 127 14, 142301 (2021)
[3] ALICE Collaboration, arXiv:2107.11183 [nucl-ex]
[4] B. Fu, L. -G. Pang, H. Song, Y. Yin in preparation
The higher-order fluctuations of conserved quantities such as net baryon number are predicted to be sensitive to the non-equilibrium correlation length, $\xi$, and thus serve as indicators of critical behavior. Experimentally, fluctuations of proton and anti-proton numbers have been shown to be reliable proxies for baryons and anti-baryons. In the first Beam Energy Scan (BES-I) at the Relativistic Heavy Ion Collider (RHIC), which was run from 2010-2014, the higher-order cumulant ratio, $C_4/C_2$, of the net-proton multiplicity distributions shows a non-monotonic energy dependence between the energies of 7.7 to 62.4 GeV with a significance of 3.1$\sigma$. Motivated by the findings of BES I, the Solenoidal Tracker at RHIC (STAR) collaboration improved the detector performance of the STAR detector and began two additional physics programs: the BES-II and the fixed-target (FXT) program. While BES-II revisits the energies of BES-I with higher statistics and improved detector performance, the FXT program extends the lowest energy from $\sqrt{s_{_{\mathrm{NN}}}}$ = 7.7 GeV to $\sqrt{s_{_{\mathrm{NN}}}}$ = 3.0 GeV.
In this talk, we present the higher-order cumulants of proton multiplicity distributions of the FXT run in Au+Au collisions at $\sqrt{s_{_{\mathrm{NN}}}}$ = 3.0 GeV. The data, 140 million minimum bias events, were recorded with the STAR detector at the RHIC facility with a 250 $\mu$m thick target (1\% interaction probability). The ratios of both cumulants and correlation functions are presented as a function of centrality, acceptance, and collision energy. We discuss the physics implications of these results with comparisons to results from the HADES experiment and a hadronic transport model.
During the evolution of a heavy ion collision, the system passes close to the O(4) critical point. The order parameter that controls the chiral symmetry is the quark condensate $\langle \bar q q \rangle \sim \phi_a $. Due to the non-zero quark mass, there is a crossover (not a second order phase transition) between the high and low temperature phases. In this talk we will introduce the hydrodynamic theory with the order parameter as an additional hydrodynamical variable and show the results of the real time simulation of this model that shares the same universality class of QCD, the so-called Model G.
Due to the presence of the superfluid pion degrees of freedom, we compute the modification of the ordinary hydrodynamic transport coefficients and estimate the expected critical enhancement of soft pion yields, which provides a plausible explanation for the excess seen in experiment relative to ordinary hydrodynamic computations. We extract the dynamical critical exponent of $\zeta=1.47 \pm 0.01 ({\rm stat})$, which is compatible with the theoretical expectation of $\zeta = d/2$ (with $d=3$).
Based on 2005.02885, 2101.10847 and 2111.03640
Lattice QCD simulations have shown unequivocally that the transition from hadrons to quarks and gluons is a crossover when the baryon chemical potential is zero or small. Many model calculations predict the existence of a critical point at a value of the chemical potential where current lattice simulations are unreliable. We show how to embed a critical point in a smooth background equation of state so as to yield the critical exponents and critical amplitude ratios expected of a transition in the same universality class as the liquid-gas phase transition and the 3D Ising model. The resulting equation of state has parameters which may be inferred by hydrodynamic modeling of heavy ion collisions in the upcoming Beam Energy Scan II at the Relativistic Heavy Ion Collider.
In the context of the search for the QCD critical point using
non-Gaussian fluctuations, we obtain the evolution equations for
non-Gaussian cumulants within hydrodynamics to leading order of the
systematic expansion in the magnitude of thermal fluctuations. We
develop diagrammatic technique in which the leading order
contributions are given by tree diagrams. We introduce the concept of
Wigner transform for multipoint correlators and derive the evolution
equations for the novel three- and four-point Wigner functions. We
demonstrate in a simple model simulation how this formalism accounts
for the effects of memory and baryon number conservation on critical
point signatures in non-Gaussian fluctuations.
References: Phys.Rev.Lett. 127 (2021) 7, 072301 and work in progress.
We show that the values of the first three cumulants of the baryon number distribution can be used to calculate the isothermal speed of sound and its logarithmic derivative with respect to the baryon number density. We discuss application of this result to heavy-ion collision experiments and address possible challenges, including effects due to baryon number conservation, differences between proton and baryon cumulants, and the influence of finite number statistics on fluctuation observables in both experiment and hadronic transport simulations. In particular, we investigate the relation between quantities calculated in infinite, continuous matter and observables obtained in simulations using a finite number of particles.
Charged particles in heavy-ion collisions have various production mechanisms, such as thermal and associated production, and the importance of each changes with the collision energy. Studying the yields of charged particles provides a way to investigate the properties of the produced QCD matter in heavy-ion collisions and the various production mechanisms. The RHIC Beam Energy Scan (BES) programs cover a wide range of energies, including the transition from a hadronic dominated medium to a partonic dominated medium. The recently completed BES-II program was designed to improve and extend upon the results from the BES-I program. Of particular interest is the high baryon density region which is accessible through the STAR fixed-target program, extending the energy reach from $\sqrt{s_{NN}}=7.7$ GeV down to $\sqrt{s_{NN}}=3.0$ GeV. This presentation reports on measurements of charged particle production in Au+Au collisions at $\sqrt{s_{NN}}=3.0$ GeV. Measurements of the proton stopping will be presented in addition to measurements of the production of $K^{+}$ in association with the $\Lambda$ baryon. Invariant yields and rapidity density distributions of $\pi^{\pm}$, $K^{\pm}$, and $p$ will also be presented, which will help to unravel the relative importance of the different particle production mechanisms. These measurements provide an in-depth study of the various production mechanisms for light hadrons and probe unique properties of the high baryon density medium produced in these low energy collisions.
We explore jet-medium interactions at various scales in high-energy heavy-ion collisions using JETSCAPE 3, a publicly available software framework for Monte Carlo event generators. In jet shower evolution, the virtuality and energy of each jet parton vary considerably. Thus, in high-energy heavy-ion collisions, jets can be used as dynamical probes to investigate the jet-medium interaction at various scales. JETSCAPE is a framework that enables simulations describing physics at varying scales involved in in-medium jet evolution. The JETSCAPE framework incorporates multiple models, each effective at an individual scale range, and switches between them at appropriate scales while mediating their communication.
As a new feature, the jet quenching strength q-hat with an explicit virtuality dependence depending on the resolution scale evolution of jets is now supported by JETSCAPE. In this talk, we demonstrate that this further extension is crucial for a simultaneous description of the nuclear modification factor for inclusive jets and leading hadrons. For more detailed discussion of the virtuality dependence in the jet-medium interaction, observables for jet substructures, heavy-flavor jets, and photon-triggered jets are also explored. Furthermore, we investigate the jet-medium interaction involved in the hadronization process.
Jets correlated with isolated photons are a promising channel to study jet quenching in heavy-ion collisions, as photons do not interact strongly and therefore constrain the $Q^2$ of the initial hard scattering. The measurement of isolated single photon production constrains NLO pQCD predictions and PDFs, and isolated photon production in Pb-Pb collisions is sensitive to initial geometrical scaling and modifications of the nucleon structure function in nuclei. We present the isolated photon distributions measured in pp and Pb-Pb collisions and isolated photon-jet correlations measured in Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV by the ALICE collaboration. The isolated-photon production is measured in the $12 < p_\mathrm{T} < 60$ GeV/$c$ range in pp collisions and in the $10 < p_\mathrm{T} < 100$ GeV/$c$ for the Pb-Pb collisions. We study correlations of isolated photons above 20 GeV/$c$ with charged-particle jets above 20 GeV/$c$, reconstructed with the anti-$k_{\rm T}$ algorithm. The correlations probe the lowest $p_\mathrm{T}$ range measured so far at LHC energies, and larger modifications due to the QGP are expected in the lower $p_\mathrm{T}$ regime.
Measurements of high $p_\mathrm{T}$ hadrons produced in hard scattering events offer insight to the modification of jet fragmentation and medium response of the quark-gluon plasma (QGP)
created in ultrarelativistic nucleus-nucleus collisions.
The hard scatter, tagged by an electroweak boson or a jet, fixes initial properties of the showering partons prior to interactions with the QGP. In large systems, modification to the parton fragmentation is an expected consequence of the strong medium interactions, while in small systems, indications of QGP
droplet formation are juxtaposed with previous observations of minimal jet quenching.
With the large luminosity from Run 2 data taking, ATLAS has performed new measurements of hadrons correlated with Z bosons in Pb+Pb collisions, notably without any formal reconstructed jet requirement which reduces any potential bias toward
particular fragmentation patterns. Theoretical models of parton evolution in the QGP, particularly those with medium response, are compared to this data. Additionally, a new measurement of jet-hadron correlations in centrality-selected $p$+Pb collisions is presented and compared with theoretical expectations. The measurement provides stringent limits on both cold nuclear matter and possible jet quenching effects.
Z bosons can be used to constrain the initial energy, direction, and the flavor of the recoiling parton before its interaction with the quark-gluon plasma. By measuring charged particle yields in Z boson events one can study the in-medium modifications of the recoiling parton showers and as well as the soft particles from medium response. The talk will present measurements of the azimuthal angular distributions, fragmentation functions and $p_\mathrm{T}$ spectra of charged particles tagged with Z bosons in pp and PbPb collisions at $\sqrt{s_{_{\mathrm{NN}}}} = 5.02\,\mathrm{TeV}$ using data collected with the CMS detector.
We report high-statistics measurements of semi-inclusive distributions of charged jets recoiling from high-$E_{\text{T}}$ direct photon ($\gamma_{\text{dir}}$) and $\pi^{0}$ triggers in $p$+$p$ and central Au+Au collisions at $\sqrt{s_{NN}} = 200$ GeV. In a semi-inclusive approach, event bias is induced solely by the choice of trigger; separately utilizing $\gamma_{\text{dir}}$ and $\pi^{0}$ triggers in this analysis therefore provides direct comparison of jet quenching effects for jet populations with different q/g fractions and different in-medium path length distributions. Jets are reconstructed from charged particles using the anti-$\text{k}_{\text{T}}$ algorithm with jet resolution parameters $R_{\text{jet}} = 0.2$ and 0.5. The large uncorrelated background in central Au+Au collisions is corrected using a mixed event technique. This enables a jet measurement extending to low $p_{\text{T}}$ and large $R_{\text{jet}}$ with well-controlled systematic uncertainties, which are of particular importance in searching for jet scattering effects. We report recoil jet yield and trigger-jet acoplanarity distributions for jets with $p_{\text{T}} > 5$ GeV/$c$. The comparison of recoil yields in Au+Au and $p$+$p$ collisions at fixed $R_{\text{jet}}$ probes energy loss in heavy-ion collisions. Moreover, the comparison of recoil yields for different $R_{\text{jet}}$ in Au+Au and $p$+$p$ collisions probes intra-jet broadening. The modification of trigger-jet acoplanarity distributions in central Au+Au collisions relative to $p$+$p$ collisions highlights the sensitivity of such a measurement to QGP transport parameters. We also search for evidence of large-angle scattering of jets off of quasi-particles in the QGP. The measured recoil yields and acoplanarity distributions are compared to theoretical calculations.
Experimentalists and theorists are pushing towards studying large radius jets in heavy ion collisions in an endeavour to decode signs of induced radiation and medium response with increasing precision. However, even at mid-rapidity, jets are not entirely composed of final state emissions, but contain a varying amount of initial state radiation. This contribution is small for small radius jets, but increases with jet radius and can become sizeable for large radius jets. I study in detail the contributions from initial state radiation to different jet substructure observables in p+p and A+A collisions, and show how they can compromise the interpretation of these observables in heavy ion collisions in terms of medium effects. This is done with an extended version of JEWEL, in which partons emitted from the initial state parton shower interact in the background medium in the same way as final state partons.
In this work, we introduce both gluon and quark degrees of freedom for describing the partonic cascades inside the medium. We present numerical solutions for the set of coupled evolution equations with splitting kernels calculated for the static, exponential and Bjorken expanding media to arrive at medium-modified parton spectra for quark and gluon initiated jets respectively. We discuss novel scaling features of the partonic spectra between different types of media. Next, we study the inclusive jet $R_{AA}$ by including phenomenologically driven combinations of quark and gluon fractions inside a jet. In addition, we have also studied the effect of the nPDF as well as vacuum like emissions on the jet $R_{AA}$. Differences among the estimated values of quenching parameter for different types of medium expansions are noted. Next, the impact of the expansion of the medium on the rapidity dependence of the jet $R_{AA}$ as well as jet $v_2$ are studied in detail. Finally, we present qualitative results comparing the sensitivity of the time for the onset of the quenching for the Bjorken profile on these observables. All the quantities calculated are compared with the recent ATLAS data.
Even though QGP, when looked at on length scales of order the inverse of its temperature, is best described as a strongly coupled liquid, when it is observed at sufficiently short length scales or probed at sufficiently high exchanged-momentum, asymptotic freedom predicts the presence of quark-like and gluon-like quasi-particles. High energy partons (e.g. those in jet showers) traversing the QGP, capable of triggering these high-momentum exchanges with the medium constituents, have the potential to reveal the presence of such quasi-particles.
In this work we present an implementation of this physics within the hybrid strong/weak coupling model in which, prior to this work, only the non-perturbative aspects of parton energy loss had been accounted for. Interaction with the quasi-particles results in elastic Moliere scatterings, leading to deflection of the direction of the jet parton that induced the process as well as the excitation of partons from the thermal medium that recoil after being kicked. Throughout the in-medium evolution, the system of jet partons and recoils, which might further re-scatter, inject energy and momentum into the QGP, producing wakes. We analyze a variety of setups, such as boson-jet systems, c-cbar systems, as well as dijet systems, at different jet transverse momenta and reconstruction radius, R. We will discuss the effect of Moliere scatterings on acoplanarity distributions, as well as on some of the most widely used groomed and ungroomed jet substructure observables.
Given the large impact of the wakes generated by the hydrodynamic response of the medium on jet observables, as well as the presence of selection biases, finding unique signatures of the presence of the elastic scatterings is a challenging task. We will present some strategies that may be followed with a view toward enhancing and isolating the effect under consideration. These include the application of momentum cuts, differential studies of the angular distribution of particles around the jet, and, most revealing, the properties of the subjets (jets within jets) distributions. A comparison against the effect of a purely Gaussian, Brownian-like transverse momentum broadening will also be presented.
Measurements of anisotropic flow in heavy-ion collisions are key to extract properties of the quark-gluon plasma (QGP). The combination of different flow harmonics relative to the participant and spectator planes provides a unique insight into the initial conditions and the space-time evolution of such collisions. In particular, the spectator plane provides novel information about the three-dimensional orientation of the colliding system. We report on the centrality and transverse momentum dependence of anisotropic flow coefficients $v_n$ measured at the central pseudorapidity region relative to the participant, using the cumulant method, and newly also the spectator planes in Pb-Pb and Xe-Xe collisions. In ALICE, the spectator plane is reconstructed from the deflection of neutron spectators using the Zero Degree Calorimeters. The scaling of ratios of $v_2$ to eccentricities of the initial state $\varepsilon_2$ with entropy density in Xe-Xe and Pb-Pb collisions are shown, which quantify non-linear effects of the QGP hydrodynamic evolution. The ratio of $v_2$ relative to the spectator plane and $v_2$ relative to the participant plane is compared to ratios of the corresponding eccentricities predicted by models of the initial state. The transverse momentum dependence of the shape of flow fluctuations, specifically the skewness and kurtosis, is presented, which allows to probe its modification inside the QGP through the comparison to hydrodynamic models.
RHIC's capability to perform relativistic collisions of various ion species provides a unique opportunity to explore and constrain neutron skin thickness and deformation parameters of nuclei.
The study of neutron skin thickness $\Delta r_{np}$ of nuclei can help us directly infer nuclear symmetry energy. Such information is of critical importance to the equation of state of dense nuclear matter in neutron stars and the medium formed in heavy-ion collisions. The $\Delta r_{np}$ has traditionally been measured in low-energy hadronic and nuclear scattering experiments over decades. An alternate recent measurement using parity-violating electroweak interactions by the PREX-II experiment has yielded a large neutron skin thickness of Pb nucleus [1] that is in tension with the world-wide data established in hadronic collisions. In isobar collisions at relativistic energies, the effect of neutron skin was predicted [2] to yield different multiplicities and elliptic flows. They, in turn, provide an unconventional but more precise method to probe the neutron skin [3]. The idea is to compare the produced hadron multiplicities ($N_{\rm ch}$) [3], the mean transverse momenta ($\langle p_\mathrm{T}\rangle$) [4], and the net charge multiplicities ($\Delta Q$) [5] to trace back to the neutron skin differences between the isobar nuclei.
Nuclear deformation, a ubiquitous phenomenon for most atomic nuclei, reflects collective motion induced by the interaction between valence nucleons and shell structure. In most cases, the deformation has a quadrupole shape that is characterized by overall strength $\beta_2$ and triaxiality $\gamma$, and/or an octuple shape $\beta_3$. In relativistic collisions of two nuclei such deformations enhance the fluctuations of bulk observables that are sensitive to initial state geometry [6]. The deformation parameters can be constrained from the precision measurements of the ratios of harmonic anisotropy coefficients $v_2$, $v_3$, mean transverse momentum $[p_\mathrm{T}]$ fluctuations (mean, variance and skewness), and their Pearson correlation coefficient $\rho(v_n^2,[p_\mathrm{T}])$ between two isobar systems [7]. In Au+Au and U+U collisions the same can be done by performing measurement of $v_2$, cumulants of $[p_\mathrm{T}]$ distributions, and $\rho(v_n^2,[p_\mathrm{T}])$ [8].
In this talk we will discuss the aforementioned measurements in Au+Au, U+U and isobar $^{96}$Ru+$^{96}$Ru and $^{96}$Zr+$^{96}$Zr collisions at $\sqrt{s_{NN}}=200$ GeV using the STAR detector. We will discuss how we extract the neutron skin thickness and the symmetry energy slope parameter from these data. We will contrast our results in the context of the global data on symmetry energy and tension with the PREX-II data. We will discuss how the significant deviations of the ratios of $v_2$ and $v_3$ from unity in isobar collisions are indicative of large quadrupole and octuple deformations in Ru and Zr nuclei, respectively [9]. We will also discuss how the relative enhancement of $[p_\mathrm{T}]$-skewness, sign-change of $[p_\mathrm{T}]$-kurtosis and the suppression of $\rho(v_n^2,[p_\mathrm{T}])$ in U+U relative to Au+Au collisions are consistent with a large prolate deformation of the uranium nuclei.
~\
[1]~D. Adhikari et al. (PREX Collaboration), Phys. Rev. Lett. 126, 172502 (2021), arXiv:2102.10767 [nucl-th].
[2]~H. j. Xu, X. Wang, H. Li et al., Phys. Rev. Lett. 121, 022301 (2018), arXiv:1710.03086 [nucl-th].
[3]~H. Li, H. j. Xu, Y. Zhou et al., Phys. Rev. Lett. 125, 222301 (2020) arXiv:1910.06170 [nucl-th].
[4]~H. j. Xu, W. Zhao, H. Li et al., arXiv:2111.14812 [nucl-th].
[5]~H. j. Xu, H. Li, Y. Zhou et al., Phys. Rev. C 105, L011901 (2022), arXiv:2105.04052 [nucl-th].
[6]~C. Zhang and J. Jia, Phys. Rev. Lett. 128, 022301 (2022), arXiv:2109.01631 [nucl-th].
[7]~J. Jia and C. J. Zhang, arXiv:2111.15559 [nucl-th].
[8]~J. Jia, S. Huang and C. Zhang, Phys. Rev. C 105, 014906 (2022), arXiv:2105.05713 [nucl-th].
[9]~M. Abdallah et al. (STAR Collaboration), Phys. Rev. C 105, 014901 (2022), arXiv:2109.00131 [nucl-ex].
The far-from-equilibrium non-abelian plasma is created in the early stage of heavy-ion collision. The “bottom-up” mechanism is the well-studied phenomenological description of its approaching to thermal equilibrium, but has been restricted to leading order coupling within kinetic theory calculation. In our recent work, we provide a next-to-leading-order (NLO) weak-coupling description of the thermalization process of far-from-equilibrium non-abelian plasmas. Starting from either over- or under-occupied initial conditions, we follow their time evolution towards thermal equilibrium by numerically solving the QCD effective kinetic equation at NLO accuracy for isotropic non-abelian plasmas . It turns out that the NLO corrections remain well under control for a wide range of couplings and that the overall effect of NLO corrections is to reduce the time needed to reach thermal equilibrium in the systems considered.
Reference:arXiv:2110.01540 [hep-ph]
We employ an effective kinetic description to study the space-time dynamics and development of transverse flow of small and large collision systems. By combining analytical insights in the few interactions limit with numerical simulations at higher opacity, we are able to describe the development of transverse flow from very small to very large opacities, realised in small and large collision systems. Surpisingly, we find that deviations between kinetic theory and hydrodynamics persist even in the limit of very large interaction rates, which can be attributed to the presence of the early pre-equilibrium phase. We discuss implications for the phenomenological description of heavy-ion collisions and the applicability of viscous hydrodynamics to describe small and large collision systems.
[1] V.Ambrus, S.Schlichting, C.Werthmann arXiv:2109.03290
Determining the multi dimensional structure of protons and nuclei at high energy is one central goal of the future Electron-Ion Collider. This fundamental information is a crucial input for models describing the initial state of heavy ion collisions. In particular the event-by-event fluctuating proton geometry should have a strong effect on the flow and multiplicity distribution in high multiplicity proton-proton and proton-nucleus collisions [1], assuming that a strongly interacting medium is formed in these events. Understanding the subnucleon structure to a high degree of precision is thus a prerequisite of determining whether quark gluon plasma is created in small system collisions.
In order to extract the proton shape fluctuations (see [2]) from HERA exclusive vector meson production data in a statistically rigorous manner, we apply Bayesian inference[3]. This approach enables us to extract likelihood distributions for the non-perturbative parameters describing the proton fluctuating profile, including their correlations. The resulting posterior distributions allow for a systematic propagation of uncertainties when simulating for example high-multiplicity proton-proton and proton-nucleus collisions.
We determine how accurately the HERA data can constrain the proton fluctuating shape, and illustrate how high multiplicity proton-proton collisions can provide complementary input to exclusive scattering data.
[1] H. Mäntysaari, B. Schenke, C. Shen, Phys. Lett. B 772 (2017) 681-686, arXiv:1705.03177 [nucl-th]
[2] H. Mäntysaari, B. Schenke, Phys. Rev. Lett. 117 (2016) 052301, arXiv:1603.04349 [hep-ph], H. Mäntysaari, Rep. Prog. Phys. 83 (2020), 082201, arXiv:2001.10705 [hep-ph]
[3] H. Mäntysaari, B. Schenke, C. Shen, W. Zhao, arXiv:2201.xxxxx, in preparation
In Bayesian analyses of heavy ion collisions up to now one usually uses the TRENTo prescription for the initial state, followed by a free streaming initial stage. In this work, we extend this picture in two ways. Firstly, we generalize the TRENTo formula so that it is able to describe binary scaling. This introduces a parameter which we subsequently use to determine whether binary scaling is compatible with experimental data. Secondly, we replace the weakly coupled free streaming initial stage by a description that interpolates between weak and strong coupling, where a parameter controls the interpolation. As with the first extension, we confront the model with data, to determine whether data favors a weakly or strongly coupled initial stage.
A new approach is presented to explore the singularity structure of lattice QCD in the complex chemical potential and fugacity plane [1, 2, 3]. Our method can be seen as a combination of the Taylor expansion and analytic continuation approaches. Its novelty lies in using rational (Padé) approximants for studying Lee Yang edge singularities, which provide valuable insights to the occur- rence of critical phenomena in the thermodynamic limit. Several numerical experiments have been performed to test and demonstrate its accuracy and stability.
We present a calculations of the densities of conserved charges as well as chiral condensates as a function of imaginary baryon number chemical potential, obtained with highly improved staggered quarks (HISQ) at temporal lattice extent of $N_\tau=4,6,8$. We construct various rational function approximations of the lattice data and discuss how the closest singularities in the complex plane can be determined from them. We confirm stability of our results under conformal mappings. We discuss the universal scaling behavior of the Lee-Yang edge singularity and its role as a brunch-cut singularity in the order parameter. We apply the scaling in the vicinity of the Roberge-Weiss and chiral phase transitions. We find a temperature scaling that is in accordance with the expected power law behavior and determine some previously unknown non-universal constants. Finally we discuss the possibility to detect also the QCD critical end-point, if it exists, by this new method.
References
Fluctuations of conserved charges in a grand canonical ensemble
can be computed on the lattice and, thus, provide theoretical
input for freeze-out phenomenology. Electric charge fluctuations
and the corresponding higher order correlators are extremely
difficult, suffering form the most severe lattice artefacts.
We present new simulation data with a novel discretization where
these effects are strongly suppressed and provide continuum
extrapolated results in the temperature region of the chemical
freeze-out.
Jet-medium interactions in the Quark-Gluon Plasma can receive large non-perturbative infrared contributions. These contributions affect transverse jet momentum broadening and jet quenching. Both are influenced by the modified in-medium dispersion of jets encoded in their asymptotic mass.
An IR-safe computation of the latter requires subtracting the unphysical UV limit of electrostatic QCD (EQCD), a long distance EFT of thermal QCD, and supplying the correct UV limit obtained from Minkowski-time QCD. We perform the first step of this procedure in calculating the necessary operators in EQCD both analytically and on the lattice. We find compelling agreement of the two methods in the ultraviolet regime.
Dense QCD matter can exhibit spatially modulated regimes. They can be characterized by particles with a moat spectrum, where the minimum of the energy is over a sphere at nonzero momentum. Such a moat regime can either be a precursor for the formation inhomogeneous condensates, or signal a quantum pion liquid. We discuss the underlying physics of the moat regime based on studies in low-energy models and preliminary results in QCD. Heavy-ion collisions at small beam energies have the potential to reveal the rich phase structure of QCD at low temperature and nonzero density. We show how moat regimes can be discovered through such collisions. Particle production is enhanced at the bottom of the moat, resulting in a peak at nonzero momentum, instead of zero, in the particle spectrum. Particle number correlations can even increase by several orders of magnitude at nonzero momentum in the moat regime.
We propose a new model for a homogeneous description of hadron-hadron, hadron-nucleus and nucleus-nucleus collisions, the Gluon Exchange Model (GEM). While technically our model can be regarded as a generalization of the Dual Parton Model by Capella and Tran Thanh Van, it is fundamentally based on the number of exchanged color octets (gluons) and significantly extends the Fock space of states available for the participating protons and nucleons.
In proton-proton collisions we provide an exact description of the final state proton and neutron spectrum. What is remarkable is that unlike the original DPM, GEM successfully describes the proton "diffractive peak" at high rapidity.
In proton-nucleus reactions we propose a statistical scheme for the process of soft color octet (gluon) exchange, based on the assumption that probabilities to form an effective diquark are equal for all allowed pairs of quarks. The latter effective diquark can form either from two valence, one valence and one sea, or from two sea quarks. Consequently we calculate the probabilities for different color configurations involving diquarks of valence-valence, valence-sea and sea-sea type. These probabilities appear to depend on the number of exchanged gluons, which results in increasing baryon stopping as a function of the number $N$ of proton-nucleon collisions in the nucleus. As such, the baryon nuclear stopping power appears to be governed by the emergence of new color configurations as a function of $N$ rather than by the energy loss of the original valence diquark.
The advantage of our approach lies in its high predictive power which makes it verifiable by the new, precise data on proton and neutron production from the CERN SPS. The latter verification, a set of predictions for the $N$-dependence of the baryon stopping process, and a discussion of implications for proton-oxygen collisions planned at the LHC, will be included in the talk.
References:
[1] M. Jeżabek and A. Rybicki, Phys. Lett. B816, 136200 (2021).
[2] M. Jeżabek and A. Rybicki, 2111.03401 [nucl-th].
Jet-induced medium response carries the information for the properties of quark gluon plasma produced in high-energy heavy-ion collision. Diffusion wake as an unambiguous part of the medium response will lead to a depletion of soft hadrons in the opposite direction of the jet propagation. New experimental data on Z-hadron correlation in Pb+Pb collisions at the Large Hadron Collider show, however, an enhancement of soft hadrons in the direction of both the Z and the jet. Using a coupled linear Boltzmann transport and hydro model, we demonstrate that medium modification of partons from the initial multiple parton interaction (MPI) gives rise to a soft hadron enhancement that is uniform in azimuthal angle that can overwhelm the signal of the diffusion wake. To make the effect of diffusion wake visible in the near-side Z-hadron correlation, we use a mixed-event procedure to subtract the contribution from MPI. We further employ the longitudinal and transverse gradient jet tomography for the first time to localize the initial jet production positions in Z/$\gamma$-jet events in which the effect of the diffusion wake is apparent in Z/$\gamma$-hadron correlation even without the subtraction of MPI.
It is well established that hard partons lose energy as they traverse the quark-gluon plasma (QGP). However, while there has been significant work to describe the mechanism by which this occurs, the relative contributions of the microscopic processes have yet to be constrained experimentally. One way to address this question is to exploit the theoretically derived relationship between the parton energy loss mechanism and its associated path-length dependence. The ALICE experiment has taken a multipronged approach to studying this path-length dependence in Pb-Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV by exploring the link between soft and hard observables. These efforts include the incorporation of flow-like observables into traditional jet measurements, as well as jet-like correlations.
This talk will present the $v_{2}$ measurement for charged particles at midrapidity in p-Pb and Pb-Pb collisions, which show significant non-zero values from high to low $\it{p}_{\rm T}$ for the first time that may be due to path-length dependent jet quenching. Jet yields measured with Event-Shape Engineering selections, which use the $q_{2}$ vector to classify events with varying anisotropies but similar bulk properties, will be shown to further explore this possibility. Additionally, results for correlations between hard trigger $\pi^{0}$s and recoil hadrons will be compared with respect to the event-plane angle. Jet constituent yields calculated from jet-hadron correlations with low momentum jets will also be considered.
It has been shown that high-energy partons lose energy when traversing the hot, dense medium produced in heavy-ion collisions. However, the mechanism of the energy loss, including its dependence on the path-length of the shower in the medium and sensitivity to the jet substructure, is not fully understood. This talk presents a new measurement of single jet yields as a function of the azimuthal angle with respect to the event plane in Pb+Pb collisions at $\sqrt{s_{NN}} = 5.02$ TeV. Because partons produced at different angles with respect to the event plane traverse, on average, different path lengths of the medium, this measurement gives insight into the path-length dependence of parton energy loss. The azimuthal angle dependence of the yields is characterized by the parameter $v_n^{jet}$, which quantifies the magnitude of the modulation of the azimuthal angle distribution with respect to the $n^{th}$ order event plane. While ATLAS has previously reported the $v_2^{jet}$ in Pb+Pb at $\sqrt{s_{NN}} = 2.76$ TeV, this is the first ATLAS measurement of higher-order $v_n^{jet}$. The $v_2$, $v_3$, and $v_4$ are measured for jets with $p_T = 71-398$ GeV as a function of $p_T$ and collision centrality. A nonzero value of $v_2$ is observed in all but the most central collisions. A smaller nonzero value of $v_3$ is measured, suggesting that fluctuations in the initial state play a small but distinct role in jet energy loss.
This talk also presents measurements of jet substructure performed using various jet (de)clustering and grooming techniques. Measurements of inclusive jet suppression ($R_{AA}$) in heavy-ion collisions are presented for the first time as a function of the jet substructure using both nominal ($R=0.4$) and large-radius ($R=1.0$) jets in Pb+Pb and $pp$ collisions at $\sqrt{s_{NN}} = 5.02$ TeV. The jet substructure is characterized using the Soft-Drop grooming procedure in order to identify subjects corresponding to the hardest parton splitting in the jet. The dynamics of jet quenching is measured and presented as a function of the transverse momentum scale ($\sqrt{d_{12}}$) and the angle of the hardest splitting in the jet. These measurements provide new information about the path-length dependence of jet quenching and the sensitivity of jet suppression to its substructure.
Jet quenching is a well-established signature of quark-gluon plasma formation in heavy ion collisions. Studies of the transverse momentum balance of back-to-back jets, as well as medium-induced modifications to jet shapes and fragmentation functions, provide important experimental constraints on quark-gluon plasma properties. Using a large sample of dijet events from 5.02 TeV PbPb and pp collisions recorded by CMS, we study quenching effects differentially with respect to the dijet transverse momentum balance. We use short range correlations between jets and charged particles to assess medium-induced modifications to jet substructures on each side of the dijet. The path-length dependent energy loss and energy density fluctuations are also probed using long range correlations between jets and charged particles.
The upcoming run of oxygen-oxygen (OO) collisions at the LHC offers unique experimental and theoretical opportunities to address the long standing question of high-momentum rescattering (jet quenching) in small collision systems. We have demonstrated previously that even small energy loss effect can be observed in nuclear modification factor thanks to high precision pQCD baseline calculations in inclusive oxygen-oxygen collisions. However currently there is no pp reference measurement planned at OO collision energy (6.37TeV). Therefore in our recent work we analyzed the reliability of several techniques for computing jet and hadron spectra at different collision energies. We computed the ratio of spectra between different pp collision energies in perturbative QCD, which can be used to construct a reference spectrum. Alternatively, it can be interpolated from measured spectra at nearby energies. We estimate the precision of both strategies for the spectra ratio relevant to the oxygen run. Finally we propose taking the ratio of OO and pp spectra at different collision energies, which cleanly separates the experimental measurement and theoretical computation.
Refs:
1. J. Brewer, A. Huss, A. Mazeliauskas, W. van der Schee, 2108.13434
2. A. Huss, A. Kurkela, A. Mazeliauskas, R. Paatelainen, W. van der Schee, U. Wiedemann, Phys.Rev.Lett. 126 (2021) 19, 192301, 2007.13754
3. A. Huss, A. Kurkela, A. Mazeliauskas, R. Paatelainen, W. van der Schee, U. Wiedemann, Phys.Rev.C 103 (2021) 5, 054903, 2007.13758
While a variety of jet substructure measurements have been performed in heavy-ion collisions, a unified understanding of how the QGP affects the angular and momentum structure of jets remains an open question. One of the prominent puzzles is that measurements indicate no significant modification of the jet mass in heavy-ion collisions relative to proton-proton collisions, but significant narrowing of the jet girth. In order to shed light on this puzzle, we present new systematic measurements of a flexible and perturbatively calculable class of jet substructure observables known as the jet angularities. We report angularities spanning a wide range of angular regimes, mapping the transition from girth to mass and beyond. In order to further study the momentum structure of jets, we present the first measurements of the momentum fraction carried by sub-jets reclustered from primary jet constituents. These measurements extend to higher $z$ than hadron fragmentation measurements, enabling access to a quark-dominated sample of jets and exposing new insights about soft medium-induced radiation. We compare both the jet angularity and sub-jet fragmentation distributions to a variety of theoretical implementations of jet quenching, providing critical information on the medium modifications of jets as a function of their angular and momentum structure.
High energy partons are well established to lose energy when traversing the hot and dense medium produced in heavy-ion collisions. This results in a modification to the transverse momentum distributions of jets, producing a phenomenon known as jet quenching. It has been previously established in Pb+Pb collisions at $\sqrt{s_\textrm{NN}}~=~2.76$~TeV that jet quenching results in significant modifications to the
transverse momentum balance of dijet pairs. More differential measurements are needed to better understand the asymmetric jet quenching observed and explore the role of energy loss fluctuations and path-length dependent energy loss.
In this talk, we report new, fully unfolded measurements of the dijet momentum balance in Pb+Pb and $pp$ collisions at $\sqrt{s_\textrm{NN}}~=~5.02$~TeV with extended kinematic reach over previous publications, as well as in Xe+Xe collisions at $\sqrt{s_\textrm{NN}}~=~5.44$~TeV.
This talk will additionally present a new observable, the nuclear modification factor of subleading and leading jets, which provides a
precise quantification of asymmetric energy loss experienced by dijets.
Crossover Scenario
If there were a first-order phase transition, some signatures could be expected, but the most challenging is an experimental confirmation of realistic crossover scenario. It is already known from the NS observations / pQCD calculations that a crossover transition to quark matter is likely to occur around 3-5 times normal nuclear density, where a slope parameter on the energy-pressure plane exhibits a significant change. The difficulty lies in the fact that the relevant density is too high even in cores of heavy NSs.
Gravitational Waves Signal
We will discuss prospects of gravitational waves to distinguish the EoSs with/without not a first-order but a crossover transition to quark matter. To this end, we will emphasize the importance of multiple observations as follows.
[Inspiral Stage]
So far, we still have to wait for the post-merger signals, and it is crucial to make analysis to clarify what we can infer from the inspiral stage (before binary NSs merge). We show the gravitational waves obtained from the numerical relativity to constrain the tidal deformation parameter from which we can make extrapolation of EOS to high density in a scenario with/without crossover.
[Post-merger Stage]
We then demonstrate the effect of crossover in the post-merger stage (after binary NSs start merging). We will report a prominent difference between two scenarios with/without crossover. The life time from the merger till the collapse into a blackhole significantly depends on the softened EOS in the dense region of 4-5 times normal nuclear density. We also quantify theoretical uncertainties from the finite-temperature extension that is parameterized by the thermal index. We point out that it is of tremendous importance to take account of the density dependence of the thermal index.
Conclusion
We can make use of already available gravitational wave signal in the inspiral stage to constrain the EOS of cold and dense matter before crossover. Since the uncertainty in this density region is sufficiently reduced (and the finite-temperature uncertainty is absent!), we can apply it to the post-merger analysis up to an hypothetical (but very likely) crossover density, which enables us to probe the crossover from the life time after the merger, which has been quantified by our simulations of the numerical relativity with the most realistic EOS.
I discuss the recent progress in state-of-the art perturbative QCD calculations of the equation of state at large chemical potential. I describe why these calculations that are reliable at asymptotically high densities constrain the equation of state at neutron star densities, and describe how the theoretical calculations can be confronted with multimessenger observations to empirically determine the equation of state. I argue that the properties of the EOS reflect the underlying phase structure and may be used to determine the phase of matter in the cores of neutron stars.
In this talk, we analyze the recent astrophysical constraints in the context of a hadronic equation of state (EoS), in which the baryonic matter is subject to chiral symmetry restoration. We show that it is possible to reconcile the modern constraints on the neutron star (NS) radius and tidal deformability (TD). We find that the softening of the EoS (required by the TD constraint) followed by a subsequent stiffening (required by the $2~M_\odot$ constraint) is driven by the appearance of $\Delta$ matter due to partial restoration of chiral symmetry. Sufficiently early onset of $\Delta$ matter lifts the tension with TD from GW170817. We argue that a purely hadronic EoS that accounts for the fundamental properties of quantum chromodynamics (QCD) linked to the dynamical emergence of parity doubling with degenerate masses can be fully consistent with the nuclear and astrophysical constraints. Therefore, the conclusion about the existence of quark matter in the stellar core may be premature.
Given the lack of empirical evidence of weakly interacting dark matter, it is reasonable to look to other candidates such as a confining dark sector with a similar number of particles as the standard model. Twin Higgs mirror matter is one such model that is a twin of the standard model with particles masses 3-6 times heavier than the standard model that solves the hierarchy problem. This generically predicts mirror neutron stars, degenerate objects made entirely of mirror nuclear matter. We find their structure using a realistic equation of state from crust (nuclei) to core (relativistic mean-field model) and scale the particle masses using lattice QCD results. We find that mirror neutron stars have unique signatures that are detectable via gravitational waves and binary pulsars, suggesting an impressive discovery potential and ability to probe the dark sector.
The strong interaction among stable and unstable hadrons is a fundamental question in nuclear physics and a key ingredient for the description of the equation of state and the understanding of the structure of dense stellar objects such as neutron stars. Traditional measurements, including scattering and hypernuclei experiments, are insufficient to provide stringent constraints to the theoretical modeling of the interaction between hadrons containing strangeness. Two-particle correlation measurements are a prominent tool to probe the strong interaction with high precision even in the multi-strangeness sector. The ALICE Collaboration has demonstrated that high-multiplicity pp collisions are particularly well suited due to the enhanced production of strangeness in such collisions. Combined with the excellent tracking and particle identification capabilities of the ALICE detector, precision studies of the strong interaction among strange hadrons are feasible. The present contribution will discuss the latest ALICE results on the study of p-$\Lambda$ ($S = -1$), $\Lambda$-K$^-$ ($S = -2$) and $\Lambda$-$\Xi^-$ ($S = -3$) interaction, which provide the most rigorous constraints in this field, and their interpretation in the context of the available theoretical predictions. The impact of these results on the equation of state of neutron stars is discussed.
We outline the role that an early deconfinement phase transition from normal nuclear matter to a color superconducting quark-gluon plasma phase plays for the phenomenology of supernova explosions and binary neutron star mergers. To this end we extend the compact star equation of state (EoS) from vanishing to moderately high temperatures that become accessible in the BM(a)N and MPD experiments at NICA as well as for CBM at FAIR. We study the connection of such hybrid EoS with the mass-radius relation of cold compact stars, including the intriguing possibility of additional families, as a consequence of the presence of an early and strong phase transition. Special emphasis is devoted to the simultaneous fulfillment of the new NICER mass and radius constraint from PSR J0740+6620 and the tidal deformability constraint from GW170817 which require the EoS to be soft at about twice saturation density and then to stiffen. Such a pattern is provided by anBlaschke early and strong deconfinement transition. Dynamical scenarios are being considered, such as binary compact star mergers including the subsequent emission of gravitational waves and supernova explosions of massive supergiant stars where neutrinos play the role of messengers.
*) This work is supported by NCN under grant number 2019/33/B/ST9/03059.
Using the second law of local thermodynamics and the first-order Palatini formalism, we formulate relativistic spin hydrodynamics for quantum field theories with Dirac fermions, such as QED and QCD, in a torsionful curved background. We work in a regime where spin density, which is assumed to relax much slower than other non-hydrodynamic modes, is treated as an independent degree of freedom in an extended hydrodynamic description. Spin hydrodynamics in our approach contains only three non-hydrodynamic modes corresponding to a spin vector, whose relaxation time is controlled by a new transport coefficient: the rotational viscosity. We study linear response theory and observe an interesting mode mixing phenomenon between the transverse shear and the spin density modes. We propose several field-theoretical ways to compute the spin relaxation time and the rotational viscosity, via the Green-Kubo formula based on retarded correlation functions.
Observations of strong azimuthal anisotropies ($v_n$)-- and their agreement with some hydrodynamic calculations-- in p+A collisions at RHIC and LHC have led to the suggestion that such collisions produce the smallest droplets of QGP. This hypothesis may be tested from a different angle through hyperon polarization measurements. In particular, central p+A collisions may naturally produce an initial state in which the longitudinal flow pattern depends on the transverse radial coordinate. The generic response of any fluid to such an initial condition is the generation of expanding vortical toroids--smoke rings. We use 3D viscous hydrodynamics (implemented in the MUSIC framework) to explore these unique structures in p+A collisions. We present an experimental observable which probes for their existence and provide quantitative predictions as a function of collision energy and system size. The effects of "lumpy" initial states, various definitions of vorticity, and newly-discovered symmetric shear terms will be discussed. Finally, experimental challenges for observing this unique structure will be discussed.
Suppression of charmonia is one of the most distinctive signatures of Quark-Gluon Plasma (QGP) in heavy-ion collisions. Suppression can also take place in hadron-nucleus collisions due to cold nuclear matter (CNM) effects where the presence of QGP is not expected. The hadron-nucleus collisions are therefore important as they help to disentangle the effects of the QGP from those due to CNM. Charmonium production in hA collisions at fixed-target energies is sensitive to the effects of nPDF and the partonic energy loss in nuclear matter. It is conveniently complemented by the well-known Drell-Yan process.
The double differential ($x_{\rm F}$, $p_{\rm T}$) cross-sections of J/$\psi$ production and Drell-Yan process have been measured by the COMPASS collaboration in hA collisions at $\sqrt{s} = 18.9$ GeV. A negative pion beam with a momentum of 190 GeV/c was impinging on ammonia, aluminum, and tungsten targets. The preliminary results for the ratios of heavy to light targets for both charmonia production and Drell-Yan show suppression towards high $x_{\rm F}$. A dependence with $p_{\rm T}$ is also investigated, which might indicate the presence of energy loss effects. COMPASS findings on the nuclear effects of the J/$\psi$ production and Drell-Yan process will be presented. The results will be compared to the available fixed-target and collider measurements in order to explore scaling behavior and energy dependence and will be followed by the comparison with theoretical model predictions.
NA61/SHINE is a fixed target experiment at the CERN Super Proton Synchrotron. The main goals of the experiment are to discover the critical point of strongly interacting matter and to study the properties of the onset of deconfinement. In order to reach these goals, a study of hadron production properties is performed in nucleus-nucleus, proton-proton and proton-nucleus interactions as a function of collision energy and size of the colliding nuclei. The experiment has recently completed data acquisition for its original programme on strong interactions. The Collaboration has gathered rich data in a two-dimensional scan: varying the beam energy and the sizes of colliding nuclei.
In this talk, the new results on identified charged kaon production in the intermediate size system (40Ar+45Sc and 7Be+9Be) collisions at SPS beam momentum range (13𝐴-150𝐴 GeV/𝑐) will be shown. Additionally, the new measurements of strange resonances (K0∗(892), Ξ0(1530) and Ξ(1530)) and strange baryons (Ξ-(1321), Ξ+(1321)) produced in p+p interactions will be presented. The kinematic distributions and measured multiplicities of identified hadrons will be compared with available world data and relevant models in the context of collision energy and system size dependence.
The ultra-peripheral collisions (UPC) of relativistic heavy ion beams lead to a diverse set of photon-nucleus interactions.
The measurements of particles and their interaction produced in photo-nuclear reactions can shed light on the QCD dynamics of novel, extremely asymmetric colliding systems, with energies between those available at RHIC and the LHC.
Understanding the hadronic fluctuation spectrum of the photon in this fashion is also critical for maximizing the precision of measurements at the future Electron-Ion Collider.
This talk presents the final measurement of two-particle long-range azimuthal correlations in photo-nuclear collisions using 5.02~TeV~Pb+Pb data collected in 2018 by ATLAS, with a dedicated photo-nuclear event trigger.
Candidate photo-nuclear events are selected using a combination of the single-sided zero-degree calorimeter activity and reconstructed pseudorapidity gaps constructed from calorimeter clusters and charged-particle tracks.
Correlation functions are constructed using charged-particle tracks, separated in pseudorapidity. A template fitting procedure is utilized to subtract the non-flow contribution.
Elliptic and triangular flow coefficients are presented as a function of charged-particle multiplicity and transverse momentum, and significant non-zero values of the flow coefficients are observed.
The results are compared to flow coefficients obtained in $pp$ and $p$+Pb collisions in similar multiplicity ranges, and to quantitative theoretical expectations.
Over the last years, evidence of collective behavior has been observed in high-multiplicity collisions of small systems, however, its origin is not yet understood. In this talk, we will present the ﬁrst measurement of ultra-long-range azimuthal angle correlations of identiﬁed particles in small collision systems by using forward detectors of ALICE, which allows the largest pseudorapidity separation of the particle correlations, $\Delta\eta\sim 5$, and signiﬁcantly suppresses the non-ﬂow contamination. We will show the $p_{\rm T}$-differential flow, $v_n$, of identiﬁed hadrons with many diﬀerent species, over a large $p_{\rm T}$ 0.2–8 GeV/c in both p–Pb and pp collisions. Strong evidence of splitting between the $v_n$ of baryons and mesons is observed in the intermediate and high-$p_{\rm T}$ regions. Such behavior can be explained by the quark coalescence mechanism, pointing to the presence of the partonic collectivity in small systems. Furthermore, we extend measurements of the $v_n$ of non-identiﬁed particles with the requirement of presence of hard probes such as jets or leading high-$p_{\rm T}$ particles in an event (“event-scale” dependence of $v_n$), as well as with the dependence of $v_n$ over a wide rapidity range with the Forward Multiplicity Detector (up to $\Delta\eta\sim 8$) and compare results with models. To further constrain the properties of the partonic matter created in small systems, the measurement of nonlinear ﬂow modes and the symmetric cumulants in pp, p–Pb, Xe–Xe and Pb–Pb collisions will be presented. The results are compared to a comprehensive collection of model calculations, including hydrodynamic and transport models. Finally, to investigate the origins of ﬂow in small systems and pin down any potential contribution from initial momentum anisotropy that appears in the color glass condensate eﬀective theory, the newly measured correlation between the mean transverse momentum and ﬂow coeﬃcients, $\rho(v_n^2,[p_{\rm T}])$, will be discussed. All the above measurements are based on the entire data taken from the LHC Run 2 by ALICE.
The azimuthal anisotropies observed in small systems can originate from the final state response to the initial geometry as well as from initial momentum anisotropies. Recently it has been proposed that the correlation between the flow coefficient $v_{2}^2$ and the mean $p_\mathrm{T}$ carries information on the origin of flow in small collision systems by showing a characteristic sign change at very low multiplicity. However, this sign change exists in PYTHIA8 events as a result of nonflow effects. To reduce the nonflow dependence , a new correlator that correlates multiparticle cumulants and mean $p_\mathrm{T}$ is suggested. In this talk, we present results for this correlator using two and four particle correlations in pp, pPb and peripheral PbPb collisions. We also report our high precision measurements of $v_{2}$ using four-, six-, and eight-particle correlations, together with $v_{3}$ from four particle correlations, in both pPb and peripheral PbPb collisions. The ratios between $v_{n}$ harmonics involving different numbers of particles are compared to model calculations to study the fluctuation-driven initial state anisotropies. The results provide insights to the origin of flow in small collision systems.
The experimental observations of anisotropic flows in proton-proton and
proton-nucleus collisions at RHIC and LHC energies has stimulated a big
interest in these small systems as a new study area for the formation and
evolution of the quark-gluon plasma. We investigate the effects of
non-equilibrium dynamics in such systems by comparing a microscopic
nonequilibrium transport approach, the Parton-Hadron-String-Dynamics
(PHSD), with a macroscopic 2D+1 viscous hydrodynamical model, VISHNew,
that describes a locally approximately equilibrated medium. The initial
conditions for the hydro evolution are taken from PHSD at different
starting times in order to study its impact on the subsequent evolution of
the short-lived QGP created in proton-nucleus collisions, investigated in
terms of energy density, viscous corrections, spatial and momentum
eccentricities. The latters have been linked to the development of
collective flows, whose origin is high-multiplicity proton-nucleus
collisions is still under debate. We address this issue also by means of a
new and more differential observable, the transverse spherocity, which
classifies final-state event topologies and allows to isolate hard and
soft effects. The investigation of such quantity in both transport and
hydro frameworks permits to gain further insights into the mechanisms
responsible for the QGP-like effects in small systems.
Small collision systems exhibit features that are characteristic of collective flow, a hallmark of QGP formation. However, jet quenching in small systems has not yet been observed, and quantifying or setting limits on the magnitude of jet quenching in small systems is a key element in understanding the limits of QGP formation. In this talk we present a search for jet quenching effects in pp collisions at $\sqrt{s} = 13\ \mathrm{TeV}$ based on two jet observables: inclusive jet production, and the semi-inclusive yield of jets recoiling from a high-$p_{\rm T}$ hadron. Both measurements are carried out differentially in event multiplicity, which varies the size of the collision system. Jets are reconstructed from charged particles using the anti-$k_\mathrm{T}$ algorithm, with $R$ between 0.3 and 0.7. The $R$-dependent inclusive jet cross section is compared to pQCD calculations. To search for jet quenching effects, the shape of the inclusive jet yield in different multiplicity intervals is compared to the one obtained in minimum-bias (MB) events. The jet yield increases as a function of charged-particle multiplicity, which is similar to the one observed from soft sectors. In the semi-inclusive analysis, recoil jet acoplanarity is measured for events selected on high multiplicity (HM) and compared to the MB population. A striking modification of the acoplanarity distribution, which is nominally characteristic of jet quenching, is observed in the measured HM population. Its origin is elucidated by comparison to model calculations, with implications for the larger LHC small-systems program.
In this talk we extend the novel expansion scheme introduced in [1] to explore the impact of a strange and electric charge chemical potential. We focus on the equation of state along the strangness neutral line, which allows us to match conditions in heavy ion collision experiments. We are also able to extrapolate different thermodynamic quantities to values of the strangeness and electric charge densities beyond those corresponding to the strangeness neutrality conditions.
[1] S. Borsányi et al., Phys.Rev.Lett. 126 (2021) 23, 232001
Compared to the earlier calculation of the equation of state of QCD with physical light and strange quark masses, performed in 2017, the HotQCD collaboration has accumulated an order of magnitude larger statistics for up to 8th order cumulants on lattices with temporal extent Nt=8 and 12 and added results for Nt=16 that were not available previously. We use these high statistics results on Taylor expansion coefficients for an updated calculation of the equation of state in (2+1)-flavor QCD at non-zero net baryon-number density. We show that previously observed ''wiggles'' in the Taylor series for e.g. the net baryon-number, smoothen out with increasing statistics, confirming that there is no hint of a breakdown of the Taylor expansion up to baryon chemical potential $\mu_B/T=2.5$.
We compare calculations for pressure, energy and entropy densities as well as net baryon-number densities with HRG model calculations based on the recently constructed QMHRG2020 hadron list, which in addition to the hadronic resonances listed by the Particle Data Group, also includes resonances calculated in relativistic quark models. We discuss the sensitivity of the QCD equation of state to the choice of the hadron spectrum. We also provide an update for the speed-of-sound and use lattice QCD results for (2+1)-flavor QCD to discuss the sensitivity of the dip in the speed-of-sound to the occurrence of a chiral phase transition in the universality class of O(N) spin models.
One of the central goals in QCD with non-vanishing conserved charge densities is to find evidence
for the existence of the so-called critical end point (CEP) in the QCD phase diagram. Lattice QCD
calculations at smaller than physical quark masses, combined with our model based understanding of
the QCD phase diagram, suggest that this critical point, if it exists, needs to be searched for at
temperatures below $T\sim 140~$MeV. Following the rather small decrease of the QCD pseudo-critical
temperature with increasing baryon chemical potentials ($\mu_B$), it is expected that such low
temperatures will only be reached for $\mu_B\ge 400~$MeV. These large values of $\mu_B$ and low
temperatures are reached at freeze-out in heavy ion collisions only for beam energies less than 10 GeV.
In this low temperature, high density regime studies of QCD thermodynamics with straightforward
Taylor expansions are likely to fail. We extend our Taylor series expansion down to temperatures of
125~MeV and use the high statistics results for conserved charge cumulants up to 8th order, obtained
by the HotQCD collaboration, to resum Taylor expansions of the logarithm of the QCD partition function.
We will construct resumed results for cumulants of conserved charge fluctuations at low temperature and
high density and show that the diagonal Pade-approximants for 8th order Taylor series in all three conserved
charge chemical potentials can have real poles, signaling the occurrence of a phase transition, only at
temperatures below 140 MeV. This gives further support for a CEP at low temperatures.
We will use these Taylor expansions and their Pade resummations for a determination of freeze-out conditions
through QCD calculations of the mean, variance as well as the ratio of mean and variance of conserved charge
fluctuations. We compare these calculations with experimental determinations of freeze-out parameters obtained
by STAR in the BES at RHIC. In particular, we find good agreement between QCD results and the STAR measurement
of the ratio of strangeness and baryon chemical potentials. We point out that this ratio of chemical potentials
combined with the statement that freeze-out occurs close to the pseudo-critical temperature $T_{pc}(\mu_B)$
determined by ALICE and STAR is consistent with HRG model calculations only when additional strange baryon resonances
are included in the spectrum used for HRG model calculations.
We present a novel approach to nonperturbatively estimate the heavy quark momentum diffusion coefficient, which is a key input for the theoretical description of heavy quarkonium production in heavy ion collisions, and is important for the understanding of the elliptic flow and nuclear suppression factor of heavy flavor hadrons. In the heavy quark limit, this coefficient is encoded in the spectral functions of color-electric and color-magnetic correlators that we calculate on the lattice to high precision by applying gradient flow. In our study we consider quenched QCD at $1.5\,T_c$, where we perform a detailed study of the lattice spacing and flow time dependence of color-electric and color-magnetic correlators, and, using theoretically well-established model fits for the spectral reconstruction,
we estimate the heavy quark diffusion coefficient. Equipped with the experience obtained in quenched QCD, we estimate the heavy quark diffusion coefficient from 2+1 flavor QCD ensembles at small but finite lattice spacing.
The jet transport coefficient $\hat{q}$ is the leading coefficient that characterizes the transverse broadening of the hard parton traversing QGP. The transverse kicks received from the medium changes the off-shellness of the hard parton, which leads to enhancement in the gluon emissions. Since the transverse broadening is the dominant mechanism responsible for the suppression of the high-transverse momentum charged-hadrons and jets, understanding the temperature and parton’s momenta dependence of $\hat{q}$ are crucial.
In this talk, we present for the first time a lattice QCD calculation of $\hat{q}$ in pure gluonic plasma and $n_{f}=3$ QCD plasma. In this formalism[1,2,3], we considered a light-like hard quark undergoing a single scattering with the plasma. $\hat{q}$ is factorized and expressed in terms of matrix elements for transverse broadening and field-strength field-strength correlator. The presence of the hard scale allows one to carry out Taylor expansion of the correlator after the analytic continuation to deep-Euclidean region. The leading twist operator in the operator-product expansion is computed on both quenched and unquenched lattices for a wide range of temperatures, ranging from 200MeV < T < 1GeV. The lattice extracted $\hat{q}$ from our formalism is compared with the existing (non) perturbative calculations and phenomenology-based extractions of $\hat{q}$. The computed $\hat{q}$ shows a temperature dependence similar to the entropy density and shows considerable agreement with phenomenology-based extractions carried out by the JET and JETSCAPE collaboration.
[1] A. Kumar, A. Majumder, J. H. Weber, arXiv:2010.14463 [hep-lat] (2020).
[2] A. Kumar, A. Majumder, C. Nonaka, PoS LATTICE2018 169 (2018).
[3] A. Majumder, Phys. Rev. C87 034905 (2013).
In this talk I shall review how the S-matrix formalism can be applied to study the thermal properties of interacting hadrons.
The central idea of this approach is to compute an effective density of state from the scattering phase shifts. As the phase shifts encode a wealth of information about the hadronic interactions, the method can robustly handle many dynamical structures, e.g. overlapping resonances, poles and roots, and assess their influences on thermal observables.
As an application I will present an analysis on proton and Lambda yields from the heavy ion collision experiments at the LHC. I will discuss how inconsistencies between theory and experiment, e.g. the proton puzzle and the proton to Lambda ratio, may be resolved by considering some essential features of the empirical baryon spectrum. These dynamical features are also crucial for understanding the Lattice results on thermal QCD, such as the baryon electric charge correlation.
Lastly I will report on some recent progress in analyzing in-medium effects within the S-matrix formalism.
The STAR Collaboration has successfully completed the upgrade of the forward detector system located between 2.5 $< \eta <$ 4.0. This upgrade comprises a Forward Calorimeter System, which contains an Electromagnetic Calorimeter and Hadronic Calorimeter; and a Forward Tracking System which consists of a Forward Silicon Tracker and Forward small-strip Thin Gap Chambers. The forward detector upgrade will have excellent detection capability for neutral pions, photons, electrons, jets, and hadrons. A combination of soft and hard probes collected during 2023-25 will be used to probe the QGP’s microstructure and will enable a unique forward physics program via the collection of high statistics Au+Au, p+Au, and pp data at $\sqrt{s_{_{\mathrm{NN}}}}$ = 200 GeV. With the extended acceptance and the enhanced statistics, STAR will be positioned to perform correlation studies in heavy-ion collisions, e.g., the pseudorapidity dependence of azimuthal correlations and the pseudorapidity dependence of global hyperon polarization. The STAR forward detector upgrade will also enable an extensive suite of measurements probing the quark-gluon structure of heavy nuclei.
In this talk, we will present the current status of the forward detector system and discuss its performance during data taking with cosmic ray and pp collisions at $\sqrt{s_{_{\mathrm{NN}}}}$ = 510 GeV.
The LHCb experiment has recently undergone a series of major upgrades: the entire tracking system has been replaced with higher-granularity sensors, the readout electronics have been upgraded, and all hardware triggers have been removed in favor of a new state-of-the-art streaming readout system. In addition, the gaseous target SMOG system has been upgraded with a dedicated storage cell to greatly increase the rate of fixed target collisions at LHCb. This talk will include the first performance results from the new LHCb tracking system, the streaming readout system, and SMOG II, with a focus on how these upgrades directly impact the LHCb heavy ion physics program. Further upgrades planned for LHC Run 4 and 5 will also be discussed.
After 9 years of successful data-taking in proton-proton and heavy ion collisions at a variety of energies, the ATLAS detector started in 2018 the preparations for an ambitious physics project, aiming the exploration of very rare processes and extreme phase spaces, an endeavor that will require a substantial increase in the amount of data to be taken. To accomplish this purpose, a comprehensive upgrade of the detector and associated systems was devised and planned to be carried out in two phases. The Phase-I upgrade program foresees new features for the muon detector, for the electromagnetic calorimeter trigger system and for all trigger and data acquisition chain. These upgrades are expected to be fully functional in 2021 and will enable ATLAS to carry on its physics program at a two fold increased luminosity. For the heavy-ions program, a new Zero Degree Calorimeter (ZDC) with improved radiation hardness will be installed. Upon reaching an integrated luminosity of 350 fb-1, the LHC will undergo a new upgrade, becoming the High-Luminosity LHC (Hl_LHC). The HL-LHC will reach an instantaneous ultimate luminosity of 7.5x1034 cm-2s-1, which will enable the experiments to accumulate 4 ab-1 of integrated luminosity in about 10 years of operation. The challenges the ATLAS experiment will face during the HL-LHC stage are paramount, as it will have to cope with more than 200 simultaneous collisions per bunch crossing with many subsystems exposed to very high radiation levels. To preserve its performance, the ATLAS detector will require a major upgrade program, known as Phase-II upgrade program. During the Phase-II upgrade, a completely new all-silicon tracker with extended rapidity coverage will replace the current inner tracker detector; the calorimeters and muon systems will have their trigger and data acquisition systems fully redesigned, allowing the implementation of a free-running readout system. Finally, a new subsystem called High Granularity Timing Detector will aid the track-vertex association in the forward region by incorporating timing information to the reconstructed tracks. This presentation will summarize the expected performance of the aforementioned projects, as well as the new insights gained during the construction phase.
The sPHENIX detector at the BNL Relativistic Heavy Ion Collider (RHIC) is currently under construction and on schedule for first data in early 2023. Built around the excellent BaBar superconducting solenoid, the central detector consists of a silicon pixel vertexer adapted from the ALICE ITS design, a silicon strip detector with single event timing resolution, a compact TPC, novel EM calorimetry, and two layers of hadronic calorimetry. The plan is to use the combination of electromagnetic calorimetry, hermetic hadronic calorimetry, precision tracking, and the ability to record data at high rates without trigger bias to make precision measurements of Heavy Flavor, Upsilon and jets to probe of the Quark Gluon Plasma (QGP) formed in heavy-ion collisions. These measurements will have a kinematic reach that not only overlaps those performed at the LHC, but extends them into a new, low-pT regime. sPHENIX will significantly expand the observables and kinematic reaches of these measurements at RHIC and provide a comparison with the LHC measurements in the overlapping kinematic region. The physics program, its potential impact, and recent detector development will be discussed in this talk.
The Compressed Baryonic Matter (CBM) experiment is one of the major scientific pillars of the Facility for Antiproton and Ion Research (FAIR), which is expected to become operational in 2025-26. The goal of CBM is to explore the QCD phase diagram in the region of high baryon densities using nucleus-nucleus collisions in the energy range \sqrt{s_{NN}} = 2.9 - 4.9 GeV. CBM will be utilizing peak interaction rates of up to 10 MHz and an advanced triggerless data acquisition scheme, giving access to rare physics probes not studied before.
This contribution will give an overview of the CBM physics goals among which the equation-of-state of dense nuclear matter, the possible phase transition from hadronic to partonic phase, and chiral symmetry restoration play a major role. The CBM physics performance in terms of (multi-) strange particle production, dilepton spectroscopy, collective flow and femtoscopic observables will be discussed. In addition, the status of the comprising detector sub-systems will be presented. This includes their performance in FAIR Phase-0 experiments, especially in the currently operated demonstrator mCBM at SIS18.
The Multi-Purpose Detector (MPD) is the first experiment at the
NICA Collider, which is in construction at the Joint Institute for
Nuclear Research in Dubna. During initial stage of operation the
complex
will study collisions of heavy ions in for sqrt(s_NN) of 4-11 GeV,
with Bi+Bi collisions at 9.2 GeV, in particular planned for first run.
It is expected that an excited QCD matter with high baryonic density
will be created in these collisions. In this talk I will present the
general MPD capabilities to study this exotic state of matter.
MPD is an international collaboration consisting of 44 institutions
from 13 countries. The construction and commissioning of the detector
is planned for 2022 and 2023, with the first data expected in
2023. The status of all subsystem preparations as well as their design
performance will be presented. MPD aims to study the phase diagram of
QCD matter at maximum baryonic density, determine the nature of the
phase transition between the deconfined and hadronic matter and search
for the critical end point. The physics programme, with emphasis on
potential first physics measurements with initial beams will be
discussed and it will be shown how MPD results can be used to
characterize the QCD matter created in heavy-ion collisions, including
the relevance of these investigations to other physics areas such
as astrophysics, particle physics and neutron star composition.
The already existing Baryonic Matter at Nuclotron (BM@N) experiment is
being upgraded for measurements of Au+Au collisions up to a kinetic
beam energy of 3.8A GeV in order to investigate the equation-of-state
and the microscopic degrees-of-freedom of QCD matter at neutron star
core densities.
In this contribution, the final measurements of the centrality dependence of $R_{\rm AA}$ of non-prompt $\mathrm{D}^0$ and electrons from beauty hadron decays in Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV will be presented. These measurements provide important constraints to the in-medium mass-dependent energy loss and hadronization of the beauty quark. The integrated non-prompt $\mathrm{D}^0$ $R_{\rm AA}$ will be presented for the first time and will be compared with the prompt $\mathrm{D}^0$ one. This comparison will shed light on possible different shadowing effects between charm and beauty quarks. In addition, the first measurements of non-prompt $\mathrm{D}_{s}$ production in central and semi-central Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV will be discussed. The non-prompt $\mathrm{D}_{s}$ measurements provide additional information on the production and hadronization of $\mathrm{B}_{s}$ mesons. Finally, the first measurement of non-prompt D-mesons elliptic flow in Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV will also be discussed. It will help to further investigate the degree of thermalization of beauty quark in the hot and dense QCD medium.
Evidence for the production of top quarks in heavy ion collisions is reported in a data sample of lead-lead collisions recorded in 2018 by the CMS experiment at a nucleon-nucleon center-of-mass energy of $\sqrt{s_{_{\mathrm{NN}}}} =$ 5.02 TeV, corresponding to an integrated luminosity of $1.7\pm0.1\,\mathrm{nb}^{-1}$. Top quark pair ($\mathrm{t\bar{t}}$) production is measured in events with two opposite-sign high-$p_\mathrm{T}$ isolated leptons ($\ell^\pm\ell^\mp =\,\mathrm{e}^{+} \mathrm{e}^{-},\,\mu^{+} \mu^{-},\,\mathrm{and}\,\mathrm{e}^{\pm} \mu^{\mp}$). We test the sensitivity to the $\mathrm{t\bar{t}}$ signal process by requiring or not the additional presence of b-tagged jets, and hence demonstrate the feasibility to identify top quark decay products irrespective of interacting with the medium (bottom quarks) or not (leptonically decaying W bosons). To that end, the inclusive cross section ($\sigma_\mathrm{t\bar{t}}$) is derived from likelihood fits to a multivariate discriminator, which includes different leptonic kinematic variables with and without the b-tagged jet multiplicity information. The observed (expected) significance of the $\mathrm{t\bar{t}}$ signal against the background-only hypothesis is 4.0 (5.8) and 3.8 (4.8) standard deviations, respectively, for the fits with and without the b-jet multiplicity input. After event reconstruction and background subtraction, the extracted cross sections are $\sigma_\mathrm{t\bar{t}} = 2.03 ^{+0.71}_{-0.64}$ and $2.54 ^{+0.84}_{-0.74}\,\mu\mathrm{b}$, respectively, which are lower than, but still compatible with, the expectations from scaled proton-proton data as well as from perturbative quantum chromodynamics predictions. This measurement constitutes the first crucial step towards using the top quark as a novel tool for probing strongly interacting matter.
We make predictions for rapidity densities of beauty hadrons produced in Pb-Pb collisions at LHC energy. The approach follows that outlined for charm in JHEP 07 (2021) 035, with the canonical suppression as an important ingredient. The hadronic mass spectrum is taken from PDG 2020, with 48 b mesons and 46 b baryons in total. As further input we use the measured cross section for $\mathrm{b\bar{b}}$ production in pp collisions at 5.02 TeV $\mathrm{d} \sigma_\mathrm{b\bar{b}}/\mathrm{d} y =34.5\pm 2.4^{+4.7}_{-2.9}\, \mu\mathrm{b}$, taken from ALICE coll. (arxiv:2102.13601) and shadowing based on reweighted nPDFs (Kusina et al, PRD, 104 (2021) 014010).
Assuming full thermalization of b-quarks at the chemical freeze-out temperature $T_{ch}$ = 156.5 MeV we overpredict the measured rapidity densities of $\Upsilon(1S)$ for central Pb-Pb collisions by a factor of 2-3. This result could indicate that a sizeable fraction of b-quarks is not thermalized.
We will discuss these results in the context of the current understanding of Debye screening scenarios for the $\Upsilon$ family and also provide new results for the production of $B_c$ hadrons.
The early production of heavy-flavour partons makes them an excellent probe of the dynamical evolution of QCD systems. Jets tagged by the presence of a heavy-flavour hadron give access to the kinematics of the heavy partons, and along with correlation measurements involving heavy-flavour hadrons allow for comparisons of their production, propagation, and fragmentation across different systems. The properties of heavy-flavour parton showers are driven by the large dead cone of heavy quarks, the presence of which is directly measured for the first time, using jets tagged with a fully reconstructed D$^{0}$ meson amongst their constituents, in pp collisions. Whilst traversing the QGP, these partons are expected to lose energy through interactions with the medium, at a different rate to their inclusive counterparts. To constrain the energy loss in the QGP, measurements of the nuclear modification factor of D$^{0}$ meson-tagged jets are presented in the 0-10$\%$ most central Pb-Pb collisions. Properties of heavy-flavour jets are also investigated in small systems through measurements of the production and substructure of jets tagged with D$^{0}$ mesons or electrons originating from heavy-flavour decays. Measurements of the fragmentation function and radial shape of jets containing a $\Lambda^{+}_{c}$, probing different dimensions of the hadronisation dynamics of charmed baryons, are also presented in pp collisions. Additionally, measurements of D$^{0}$-hadron correlations and the correlation of electrons from heavy-flavour decays with hadrons are presented, in both pp and p-Pb collisions, probing the impact of cold nuclear effects and providing a baseline for future Pb-Pb measurements.
Transverse momentum broadening and energy loss of a propagating parton are dictated by the space-time profile of the jet transport coefficient $\hat{q}$ in a dense QCD medium. The spatial gradient of $\hat{q}$ perpendicular to the propagation direction can lead to a drift and asymmetry in parton transverse momentum distribution. Such an asymmetry depends on both the spatial position along the transverse gradient of the dense matter and path length of a propagating parton as shown by numerical solutions of the Boltzmann transport in the simplified form of a drift-diffusion equation. In high-energy heavy-ion collisions, this asymmetry with respect to a plane defined by the beam and trigger particle (photon, hadron, or jet) with a given orientation relative to the event plane is shown to be closely related to the transverse position of the initial jet production in full event-by-event simulations within the linear Boltzmann transport model. Such a gradient tomography can be used to localize the initial jet production position for a more detailed study of jet quenching and properties of the quark-gluon plasma along a given propagation path in heavy-ion collisions.
The jet quenching phenomenon, one of the signatures of the quark-gluon plasma, is well established through experimental measurements at RHIC and LHC. However, the details of the expected dependence of jet-medium interactions on the flavor of the parton initiating the shower are not yet settled. This talk presents the first b jet shapes measurements from 5 TeV PbPb and pp collisions collected by the CMS. Comparisons made with jet shapes of inclusive jets, produced predominantly by light quarks and gluons, allow experimental observations of a “dead cone” effect in suppressing in-jet transverse momenta of constituents at small radial distance R from the jet axis. A similar comparison for large distances provides insights on the role of parton mass in the energy loss and possible mass-dependence of medium response.
In this contribution, the similarity between small and large collision systems will be further explored using the underlying event (UE) charged-particle density, $N_{\rm T}$ and the self-normalized version, $R_{\rm T}$. By selecting on $N_{\rm T}/R_{\rm T}$ and topological region, different microscopic processes contributing to the inclusive production can be isolated.
Final measurements of charged particle production as a function of $N_{\rm T}$ in pp, p-Pb and Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV will be presented in the toward, away and transverse regions. The UE contributions measured in the transverse region are subtracted from the toward and the away regions to search for jet-like modifications in small systems. The jet-like signals are studied both as a function of $N_{\rm T}$ and leading particle $p_{\rm T}$.
Final results of $\pi$, K and p production as a function of $R_{\rm T}$ in pp collisions at $\sqrt{s}$ = 13 TeV are presented to explore the particle species dependence. In particular, the focus will be on very low (high) $R_{\rm T}$ to isolate the ``jet'' (UE) contributions.
All the above results are compared with predictions from QCD-inspired Monte Carlo event generators such as PYTHIA and EPOS LHC.
Jets have become a prominent tool for studying properties of the quark-gluon plasma through observations of in-medium modifications of parton showers and energy loss patterns in heavy-ion collisions. These effects, termed jet quenching, were expected to depend on the color-charge and/or flavor of the parton initiating the shower. The jet charge observable, defined as the momentum-weighted sum of in-jet particle charges, is sensitive to the electric charge of the original parton and can be used to discriminate between gluon-initiated and quark-initiated jets in proton-proton collisions. In this talk, the first measurements of jet charge distributions from 5 TeV PbPb collisions compared with matching energy pp data and predictions from leading and next-to-leading-order generators. The measurements performed with the CMS experiment show no significant modification to the components of the jet charge distribution between pp and PbPb collisions.
The JETSCAPE Collaboration reports a new determination of jet transport coefficients in the
Quark-Gluon Plasma, using both reconstructed jet and hadron data measured at RHIC and the
LHC. The JETSCAPE framework incorporates detailed modeling of the dynamical evolution of
the QGP; a multi-stage theoretical approach to in-medium jet evolution and medium response;
and Bayesian inference for quantitative comparison of model calculations and data. The multi-
stage framework incorporates multiple models to cover a broad range in scale of the in-medium
parton shower evolution, with dynamical choice of model that depends on the current virtuality
or energy of the parton.
We will discuss the physics of the multi-stage modeling, and then present a new Bayesian
analysis incorporating it. This analysis extends the recently published JETSCAPE determination
of the jet transport parameter $\hat{q}$ that was based solely on inclusive hadron suppression
data [1], by incorporating reconstructed jet measurements of quenching. We explore the
functional dependence of jet transport coefficients on QGP temperature and jet energy and
virtuality, and report the consistency and tensions found for current jet quenching modeling with
hadron and reconstructed jet data over a wide range in kinematics and $\sqrt{s}$. This analysis
represents the next step in the program of comprehensive analysis of jet quenching
phenomenology and its constraint of properties of the QGP.
[1] JETSCAPE Collaboration (S. Cao et al.), Phys. Rev. C104 (2021) 1, 024905
In the last few years, several frameworks have achieved the evaluation of the medium-induced gluon radiation spectrum (or rate) with all-order resummation of multiple scatterings for static media. However, conceptual and computational issues arise when embedding approaches including multiple scatterings into dynamic plasmas. In this talk, we will show several paths to overcome these difficulties and present results on the fully-resumed spectrum for longitudinally evolving media. Furthermore, we will quantify the accuracy of the different methods and analyze their performance in realistic set-ups as those employed in phenomenological analyses.
We present the measurement of two-particle correlations in hadronic $e^+e^-$ collisions data collected by the Belle detector at KEKB. The clean $e^+e^-$ collision system is conducive for the unambiguous investigation of the azimuthal anisotropy of final-state charged particles found in various heavy ion and proton-proton collisions. Following up on the first examination in $e^+e^-$ annihilation events using the small archived ALEPH dataset, high-statistics Belle datasets at center-of-mass energies of $\sqrt{s} = 10.52$ GeV ($89.5 {\rm fb}^{-1}$) and 10.58 GeV on the $\Upsilon(4S)$ resonance ($333.2 {\rm fb}^{-1}$) are analyzed. The larger statistics also enables the study of very rare events of the multiplicity distribution tail. Measurements are reported as a function of the charged particle multiplicity over the full relative azimuthal angle $(\Delta \phi)$ and three units of pseudorapidity $(\Delta \eta)$. Correlation functions calculated in two coordinate systems with respect to different reference axes –– the conventional beam axis and the event thrust axis –– are measured. The thrust-reference-axis coordinate is the more natural representation for $e^+e^-$ annihilating into a quark-antiquark pair for providing sensitivity to the color activity emitted transverse to the diquark fragmentation. In this talk, we also present a qualitative understanding for the measured correlation structure based on Monte Carlo simulations. We will discuss the correlations for jet fragmentation in this low energy regime and for the special scenario of $b\bar{b}$ bound state decays.
The first measurement of $anti-k_{T}$ jets and two-particle angular correlations of charged particles emitted in high energy $e^+e^−$ annihilation is presented. The archived data at a center-of-mass energy of 91-209 GeV were collected with the ALEPH detector at LEP between 1992 and 2000.
At 91 GeV, no significant long-range correlation was observed in either the lab coordinate analysis or the thrust coordinate analysis, where the latter is sensitive to a medium expanding transverse to the color string between the outgoing $q\bar{q}$ pair from Z boson decays. We also present the first measurement of anti-$k_{T}$ jet energy spectra and substructures compared to various event generators, NLO, and NLL'+R resummation calculations.
The correlation functions are measured over a broad range of pseudorapidity and full azimuth as a function of charged particle multiplicity for the first time with LEP2 data. This data set provides higher event multiplicity reach up to around 50 and a chance to sample different underlying hard-scattering processes. Studies of the high energy annihilation data will expand our search for collective phenomena in $e^+e^−$ collisions to a new phase space for a potential discovery.
The creation of fluid-like quark-gluon plasma in small collision systems has been investigated via elliptical azimuthal anisotropy of emitted particles in these interactions. A novel search for QCD collective effects in hard probes is presented using high-$p_\mathrm{T}$ jets in 13 TeV pp collisions at CMS. Studies of short- and long-range azimuthal correlations inside a jet produced with very high-multiplicity charged daughters are presented, where the system is rotated to a new "jet frame" with the high-$p_\mathrm{T}$ jet direction being the beam z axis. We also report the first measurement of the azimuthal anisotropy for the $\Upsilon$(1S) meson in pPb collisions at 8.16 TeV. The dimuons used to reconstruct the $\Upsilon$(1S) meson are coupled with charged hadrons using the long-range two-particle correlation method. The results are discussed in terms of collectivity and modification of bottom quarks.
The suppression of the $J/\psi$ nuclear modification factor has been seen as a trademark signature of final state effects in large collision systems for decades. In small systems, deviations of the nuclear modification from unity had been attributed to cold nuclear matter effects until the observation of strong differential suppression of the $\psi(2S)$ state in $p/d$+A collisions, which suggests the presence of final state effects. In this talk, we present results of $J/\psi$ and $\psi(2S)$ measurements in the dimuon decay channel for $p$+$p$, $p$+Al, and $p$+Au collision systems at $\sqrt{s_{NN}}$ = 200 GeV. Key results include the nuclear modification factors $R_{pA}$ as function of $p_T$ and rapidity. The measurements are compared with shadowing and transport model predictions, as well as to complementary measurements at LHC energies.
Measurements of quarkonia and open-heavy ﬂavor in hadronic collisions provide a unique testing ground for understanding quantum chromodynamics (QCD). Although recently there was signiﬁcant progress, our understanding of hadronic collisions has been challenged by the observation in high-multiplicity proton-proton (pp) collisions of intriguing effects, such as collective phenomena.
The excellent particle identiﬁcation, track and decay-vertex reconstruction capabilities of the ALICE experiment are exploited to measure quarkonia both at midrapidity and forward rapidity, as well as open-beauty hadron production at midrapidity, the latter accessed through different analyses strategies, some of them employing machine-learning techniques.
In this contribution, the ﬁrst measurements of the elliptic ﬂow ($v_2$) of J/$\psi$ at high multiplicity as well as J/$\psi$ pair production in pp collisions at $\sqrt{s} = 13$ TeV, will be shown. A comprehensive set of new measurements of quarkonium and open-beauty hadron production in pp and p-Pb collisions will also be discussed. Among the results, we will highlight the ﬁrst measurement of non-prompt D$^*$ polarization in pp collisions at 13 TeV, as well as the ﬁrst measurement of non-prompt $\Lambda_c$ in pp collisions and the latest measurements of b-tagged jets, non-prompt D mesons and non-prompt J/$\psi$ in pp and p-Pb collisions, at different collision energies. Recently published inclusive quarkonium production cross sections at midrapidity and forward rapidity in pp collisions at 5 and 13 TeV will be presented as well. An overview of multiplicity dependent results in pp and p-Pb collisions, including the ﬁrst analysis of non-prompt D meson fractions at midrapidity, ground and excited quarkonium states at forward rapidity, will be shown. The comparison of results with available models will also be discussed.
The observation of collectivity in small hadronic collisions raises the question whether the tiny droplet of quark gluon plasma can form in systems with size significantly smaller than nucleus-nucleus collisions. Dynamics and hadronization of heavy flavor quarks in small-system collisions provide a powerful tool to address the origin of observed collective phenomena because of their early production time and sensitivity to the finite system size effect. A comprehensive study of charm (prompt $D^0$, J/$\psi$ mesons) and bottom (via non-prompt $D^0$ mesons) hadron elliptic flow in proton-proton and proton-lead collisions with the full LHC Run-2 data collected by the CMS experiment is presented, where a mass hierarchy is observed. New measurements of the charm baryon $\Lambda_c^{+}$ yields and ratios to prompt $D^0$ yields are also presented as functions of $p_\mathrm{T}$ and event multiplicity, and are directly compared with light flavor strange baryon-to-meson ratios to provide constraints on the charm hadronization in small systems. These results are compared to theoretical models, which provide crucial new insights to charm hadronization mechanisms and possible QGP medium effects in high-multiplicity small-system collisions.
The medium modification to particle spectra and the origin of collectivity in small collision systems are widely debated topics in our community.
To address these open questions we propose the study of particle production and collectivity for varying system sizes, presented in decreasing order (Au+Au $>$ Ru+Ru/Zr+Zr $>$ $^{3}$He+Au $>$ d+Au $>$ p+Au $>$ p+p $>$ $\gamma$+Au), available at RHIC using the STAR detector.
We present the first measurements of charged hadron yields in isobar (Ru+Ru and Zr+Zr) collisions. We perform measurements of identified particle spectra at low transverse momenta ($p_\mathrm{T}$) as a function of rapidity and event centrality. We also perform the centrality dependent measurements of nuclear modification factors ($R_\mathrm{AA}$) at high $p_\mathrm{T}$. Combined with the existing results in smaller systems (p/d+Au), these results provide an additional handle in studying system size and collision geometry dependences of the medium modification to particle production.
We also revisit the measurements of elliptic ($v_2$) and triangular ($v_3$) anisotropies in p+Au, d+Au and $^{3}$He+Au collisions at 200 GeV including a comprehensive evaluation of the non-flow effects using different subtraction methods.
In addition to the results obtained from the mid-rapidity ($|\eta|<$1), we also use the Event Plane Detectors that span over 2.1$<|\eta|<$5.1 to investigate the potential influence of longitudinal flow de-correlations in $v_n$ measurements using peripheral Ru+Ru and Zr+Zr collisions.
Our study of photonuclear ($\gamma$+Au) processes using ultra-peripheral Au+Au data can push the boundaries of small system scan at RHIC. We lastly present measurements on particle production and long-range di-hadron correlations in inclusive $\gamma$+Au-rich events that are not dominated by hadronic interactions.
Event geometry and initial state correlations have been invoked as possible explanations of long-range rapidity correlations observed in high multiplicity pp and pPb collisions. We study initial state momentum correlations and event-by-event geometry in p+Pb collisions at \sqrt{s}=5.02 TeV by following the approach of extending the impact parameter dependent Glasma model (IP-Glasma) to 3D using JIMWLK rapidity evolution of the incoming nuclear gluon distribution [1].
Investigating the non-trivial rapidity dependence of the observables, we find that geometry is correlated across large rapidity intervals whereas initial state momentum correlations are relatively short range in rapidity. Based on our results, we discuss implications for the relevance of both effects in explaining the origin of collective phenomena in small systems.
[1]. B. Schenke and S. Schlichting, Phys. Rev. C, vol. 94, no. 4, p. 044 907, 2016
[2]. B. Schenke, S. Schlichting and P. Singh, Rapidity dependence of initial state geometry and momentum correlations in p+Pb collisions [to appear]
The production mechanism of deuterons, which have a binding energy of 2.2 MeV, is a topic of current interest in high energy heavy-ion collisions, where the system undergoes kinetic freeze-out at temperatures around 100 MeV. Two possible scenarios include (a) statistical thermal process and (b) coalescence of nucleons. Cumulants of deuteron number distributions and proton-deuteron correlations are sensitive to these physics scenarios. In addition, they are also sensitive to the choice of canonical versus grand canonical ensemble in statistical thermal models.
We report the first systematic measurements of collision energy and centrality dependence of cumulants (up to fourth order) of deuteron number distributions in Au+Au collisions at $\sqrt{s_{NN}}$ = 7.7, 11.5, 14.5, 19.6, 27, 39, 54.4, 62.4, and 200 GeV. We will also discuss new measurements on proton-deuteron correlations. The measurements are performed in the STAR experiment at mid-rapidity ($|y|<$ 0.5) and within transverse momentum range 0.8 $< p_{T} ({\rm GeV}/c) <$ 4.0, using Time Projection Chamber and Time-of-Flight detectors. The experimental results are compared to the statistical thermal model calculations with a grand canonical, canonical ensemble, and the UrQMD model that incorporates the coalescence of nucleons close by in space and momentum to form deuterons. These theoretical comparisons with the experimental measurements provide key insights into the mechanism of deuteron production in high-energy heavy-ion collisions.
A valuable tool used in the search for QCD's critical point is the computation of cumulants of conserved charge. Near this point, it is expected a sharp increase of this quantity due to divergence of correlation lengths. This calculation requires high statistics, which poses a challenge to hydrodynamics simulations, which tends to be computationally expensive. The issue can be ameliorated by means of a procedure called oversampling, i.e. one repeats the Monte Carlo step of the particlization many times for a single hydro event. However, this has the drawback of removing effects of fluctuations caused during the particlization. We use a toy model to demonstrate a method to compute cumulants (developed originally by Grassi, Hirayama and Ollitrault) in a scenario where the oversampling procedure is employed and proceed to compute it in several collision energies.
Fluctuations of conserved charges are important probes to explore hot and dense medium in relativistic heavy-ion collisions. In this talk we focus on the experimentally-observed second-order cumulants of baryon number and electric charge at the top RHIC energy. We compare the ratio of these cumulants with the corresponding susceptibility ratio observed in lattice QCD numerical simulations. We show that, if one assumes that the experimental results on the cumulants are thermal, the "temperature" predicted from this comparison is significantly lower than that of the chemical freezeout. We argue that this discrepancy comes from the diffusion and resonance decays. The importance of the acceptance correction of the transverse-momentum cut is also emphasized.
We present a dynamical description of (anti)proton number cumulants and correlation functions in heavy-ion collisions by utilizing hydrodynamics simulations [1]. The cumulants are calculated via an appropriately extended Cooper-Frye procedure describing particlization of an interacting hadron resonance gas [2] while the effects of global baryon number conservation are taken into account using a generalized subensemble acceptance method [3]. The experimental data of the STAR and ALICE Collaborations are consistent at $\sqrt{s_{\rm NN}} \geq 20$ GeV with simultaneous effects of global baryon number conservation and repulsive interactions in baryon sector, the magnitude of the latter being in line with the behavior of baryon number susceptibilities observed in lattice QCD. The STAR and HADES data at lower collision energies show indications for notable multi-particle correlations, which can indicate sizable attractive interactions among baryons due to the QCD critical point in baryon-rich region as well as the influence of volume fluctuations. We also clarify differences between cumulants and correlation functions (factorial cumulants) of (anti)proton number distribution, proton versus baryon number fluctuations, as well the effects of hadronic afterburner and multiple conserved charges.
[1] V. Vovchenko, V. Koch, C. Shen, arXiv:2107.00163
[2] V. Vovchenko, V. Koch, Phys. Rev. C 103, 044903 (2021)
[3] V. Vovchenko, arXiv:2106.13775, to appear in Phys. Rev. C
The elliptic flow harmonic $v_{2}\{2k\}$ is determined using Q-cumulants of different orders, with $k=1,...,5$, for 5.02 TeV PbPb collisions. The results were obtained using data from the CMS experiment at the LHC. The $v_{2}\{2k\}$ values show an ordering, with $v_{2}\{2\} > v_{2}\{4\} > \approx v_{2}\{6\} > \approx v_{2}\{8\} > \approx v_{2}\{10\}$. The hydrodynamics behavior of the medium can be probed with high precision using the higher order moments of the cumulant expansion. It is found that both hydrodynamics probes $\frac{v_{2}\{6\}-v_{2}\{8\}}{v_{2}\{4\}-v_{2}\{6\}}$ and $\frac{v_{2}\{8\}-v_{2}\{10\}}{v_{2}\{6\}-v_{2}\{8\}}$ are centrality dependent. This dependence is explained by introducing previously ignored higher order moments in the Taylor expansion of the corresponding generating function of the cumulants. The higher order moments, skewness, kurtosis and the new $5^{th}$ moment are expressed through the $v_{2}\{2k\}$ ($k=1,...,5$) harmonics and measured as a function of centrality. The results bring new precision to probes sensitive to initial-state fluctuations.
We present a novel approach to quantify correlations between baryon-antibaryon, baryon-baryon, and antibaryon-antibaryon pairs. For special case of Gaussian correlations, we used the Cholesky factorization [1] of the covariance matrix, while arbitrary correlations were introduced using the well-known Metropolis and Simulated Annealing [2] methods. Our approach is general enough to be used for correlations between strange and/or charm hadrons, it can also be applied to multi-particle final states. The results obtained are systematically compared to the corresponding publications from the ALICE and STAR collaborations. One focus of our analysis is to quantify the width of correlations in rapidity space. Such investigations are key to our understanding of the mechanism of baryon production at energy scales from several GeV to several TeV.
G. H. Golub and C. F. Van Loan, Matrix Computations, Johns Hopkins University Press, 1989.
N. Metropolis, A. W. Rosenbluth, M. N. Rosenbluth A. H. Teller and E. Teller, Equation of state calculations by fast computing machines, J.Chem.Phys. 21 (1953) 1087-1092
Azimuthal angle ($\Delta\phi$) and transverse momentum ($p_\mathrm{T}$) correlations of isolated photons and associated jets, which are sensitive to medium induced parton momentum broadening, are reported for the first time with the latest high statistics pp and PbPb data recorded with the CMS detector at $\sqrt{s_{_{\mathrm{NN}}}} =$ 5.02 TeV. The fully corrected photon+jet azimuthal correlation and $p_\mathrm{T}$ imbalance in PbPb collisions are studied as a function of collision centrality and photon $p_\mathrm{T}$. In addition, a novel measurement of the decorrelation of jet axes calculated with the energy weight and the winner-take-all schemes ($\delta_{jj}$) is reported for the first time. This new observable is insensitive to the initial state radiation which significantly smears the photon+jet azimuthal correlation. A significant modification of $\delta_{jj}$ is reported, which signals a direct detection of in-medium momentum broadening of the leading parton inside the jet. Furthermore, the transverse energy spectra and nuclear modification factors ($R_\mathrm{AA}$) of isolated photons will be discussed.
Exploring the strong interaction among hadrons, the ALICE Collaboration has for the first time extended the experimental measurements from two- to three-body interactions. These measurements provide unique information on many aspects of strongly-coupled systems, like the genuine three-body interaction, the formation of light nuclei and the search for exotic bound states. Among those, many-body interactions, also including hyperons, are an important ingredient in the calculation of the equation of state of neutron stars.
The results presented in this talk are obtained using high-multiplicity pp collisions at $\sqrt{s}$ = 13 TeV measured by ALICE at the LHC. The first measured three-body correlations include p-p-p and p-p-$\Lambda$. Their genuine three-body interactions are obtained by subtracting the known two-body effects from the measured correlation functions for the triplets. In both systems, a non-zero three-particle cumulant is observed, providing a hint to the existence of a genuine three-body effect. Another class of many-body interaction studies is identified in the correlations of hadrons with light nuclei. ALICE has measured proton-deuteron (p-d) interactions, a system containing three hadrons building up a pair of a hadron and a nucleus. The experimental correlation function is compared with theoretical predictions obtained employing the scattering parameters extracted from traditional scattering experiments for the p-d system. A clear deviation is observed, which may be interpreted as a demonstration of the late formation time of (anti)deuterons in hadron-hadron collisions.
Quark number susceptibilities as computed in lattice QCD are commonly believed to provide insights into the microscopic structure of QCD matter, in particular its degrees of freedom. We generalize a previously constructed partonic $T$-matrix approach to finite chemical potential to calculate various susceptibilities, in particular for configurations containing a heavy charm quark. At vanishing chemical potential and moderate temperatures, this approach predicts large collisional widths of partons generated by dynamically formed hadronic resonance states which lead to transport parameters characteristic for a strongly coupled system. The quark chemical potential dependence is implemented into the propagators and the in-medium color potential, where two newly introduced parameters for the thermal and screening masses are fixed through a fit to the baryon number susceptibility, $\chi^B_2$. With this setup, we calculate heavy-light susceptibilities without further tuning; the results qualitatively agree with the lattice-QCD (lQCD) data for both $\chi^{uc}_{11}$ and $\chi^{uc}_{22}$. This implies that the lQCD results are compatible with a significant content of broad $D$-meson and charm-light diquark bound states in a moderately hot QGP.
Reference:
Shuai Y.F. Liu, Ralf Rapp, arXiv:2111.13620
The strong interaction among D mesons and light-flavor hadrons was completely out of experimental reach until recently. The scattering parameters governing elastic and inelastic D-pion/kaon/proton collisions are completely unknown. This poses strong limitations not only to the search of molecular states composed of charm and non-charm hadrons, but also to the study of the rescattering of charm mesons following their formation in ultrarelativistic heavy-ion collisions. In such collisions a colour-deconfined medium, called quark-gluon plasma (QGP), is formed. The system experiences a hydrodynamic expansion cooling down up to the chemical freeze-out, which is followed by a hadronic phase. The knowledge of the scattering parameters of charm hadrons with non-charm hadrons would be a crucial ingredient for models based on charm-quark transport in a hydrodynamically expanding QGP to describe the typical observables of heavy-ion collisions.
In this talk we will report on the first estimation of the scattering parameters governing the strong interaction of the D-proton channel measured by the ALICE Collaboration in high-multiplicity pp collision at $\sqrt{s}$= 13 TeV at the LHC. The strong interaction is studied by means of correlation in momentum space and the analysis is extended to D-kaon and D-pion combinations. It is demonstrated that all the relevant scattering parameters for the interaction of D mesons with light-flavor hadrons will be experimentally determined thanks to the upgrades of the ALICE experimental apparatus planned for the LHC Run 4 and 5 data taking periods.
The energy loss of quarks when travelling a QGP medium is expected to depend on their mass. Heavy quarks causes a dead cone region along the direction of the quark where no energy is lost by gluon bremsstrahlung. This effect can be measured by comparing the nuclear modification factors of hadrons made of light and heavy quarks. The heavy quark coupling with the thermalized QGP is also an important aspect in probing the thermodynamics of QGP. The coupling is studied by the measurement of the flow harmonics ($v_2$) and its dependency on the transverse momentum $p_{\rm T}$ of the probe and on the collision centrality.
The PHENIX experiment contains a set of vertex detectors which enable the measurement of the distance of the closest approach (DCA) of identified electrons and muons. By utilizing their DCAs, identification of charm and bottom semi-leptonic decays was realized in Au+Au collisions at $\sqrt{s_{NN}}$=200 GeV. The $p_{\rm T}$ and collision centrality dependence of the nuclear modification and $v_2$ for charm and bottom hadrons were thus measured at mid-rapidity region ($|\eta|<$0.35) using the large dataset from the RHIC Year-2014 run. The $p+p$ yield used as a reference was obtained via semi-leptonic decay of heavy quarks and non-prompt J/$\psi$ yield ($B\rightarrow J/\psi+X$). The final results from this work will be presented and discussed in the view of the heavy quark energy loss, its coupling to and diffusion in QGP.
In relativistic heavy-ion collisions, the directed flow ($v_1$) of hadrons can provide insights into the ultra-strong electromagnetic (EM) field~[1-2], the constituent quark content of hadrons~[3], and the role of transported quarks~[4]. Here, the first measurement is reported for rapidity-odd directed flow of $\Xi$ and $\Omega$ in Au+Au collisions at $\sqrt{s_{NN}}=$ 19.6, 27, and 200 GeV, as well as $v_1$ for identified charged hadrons with unprecedented precision in Au+Au and isobar collisions at $\sqrt{s_{NN}}=$ 200 GeV.
The coalescence sum rule is examined with various combinations of hadrons where all constituent quarks are produced. For such combinations a systematic violation of the sum rule is observed with increasing difference in the electric charge and the strangeness content of the associated constituent quarks. By comparing with the Parton-Hadron-String Dynamics model that includes an EM field, the results suggest that the constituent quark sum rule could be violated in the presence of a strong EM field that drives the $v_1$ of produced quarks and anti-quarks to opposite directions. The splitting of $v_1$ slope with rapidity ($\Delta (dv_1/dy)$) between positively and negatively charged hadrons ($\pi$, $K$, $p$) is also studied with large statistics. A clear transition of $\Delta(dv_1/dy)$ from positive in central collisions to negative in peripheral collisions is observed for protons and kaons. With the effects of both transported quarks and the EM field considered, it is found that the significant negative values in peripheral events can only be explained by the presence of an EM field with the Faraday or Coulomb effect being dominant.
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The f$_0$(980) resonance was observed several years ago in $\pi\pi$ scattering experiments. Despite a long history of experimental and theoretical studies, the nature of this short-lived resonance is far from being understood and there is no agreement about its quark structure. According to different models, it has been associated with a meson, considered as a tetraquark, or as a KK molecule. In this talk we address two complementary avenues to study the nature of the f$_0$(980) resonance.
The first method exploits the excellent tracking and particle identification of the ALICE experiment to measure the differential spectra and integrated yield of the f$_0$(980) meson produced in pp and p-Pb collisions at an energy of $\sqrt{s}$ = 5 TeV. The results are discussed in the comparison with models and the properties of other hadrons. The nuclear modification factor shows hints of final-state effects in p-Pb collisions and will be presented and discussed in this perspective. The second method involves femtoscopic correlations of non-identical charged kaons (K$^+$K$^-$), studied in Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ = 2.76 TeV. For the analysis of the K$^+$K$^-$ correlation, a comparison of the measured data to the Lednick$\acute{\rm y}$-Luboshitz interaction model allows to extract for the first time the f$_0$(980) mass and coupling parameters. The measured width and mass of the f$_0$(980) resonance are consistent with the existing PDG data.
The hydrodynamic modeling of the quark-gluon plasma (QGP) permits us today not only to perform quantitative extractions of the transport properties of the QGP, but also to strongly constrain its initial condition. A growing body of experimental evidence shows that the QGP initial condition is strongly impacted by the shape and radial structure of the colliding nuclei. We discuss the exciting prospect of using precision flow measurements as a tool to image the structure of atomic nuclei, and show how such measurements probe the quadrupole, octupole, and triaxial deformations of the colliding ions, as well as their neutron skin. Motivated by recent groundbreaking measurements from RHIC and LHC, we discuss in particular the case of collisions of isobaric nuclei, which provide the cleanest access route to the collective structure of the colliding ions. We discuss the implications of obtaining an information about the structure of nuclei from high-energy collisions that is fully complementary to that obtained in low-energy experiments, and argue that a scan of stable isobars at high-energy colliders may open a new exciting direction of research in nuclear physics.
It is frequently supposed that quark-gluon plasma created in heavy-ion collisions undergoes free streaming at early times. We examine this issue based on the assumption that a universal attractor dominates the dynamics already at the earliest stages, which offers a way to connect the initial state with the start of the hydrodynamic expansion in an approximate but conceptually transparent fashion. We demonstrate that the centrality dependence of the measured particle multiplicities can be used to quantitatively constrain the pressure anisotropy and find that it strongly depends on the model of the initial energy deposition. As an illustration, we compare three initial state models and show that they predict rather different early-time values of the pressure anisotropy. This strongly suggests that assuming free streaming prior to hydrodynamization is not necessarily compatible with a generic initial state model and that features of the pre-hydrodynamic flow need to be matched with the model of the initial state.
Exclusive vector meson production is a powerful process to probe the small Bjorken-$x$ structure of protons and nuclei, as such processes are especially sensitive to gluonic structure and also provide access to the spatial distribution of small-$x$ gluons in nuclei. A powerful theoretical framework to study vector meson production at high energy, and to describe the initial condition of heavy-ion collisions, is the Color Glass Condensate (CGC) effective field theory. So far, most calculations in the CGC framework have been done at the leading order. Recent theoretical developments on the NLO heavy vector meson wave function [1] and the NLO virtual photon light-front wave function with massive quarks [2,3] have made it possible to go beyond the leading order, allowing us to include the next-to-leading corrections in $\alpha_s$ and calculate exclusive heavy vector meson production at NLO in the dipole picture for the first time.
In this talk, we will present results from our recent work on longitudinal [4] and transverse [5] NLO vector meson production in the nonrelativistic limit. We demonstrate the cancellation of UV and IR divergences in the NLO calculation, which includes taking into account both the Balitsky-Kovchegov equation describing the rapidity evolution of the dipole amplitude and the renormalization of the leading-order vector meson wave function. The next-to-leading order corrections are found to be sizable; however, their numerical effect on vector meson production is compensated by the dipole amplitude, fitted to HERA inclusive cross section data at NLO [6]. Finally, exclusive $J/\psi$ production is calculated numerically and compared to the existing data. We find that both the NLO corrections and the first relativistic corrections, calculated in Ref. [7], are numerically important and result in a good agreement with the data. We demonstrate that it is possible to simultaneously compute consistently both inclusive and exclusive cross sections at NLO accuracy in the CGC framework.
[1] M. Escobedo and T. Lappi, Phys.Rev.D 101 (2020) 3, 034030, arXiv:1911.01136 [hep-ph]
[2] G. Beuf, T. Lappi and R. Paatelainen, Phys. Rev.D 104 (2021) 5, 056032, arXiv:2103.14549 [hep-ph]
[3] G. Beuf, T. Lappi and R. Paatelainen, in preparation
[4] H. Mäntysaari and J. Penttala, Phys. Lett.B 823 (2021), 136723, arXiv:2104.02349 [hep-ph]
[5] H. Mäntysaari and J. Penttala, in preparation
[6] G. Beuf, H. Hänninen, T. Lappi and H. Mäntysaari, Phys. Rev.D 102 (2020), 074028, arXiv:2007.01645 [hep-ph]
[7] T. Lappi, H. Mäntysaari and J. Penttala, Phys.Rev.D 102 (2020) 5, 054020, arXiv:2006.02830 [hep-ph]
Hydrodynamics is an effective theory for the description of long-wavelength phenomena of fluids, that can be expressed as a small gradient expansion of fluid velocities relative to a thermal background. Thus, hydrodynamics is expected to fail for systems which are far-from-equilibrium. The medium produced in pp collisions at LHC and RHIC energies is an example of such a system. However, recent experimental results of high energy pp collision have shown evidence of collectivity similar to those observed in heavy-ion collisions. The unprecedented success of hydrodynamics to describe collectivity in heavy-ion collisions, as well as small systems, can be attributed to the fact that there exists a stable universal attractor which makes the dynamical equations to quickly converge and enter a hydrodynamic regime, at a time scale much smaller than the typical isotropization time scales. In the present work, we go beyond the previous
works which considered 1+1d longitudinal boost invariant systems, by considering a system undergoing Gubser flow which has a simultaneous transverse and longitudinal expansion.
To investigate the dynamics of such a system, the Boltzmann equation is solved in the relaxation time approximation using a hierarchy of angular moments of the distribution function. The dynamics of transition is described by the presence of fixed points which describes the evolution of the system in various stages. We found that unlike 1+1d Bjorken flow which has late-time thermalization (hydrodynamization), Gubser flow is intrinsically a 3+1d expanding system with dynamics such that the system goes from early time free-streaming regime to intermediate thermalization (hydrodynamization) and back to free-streaming in the late time regime. The attractor solution is found for various orders of moments as an interpolation between these fixed points.
Event-by-event pseudorapidity distributions in heavy-ion collisions are sensitive to longitudinal fluctuations. Their shapes can be decomposed using Legendre polynomials, analogous to the Fourier decomposition for anisotropic flow. A longitudinal decomposition for Xe—Xe collisions at $\sqrt{s_{\mathrm{NN}}}=5.44$ TeV and Pb—Pb collisions $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV measured with the ALICE detector is presented for event-by-event pseudorapidity distributions and compared to models. A significant forward-backward asymmetry in the particle production is observed, which is quantified via the first-order coefficient, $a_{1}$, from the longitudinal decomposition. Such an asymmetry shows a breaking of boost invariance, which is assumed in various models that describe the initial state of a heavy-ion collision.
Fluid-dynamical theories are always constructed in terms of an expansion around a given, yet arbitrary, local equilibrium state. This is implemented by the choice of the so-called matching conditions which define the temperature, chemical potential, and velocity of a viscous fluid. Matching conditions are an essential feature of nonequilibrium systems and their consequences to the emergence of hydrodynamic behavior have not been explored. In particular, the interplay between matching conditions and fluid-dynamical attractors [1] are far from understood.
We investigate for the first time how fluid-dynamical attractors in Bjorken flow are affected by choices of matching conditions, considering several formulations of fluid dynamics and kinetic theory. We show that the effect considerably worsens the agreement between solutions of first-order [2] and second-order fluid dynamics [3] and kinetic theory. These results directly affect the modeling of ultrarelativistic heavy-ion collisions where a fluid dynamical approximation is thought to be valid even at early times when the system is far from equilibrium.
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[2] F. S. Bemfica, M. M. Disconzi and J. Noronha, Phys.Rev.D 98, no.10, 104064 (2018).
[3] W. Israel, J. M. Stewart, Ann. of Phys., 118 (2), 341-372 (1979); G.S.Denicol, H. Niemi, E. Molnar and D. H. Rischke, Phys. Rev. D 85, 114047 (2012).
Transverse-momentum ($p_T$) dependence of elliptic flow ($v_2$) for identified particles at the top RHIC energy has well established a number-of-constituent-quark (NCQ) scaling [1], supportive of the coalescence sum rule that determines $v_2$ of mesons and baryons as the sum of contributions from their constituent quarks. The NCQ scaling implicitly assumes that the produced drops of partonic matter are thermally equilibrated, and constituent quarks at hadronization do not remember their past history. This assumption is found to be violated with the RHIC Beam Energy Scan (BES) data [2]. In this talk, we will focus on the $p_T$-integrated $v_2$ data instead of $v_2(p_T)$, and quantitatively explain the results on $\pi^\pm$, $K^\pm$, $p$, $\bar{p}$, $\Lambda$ and $\bar{\Lambda}$ in Au+Au collisions at $\sqrt{s_{\rm NN}} = 7.7$ GeV to 62.4 GeV by differentiating quarks produced in the collision and those transported from the initial-state nuclei. After verification of the coalescence sum rule, we extract $v_2$ for both produced ($u/d/{\bar u}/{\bar d}$, $s$, and ${\bar s}$, separately) and for transported ($u/d$) quarks. The latter is found to be larger than the former, and accounts for the observed $v_2$ difference between particles and antiparticles. We also relate the $v_2$ measurements of multi-strange hadrons to the different formation times of $\phi$, $\Omega^\pm$ and $\Xi^+$. At the RHIC BES regime, although the coalescence sum rule seems to be valid and the partonic degrees of freedom are important, the produced nuclear matter may not reach a full thermal equilibrium. We will discuss the beam energy dependence of the baryon transport and its impact on thermal equilibration.
[1] J. Adams et al. (STAR Collaboration), Nucl. Phys. A 757 (2005) 102.
[2] L. Adamczyk et al. (STAR Collaboration), Phys. Rev. C 93, 014907 (2016).
The rapidity dependence of particle production contains information on the partonic structure of the projectile and target and is, in particular at LHC energies, sensitive to non-linear QCD evolution in the initial state. At LHC, collision final states have been mainly studied in the central kinematic region, however, there is a rich opportunity for measurements in the forward direction, which probe the nucleon structure at small Bjorken-x values. Moreover, investigating the system-size dependence of the particle production at the same collision energy is particularly important for directly studying nuclear effects.
In the first part of the talk, the final Run 1 and 2 particle-production results at forward rapidities will be presented for pp, p-Pb, and Pb-Pb collision systems, where ALICE has unique coverage. When combined, the Forward Multiplicity and the Silicon Pixel Detectors can measure charged particles over a wide range of $-3.4<\eta<5.0$. The Photon Multiplicity Detector has complementary coverage for neutral-particle production over the kinematic range $2.3<\eta<3.9$.
In the second part of the presentation, we will introduce the upgraded Run 3 ALICE configuration. The new Monolithic Active Pixel Sensors-based Inner Tracking System allows full tracking and vertexing for $|\eta|<2.5$. When combined with the new Muon Forward Tracker, the tracking can be extended to cover $-3.6<\eta<2.5$. The performance of the new detectors and the tracking/matching algorithms will be presented for the $\sqrt{s}=900$ GeV pp pilot-beam data taking in autumn 2021.
A system of gluon fields generated at the earliest phase of relativistic heavy-ion
collisions can be described in terms of classical fields. Numerical simulations show that the system is unstable but a character of the instability is not well understood. With the intention to systematically study the problem, we analyze a stability of classical chromomagnetic and chromoelectric fields which are constant and uniform. We consider the Abelian configurations discussed in the past where the fields are due to the single-color potentials linearly depending on coordinates. However, we mostly focus on the nonAbelian configurations where the fields are generated by the multi-color non-commuting constant uniform potentials. We derive a complete spectrum of small fluctuations around the background fields which obey the linearized Yang-Mills equations. The spectra of Abelian and nonAbelian configurations are similar but different and they both include unstable modes. We briefly discuss the relevance of our results for fields which are uniform only in a limited spatial domain.
We employ an effective kinetic theory of Quantum Chromo Dynamics (QCD) to study the pre-equilibrium dynamics of the Quark-Gluon Plasma (QGP) at zero and finite net-baryon density. By investigating the dynamics of the energy-momentum tensor and the conserved charges, we determine the relevant time and temperature scales for the onset of viscous hydrodynamics, and quantify the evolution of the chemical composition of the QGP at early times [1,2]. We address phenomenological consequences regarding the role of the pre-equilibrium phase at different collision energies [1], and discuss how the improved understanding of the pre-equilibrium phase can be used to constrain hot and cold nuclear matter properties from heavy-ion collisions [3].
[1] X. Du, S. Schlichting, PRL 127 (2021) 122301
[2] X. Du, S. Schlichting, PRD 104 (2021) 054011
[3] X. Du, S. Schlichting, work in progress
We propose a novel way to study the Weizsäcker-Williams gluon distribution
using the Electron Ion Collider. We examine the interplay between the effect
of the soft gluon emissions, or Sudakov effects, and gluon saturation effects
with the help of the azimuthal correlations between the total transverse
momentum of the dijet system and the scattered electron. Kinematic cuts are
imposed such that the dijet system is produced in the forward direction of the
laboratory frame, which sets an upper bound on the probed longitudinal
fractions of the hadron momentum carried by scattered gluons. Further cuts are
chosen such that they allow us to use the factorization formalism that directly
involves the unpolarized Weizsäcker-Williams gluon distribution. We find that
this observable is very sensitive to the soft gluon emission and moderately
sensitive to the gluon saturation. The former sensitivity is, however, greatly
reduced in the nuclear modification ratio when comparing proton and heavy ions
as targets, opening the possibility to disentangle Sudakov effects and
saturation effects.
We perform an analytic calculation of the 3+1D structure of the initial state in heavy-ion collisions by considering the collision of longitudinally extended nuclei in the dilute limit of the Color Glass Condensate effective field theory of high-energy QCD. Based on general analytic expressions for the color fields in the future light cone, we compute the non-trivial rapidity profiles of the transverse pressure at early times. We validate our (semi-) analytic results against non-perturbative 3+1D classical Yang-Mills simulations [2,3] and discuss prospects for the development of 3+1D initial state models based on our calculation.
[1] A. Ipp, D. Müller, S. Schlichting and P. Singh, arxiv:2109.05028
[2] S. Schlichting, P. Singh, 3-D structure of the Glasma initial state – Breaking boost-invariance by
collisions of extended shock waves in classical Yang-Mills theory, Phys. Rev. D 103 (1) (2021) 014003.
[3] A. Ipp, D. Müller, Broken boost invariance in the Glasma via finite nuclei thickness, Phys. Lett. B 771 (2017) 74–79.
With the tremendous accomplishments of RHIC and the LHC experiments and the advent of the future Electron-Ion Collider on the horizon, the quest for compelling evidence of the color glass condensate (CGC) has become one of the most aspiring goals in the high energy quantum chromodynamics research. Pursuing this question requires developing the precision test of the CGC formalism. By systematically implementing the threshold resummation, we significantly improve the stability of the next-to-leading-order calculation in CGC for forward rapidity hadron productions in pp and pA collisions, especially in the high pT region, and obtain reliable descriptions of all existing data measured at RHIC and the LHC across all pT regions. Consequently, this technique can pave the way for the precision studies of the CGC next-to-leading-order predictions by confronting them with a large amount of precise data.
Heavy-ion collisions can produce an ultra-strong magnetic field, the evolution of which was predicted to decrease (increase) the directed flow slope, $dv_1/dy$, for positively (negatively) charged particles [1, 2]. In this work, we study this effect with large statistics datasets accumulated for Au+Au, $^{96}_{44}$Ru+$^{96}_{44}$Ru, and $^{96}_{40}$Zr+$^{96}_{40}$Zr isobar collisions at $\sqrt{s_{NN}}=$ 200 GeV, and Au+Au collisions at $\sqrt{s_{NN}}=$ 27 GeV. The charge dependent $dv_1/dy$ splitting, $\Delta(dv_1/dy)$, will be presented for $\pi^{\pm}$, $K^{\pm}$, and (anti)proton. A finite $\Delta(dv_1/dy)$ between protons and anti-protons has been observed and it changes from positive to negative as a function of centrality from central to peripheral collisions. This is the first observation of a significant negative $\Delta(dv_1/dy)$ between proton and anti-protons. A similar decreasing trend of slope difference between $K^+$ and $K^-$ has also been observed in Au+Au collisions at $\sqrt{s_{NN}}=$ 200 GeV and 27 GeV, and in isobar collisions with less significance. The slope difference between $\pi^+$ and $\pi^-$ is negative and decreases as a function of centrality in Au+Au collisions at $\sqrt{s_{NN}}=$ 27 GeV, while no significant slope difference is observed in Au+Au and isobar collisions at $\sqrt{s_{NN}}=$ 200 GeV. Our measurements of significant negative $\Delta(dv_1/dy)$ cannot be explained by conventional mechanisms (e.g. transported quarks), but qualitatively agree with the theoretical prediction with an ultra-strong electromagnetic field in peripheral collisions.
[1] U. Gursoy, D. Kharzeev, E. Marcus $et~al.$, Phys. Rev. C ${\bf 98}$, 055201 (2018).
[2] U. Gursoy, D. Kharzeev and K. Rajagopal, Phys. Rev. C $\bf{89}$, 054905 (2014).
The pre-hydrodynamic evolution of a heavy-ion collision can have a considerable effect on final state observables, notably those related to the transverse momentum spectra of final particles [1]. In this work, we study two different collision systems, namely Pb-Pb and p-Pb, utilising a state-of-the-art hybrid model. We extend our previous results [1] on the effects of pre-hydrodynamical models on final state observables to small systems. We show that the artificial bulk pressure generated at the switch to hydrodynamics by the assumption of scaling invariance during the pre-hydrodynamical evolution is enhanced for smaller systems sizes. We also show that its magnitude is dependent on the duration of the pre-hydrodynamical phase, growing with longer evolution times. These results further reinforce the need for non-conformal pre-hydrodynamical models, particularly in light of concerns about the validity of hydrodynamics at early times and the resulting need for accurate pre-hydrodynamic evolution, and the growing interest in small systems. Finally, we investigate whether a free-streaming pre-hydrodynamical evolution with a velocity smaller than $c$ [2], thus effectively breaking the assumption of scale invariance, can alleviate the effects of this artefact when performing the extraction of transport coefficients and initial state parameters from hybrid simulations.
[1] Tiago Nunes da Silva, David Chinellato, Mauricio Hippert, Willian Serenone, Jun Takahashi, Gabriel S. Denicol, Matthew Luzum, Jorge Noronha, Phys. Rev. C 103, 054906 (2021)
[2] Govert Nijs, Wilke van der Schee, Umut Gürsoy, Raimond Snellings, Phys. Rev. C 103, 054909 (2021)
Recently, STAR reported the isobar (${^{96}_{44}\text{Ru}}+{^{96}_{44}\text{Ru}}$, ${^{96}_{40}\text{Zr}}+{^{96}_{40}\text{Zr}}$) results for chiral magnetic effect (CME) search [1]. The Ru+Ru to Zr+Zr ratio of the CME-sensitive observable $\Delta\gamma$, normalized by elliptic anisotropy ($v_{2}$), is close to the inverse multiplicity ($N$) ratio. In other words, the ratio of the $N\Delta\gamma/v_{2}$ observable is close to the naive background baseline of unity. However, non-flow correlations are expected to cause the baseline to deviate from unity. To further understand the isobar results, we study non-flow effects using the same isobar data by two-particle correlations as functions of pseudorapidity and azimuthal angle differences ($\Delta\eta$, $\Delta\phi$) of the pairs. We extract the charge-dependent correlations by the difference between the opposite-sign (OS) and same-sign (SS) charge pairs, properly normalized such that the correlations vanish at $|\Delta\eta|\rightarrow \infty$. These charge-dependent correlations come primarily from resonance decays, intra-jet (near-side) correlations, and Coulomb effects. We study the charge-independent correlations by examining the small and large $|\Delta\eta|$ behaviors of the SS correlations. The intra-jet (near-side) can be well isolated at small $|\Delta\eta|$ and $|\Delta\phi|$. We investigate the inter-jet (away-side) correlations by exploiting Pythia and HIJING simulations, together with the knowledge of near-side correlations obtained from the data. By comparing the two isobar systems, many systematic uncertainties can be minimized. By studying how non-flows differ between the two isobar systems, we can gain insights into the baseline of the CME.
[1] M. Abdallah et al. [STAR], [arXiv:2109.00131 [nucl-ex]].
We investigate the angular momentum in heavy-ion collisions applying the hadronic transport approach SMASH. In contrast to geometrical models (e.g. a Glauber approach) our transport approach allows to access the full phase-space information of every particle at any time. The importance of understanding the non-equilibrium angular momentum transferred to the fireball and in turn the quark-gluon plasma (QGP) was highlighted by recent results of the STAR experiment at the Relativistic Heavy Ion Collider (RHIC). The spin polarization measurement of the $\Lambda$-hyperon revealed a high angular momentum of the heavy ions and provided experimental evidence for vorticity in the QGP for the first time. Therefore, a systematic exploration of the angular momentum within a dynamic calculation for beam energies from $\sqrt{s}_{NN}= 2.41GeV$ to $\sqrt{s}_{NN}= 200GeV$ is a crucial step towards the full description of vorticity as a fundamental property of the QGP. Results for the angular momentum of Au-Au collisions as function of the impact parameter are presented and the influence of the initial Fermi momentum is studied. Moreover, it is shown that the angular momentum exhibits a distinct maximum for a specific impact parameter, independent of the beam energy. We show that the remaining angular momentum $L_r$ of the system grows with increasing system size in a range of $A=16$ $(^{16}_8O)$ to $A=208$ $(^{208}_{82}Pb)$ while we observe that for smaller beam energies a larger fraction of the initial angular momentum is transferred to $L_r$. The findings are important to guide future experimental programs and indicate where the largest transfer of angular momentum is expected.
Recent relativistic heavy-ion collision experiments have found evidence for the generation of strong magnetic field and global angular momentum. The numerical simulation of evolution of the QCD medium is based on either magnetohydrodynamics or spin-hydrodynamics for calculation of observables pertaining to magnetic field or global angular momentum, respectively. However, these two effects are not entirely separable due to the possible spin alignment of medium constituents in the presence of magnetic field, similar to the Einstein-de Haas effect. Therefore a unified framework of “spin-magnetohydrodynamics” needs to be developed for precise calculation of experimental observables. Here we present the first formulation of this unified framework in relativistic context.
Starting from the classical description of spin, a kinetic theory of massive spin-1/2 particles in the presence of a magnetic field is obtained in the small polarization limit. We use a relaxation time approximation for the collision kernel in the relativistic Boltzmann equation and obtain the correction to phase-space distribution function. Building on the kinetic description, we then formulate a non-resistive, relativistic dissipative spin-magnetohydrodynamics for a fluid, whose constituent particles are considered to be spin-polarizable but non-magnetizable. We find multiple novel transport coefficients and show that all dissipative currents i.e. particle diffusion, shear stress tensor, bulk viscous pressure and non-equilibrium spin-tensor contain coupling between spin and magnetic field.
The study of the production of $\phi$ meson has always been of great interest both in elementary and heavy-ion collisions. As observed by ALICE, strangeness enhancement in high-multiplicity pp collisions is one of the potential manifestations of QGP formation. Recent results at LHC suggest that $\phi$ behaves like a particle with net strangeness between 1 and 2 in small systems. These observations unfold new directions for theoretical and experimental studies of $\phi$ meson production in small systems. Polarization measurements of vector mesons are crucial for understanding the particle production mechanisms in high-energy collisions. In non-central heavy-ion collisions, the presence of a large initial angular momentum can polarise the vector mesons. This might be either due to spin-orbital-angular-momentum interaction or by hadronization from polarized quarks. The $\phi$ meson polarisation in pp collisions could be used as a reference while interpreting the results from heavy-ion collisions. The huge data sample collected during the Run 2 of the LHC measurements gives access to look for the multiplicity dependence of this measurement. This poster will present the perspectives for new results on the multiplicity dependence of $\phi$ meson polarisation in pp collisions at $\sqrt{s}$ = 13 TeV in the helicity reference frame.
The proton spin decomposition provides key information about the structure of the nucleons. Since the late 1980s, experiments showed that the quark spin contributes only $\sim$30\% to the proton spin, with the remaining part coming from the gluon spin as well as the quark and gluon orbital angular momentum. While the quark spin contribution was better constrained by polarized deep inelastic scattering, the gluon spin contribution remains less known. The Relativistic Heavy Ion Collider (RHIC) is the only collider capable of producing two longitudinal polarized proton ($\vec{p}+\vec{p}$) beams. Direct photon, jet, and charged pion production in $\vec{p}+\vec{p}$ collisions can probe the gluon spin at leading order. Compared with hadron production, direct photon production is the most "clean" channel, since there is little fragmentation involved. However, the relatively small direct photon cross section compared to the hadron production makes it a challenging observable. To achieve this "golden channel" measurement, we utilize the RHIC 2013 run, which provides the largest integrated luminosity (155 pb$^{-1}$) in $\vec{p}+\vec{p}$, along with the PHENIX Electromagnetic Calorimeter, which has fine granularity to separate the two $\pi^0$ decay photons up to $\pi^0$ transverse momentum $p_T$ of 12 GeV/c. A shower profile analysis extends the $\gamma/\pi^0$ discrimination to beyond 20 GeV/c. This poster will present the direct photon cross section and double helicity asymmetry for the direct photon $p_T$ of 6--30 GeV/c and 6--20 GeV/c, respectively. When included in future global analyses, our results will provide an independent constraint on the gluon spin contribution to the proton spin.
Non-trivial collective velocity field due to anisotropic flow leads to vorticity along the beam direction in heavy-ion collisions. Polarization of $\Lambda$ and $\bar{\Lambda}$ hyperons along the beam direction relative to the elliptic flow plane has been observed in Au+Au collisions at RHIC and Pb+Pb collisions at the LHC. However, unlike for the case of the global polarization originating from the initial orbital angular momentum, theoretical models fail to describe its magnitude and sign, which is currently under intense discussion. Measurements of the hyperon polarization in colliding systems smaller than Au+Au may shed light on this problem. One can also expect a local polarization arising from higher harmonic flow, which provides new insight into the vorticity and polarization phenomena.
We present the first measurements of $\Lambda$ hyperon local polarization relative to the second and third order event planes in Ru+Ru and Zr+Zr collisions at $\sqrt{s_{\rm NN}}$ = 200 GeV. The results will be compared to those in Au+Au collisions at $\sqrt{s_{\rm NN}}$ = 200 GeV and the physics implications will be discussed.
We have studied local spin polarization and helicity polarization in the relativistic hydrodynamic model. Generalizing the Wigner functions previously obtained from chiral kinetic theory to the massive case, we present the possible contributions up to the order of hbar from thermal vorticity, shear viscous tensor, other terms associated with the temperature and chemical-potential gradients, and electromagnetic fields to the local spin polarization and helicity polarization. We then implement the (3+1) dimensional viscous hydrodynamic model to study the spin polarizations from these sources with a small chemical potential and ignorance of electromagnetic fields by adopting an equation of state different from those in other recent studies. Although the shear correction alone upon local polarization results in the sign and azimuthal-angle dependence more consistent with experimental observations, as also discovered in other recent studies, it is mostly suppressed by the contributions from thermal vorticity and other terms that yield an opposite trend. It is found that the total local spin polarization could be very sensitive to the equation of states, the ratio of shear viscosity over entropy density, and freezeout temperature.
Semi-classical evolution equations for the scalar and axial-vector components of the Wigner function are treated in the relaxation time approximation to introduce a framework of relativistic dissipative hydrodynamics of particles with spin 1/2. We show that a classical treatment of spin is consistent with earlier calculations using the Wigner function approach with a quantum description of spin. We then derive non-equilibrium corrections to the spin tensor. The detailed structure of the non-equilibrium spin tensor reveals the existence of multiple spin transport coefficients [1,2]. This development indicates that it might be necessary to incorporate the effects of the multiple hydrodynamic gradients to properly characterize the nature of spin-polarization observed in heavy-ion collisions.
[1] Bhadury, S. et. al. Phys.Lett.B 814 (2021) 136096.
[2] Bhadury, S. et. al. Phys.Rev.D 103 (2021) 1, 014030.
In non-central heavy-ion collisions, a large orbital angular momentum is created along the direction opposite to the reaction plane, which will be transferred to the spin of quarks through the spin-orbit coupling in parton scatterings. In our recent work, we formulate an improved coalescence model through spin density matrix with phase space dependence, which provides a uniform way to compute spin alignments of vector mesons and polarizations of baryons from polarizations of quarks and antiquarks. Within this model, various sources of spin polarization are studied, including vorticity fields, electromagnetic fields, and mean fields of vector mesons. We find that the electric part of the vector $\phi$ field can qualitatively explain the positive deviation from 1/3 for the spin alignment of $\phi$ mesons measured by the STAR collaboration. On the other hand, the spin alignment of $K^{*0}$ mesons is dominated by the electric part of vorticity fields and our model prediction also qualitatively agrees with experimental results.
We derive Boltzmann equations for massive spin-1/2 fermions with local and nonlocal collision terms from the Kadanoff--Baym equation in the Schwinger--Keldysh formalism, properly accounting for the spin degrees of freedom. The Boltzmann equations are expressed in terms of matrix-valued spin distribution functions, which are the building blocks for the quasi-classical parts of the Wigner functions. Nonlocal collision terms appear at next-to-leading order in ℏ and are sources for the polarization part of the matrix-valued spin distribution functions. The Boltzmann equations for the matrix-valued spin distribution functions pave the way for simulating spin-transport processes involving spin-vorticity couplings from first principles.
No chiral-magnetic effect signature has been observed in the experimental analysis of the isobar run at RHIC [1]. In this talk, based on [2], we highlight the influence of the nuclear structure of the isobar systems on the CME search within a relativistic hadronic transport approach (SMASH). We show that the quadrupole deformation of Ru enhances the eccentricity ratio between the isobars in ultra-central collisions up to 10%, thus leading to different CME backgrounds in the two isobar systems. In addition, the neutron skin of Zr reduces a factor of 2, from 10% to 5% , the magnetic field strength difference in peripheral collisions. These two predictions suggest a significantly smaller CME signal to background ratio than previously expected, as has been confirmed experimentally.
[1] 2109.00131
[2] Phys.Rev.C 101 (2020) 6, 061901
The interplay of the chiral anomaly and the strong magnetic field (~10$^{15}$ T) created in the off-central heavy-ion collisions could give rise to a collective excitation in the quark-gluon plasma called the Chiral Magnetic Wave (CMW), which can be experimentally sought by the charge asymmetry ($A_{\rm ch}$) dependence of elliptic flow $v_2$ of positively and negatively charged hadrons. However, non-CMW mechanisms such as local charge conservation (LCC) intertwined with collective flow can also lead to a similar dependence of $v_2$ on $A_{\rm ch}$. The measurement with triangular flow ($v_3$) thus serves as a reference as it is not expected to be affected by the CMW.
In this talk, we present new ALICE measurements of $v_2$ and $v_3$ of inclusive and identified hadrons as functions of $A_{\rm ch}$ in Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ = 2.76 and 5.02 TeV. The slope parameters of $\Delta v_2$-$A_{\rm ch}$ and $\Delta v_3$-$A_{\rm ch}$ correlations, where the $\Delta v_n$ are the differences between $v_n$ of positive and negative particles, are normalized and then compared with results from other experiments and models. In addition, the Event Shape Engineering (ESE) technique is adopted for the first time to quantitatively distinguish the CMW signal and the LCC background. The upper limit of the CMW signal contribution is further extracted. Our measurements reveal that the background effect is dominant in the search for the CMW in heavy-ion collisions.
This talk presents a novel instability in the Chern-Simons (or axionic) magnetohydrodynamics (MHD), arising from the spatial inhomogeneity of the axion-like field. In particular, this instability amplifies the Alfven waves in certain regions of spacetime in a way that is clearly parity-violating. The Aflven velocity reaches the speed of light in such regions, but it never exceeds it.
Recent observations of the spin polarization of weakly decaying Lambda hyperons have opened up a new direction to explore non-trivial vortical structures of strongly interacting matter produced in the heavy-ion experiments. A consistent framework of relativistic hydrodynamics with spin degrees of freedom (spin hydrodynamics) is under construction now to allow for future dynamic simulations of the spin polarization. This type of hydrodynamic description is based on the conservation of the total energy and linear momentum as well as the total angular momentum which includes both the orbital and spin parts.
The phenomenological approach used to construct the framework of spin hydrodynamics commonly uses a simplified form of the spin tensor [1,2]. This form does not posses an expected symmetry, namely, it is not totally anti-symmetric (which is a direct consequence of Noether's Theorem applied to the Dirac Lagrangian). Consequently, in this approach the connection between the spin hydrodynamics and the underlying field-theoretic arguments is obscured.
In our recent work [3] we demonstrate how one can connect the spin hydrodynamics constructed with a totally antisymmetric (canonical) spin tensor with the phenomenological approach used by other authors. We show that the two frameworks are not connected only by a pseudo-gauge transformation (what most people implicitly assume) but an additional subtraction of a specific divergence-free term to the canonical energy-momentum tensor should be done.
Our results help us to find and clarify connections between different formulations of spin hydrodynamics, which is important for the construction of a final, fully consistent formalism.
Since the first positive measurement of the Λ-hyperon global spin polarization in heavy-ion collisions by STAR collaboration in 2017, the understanding of the nature of this phenomenon is one of the most intriguing challenges for the heavy-ion physics community. As relativistic fluid dynamics celebrates multiple successes in describing collective dynamics of the QCD matter in such reactions, the natural question arises whether the spin dynamics can also be modeled in such a framework. In this talk, we will discuss the theoretical aspects of the relativistic spin hydrodynamics framework which is based on the de Groot - van Leeuwen - van Weert forms of energy-momentum and spin tensors. We will also show how this formalism can be used to determine observables describing the spin polarization of particles measured in the experiment.
Based on a holographic far-from-equilibrium calculation of the Chiral Magnetic Effect (CME) in an expanding quark-gluon plasma, we study collisions at various energies. We compute the time evolution of the CME current in the presence of a time-dependent axial charge density and subject to a time-dependent magnetic field. The plasma expansion leads to a dilution and eventual annihilation of the CME current after approximately 5 fm/$c$. We study distinct combinations of how the initial magnetic field and initial axial charge behave with changing initial energy as proposed in the previous literature. Most scenarios we consider lead to an increasing CME current, integrated over time, when increasing the initial energy. This would make it more likely to observe the CME at higher collision energies. However, in the scenario that the axial charge and magnetic field are fixed while the initial energy is decreased, the holographic plasma shows an increasing time-integrated CME current. This is the only one of the six scenarios which we studied which would suggest the CME to be more likely found at lower collision energies.
The search for the chiral magnetic effect (CME) in isobaric collisions of Ru+Ru and Zr+Zr at RHIC was motivated by the assumed similarity of the backgrounds (e.g., $v_n$, $N_{\rm chg}$) for the two isobars. The effects of nuclear structure differences and deformation can lead to essential differences in the backgrounds for the two isobars. Here, we use a quark Glauber model, validated in earlier studies of small and large systems, to study the effects of nuclear distribution uncertainties, especially the details of a Zr halo and nuclear shape fluctuations, on background-related variables such $\varepsilon_n$, and particle production for the isobars. The influence of such uncertainties on both particle production and $\varepsilon_n$ is significant; they provide invaluable constraints for pinning down the requisite differences for CME study in isobaric collisions. They also show that a data-model study of relevant bulk observables could constrain nuclear distribution parameters.
To probe the Chiral Magnetic Effect (CME) in heavy-ion collisions, a new technique, Sliding Dumbbell Method (SDM) [1] is developed to search for the back-to-back charge separation on event-by-event basis. The SDM helps in selecting the events corresponding to different charge separations ($f_{DbCS}$). The charge separation distributions for each collision centrality is divided into 10 percentile bins to select potential CME-like events corresponding to the maximum charge separation (e.g. top 10$\%$) in a given collision centrality. Results will be discussed for two- and three-particle correlators with respect to each bin of $f_{DbCS}$ for each collision centrality for isobaric and Au$+$Au collisions at $\sqrt{s_\mathrm{NN}} = 200$ GeV. The background contribution due to statistical fluctuations is obtained by shuffling the charges of particles in a given collision centrality. The correlated background amongst the produced particles which got removed due to shuffling is determined by restoring the shuffled charges.
References
[1] J. Singh, A. Attri, and M. M. Aggarwal, Proceedings of the DAE Symp. on Nucl. Phys. 64, 830 (2019) "http://www.sympnp.org/proceedings/64/E66.pdf".
We introduce a novel freeze-out procedure connecting the hydrodynamic evolution of a droplet of quark-gluon plasma (QGP) that has, as it expanded and cooled, passed close to a critical point on the QCD phase diagram with the subsequent kinetic description in terms of observable hadrons. The procedure converts out-of-equilibrium critical fluctuations described by extended hydrodynamics, known as Hydro+, into cumulants of hadron multiplicities that can be subsequently measured. We introduce a critical sigma field whose fluctuations cause correlations between observed hadrons due to the couplings of the sigma field to the hadrons. We match the QGP fluctuations obtained via solving the Hydro+ equations describing the evolution of critical fluctuations before freeze-out to the correlations of the sigma field. In turn, these are imprinted onto fluctuations in the multiplicities of hadrons, most importantly protons, after freeze-out via a generalization of the familiar half-a-century-old Cooper-Frye freeze-out prescription which we introduce. This framework allows us to study the effects of critical slowing down and the consequent deviation of the observable predictions from equilibrium expectations quantitatively. We can also quantify the suppression of cumulants due to the conservation of baryon number. We demonstrate the prescription in practice by freezing out the Hydro+ simulation in a simplified azimuthally symmetric and boost invariant background discussed previously.
We construct a family of equations of state for QCD, which reproduce the lattice results at small chemical potential and include a critical point in the 3D Ising model universality class. These equations of state, based on the original formulation developed in [1], include the constraint of strangeness neutrality, which is phenomenologically relevant for heavy-ion collisions [2]. We then use our parametrization to study the quartic cumulant of the baryon number $\chi_4^B$, which can be accessed experimentally via net-proton fluctuation kurtosis measurements. It was originally predicted, through universality arguments based on the leading singular contribution, that $\chi_4^B$ should show a specific nonmonotonic behavior due to the critical point. In particular, when following the freeze-out curve on the phase diagram by decreasing beam energy, the kurtosis is expected to dip, and then peak, when the beam energy scan passes close to the critical point. We find that, while the peak remains a solid feature, the presence of the critical point does not necessarily cause a dip in $\chi_4^B$ on the freeze-out line below the transition temperature [3].
References
[1] P. Parotto et al., Phys. Rev. C101 (2020) 3, 034901.
[2] J. M. Karthein et al., Eur. Phys. J. Plus 136 (2021) 6, 621.
[3] D. Mroczek et al., Phys. Rev. C103 (2021) 3, 034901.
We explore the transport properties of the QGP matter in the high $\mu_B$ region, where a CEP is incorporated. To this aim we extend the effective dynamical quasi-particle model (DQPM) - constructed for the description of non-perturbative QCD phenomena of the strongly interacting quark-gluon plasma (QGP) - to large baryon chemical potentials, $\mu_B$, including a critical end-point and a 1st order phase transition.
The DQPM is based on covariant propagators for quarks/antiquarks and gluons that have a finite width in their spectral functions (imaginary parts of the propagators). In DQPM the determination of complex selfenergies for the partonic degrees-of-freedom at zero and finite $\mu_B$ has been performed by adjusting the entropy density to the lattice QCD (lQCD) data. The temperature-dependent effective coupling (squared) $g^2(T/T_c)$, as well as the effective masses and widths or the partons are based in this adjustment.
The novel extended dynamical quasi-particle model, named "DQPM-CP", makes it possible to describe thermodynamical and transport properties of quarks and gluons in a wide range of temperature, $T$, and baryon chemical potential, $\mu_B$, and reproduces the equation-of-state (EoS) of lattice QCD calculations in the crossover region of finite $T, \mu_B$.
We apply a scaling ansatz for the strong coupling constant near the critical endpoint CEP, located at ($T^{CEP}$, $\mu^{CEP}_B) = (0.100, 0.960)$ GeV. We show the equation-of-state as well as the speed of sound for $T>T_c$ and for a wide range of $\mu_B$, which can be of interest for hydrodynamical simulations. Moreover, one of the advantages of the quasi-particle models is a simple implementation in transport models.
Furthermore, we consider two settings for the strange quark chemical potentials (I) $\mu_q=\mu_u=\mu_s=\mu_B/3$ and (II) $\mu_s=0,\mu_u=\mu_d=\mu_B/3$. The isentropic trajectories of the QGP matter are compared for these two cases.
Despite that the phase diagram of the DQPM-CP is close to the PNJL calculations the transport coefficients of both approaches differ. This elucidates that the knowledge of the phase diagram alone is not sufficient to describe the dynamical evolution of strongly interacting matter.
Transport properties of the matter created in heavy-ion collisions, the quark-gluon plasma (QGP), contain essential information about quantum chromodynamics (QCD). To deepen our understanding of QCD, it is crucial to estimate these transport properties (for instance, specific shear and bulk viscosity) in the light of experimental data as accurately as possible. In this talk, we present our latest study in inferring the transport properties of QGP by an improved Bayesian analysis using the CERN Large Hadron Collider Pb-Pb data at $\sqrt{s}_{NN}=2.76$ and 5.02 TeV. To improve the uncertainties, we include new observables sensitive to specific shear and bulk viscosity, reflecting mostly nonlinear hydrodynamic responses. We show that the uncertainty of the transport coefficients is significantly reduced by including the latest flow harmonic measurements. The analysis also reveals that higher-order harmonic flows and their correlations have a higher sensitivity to the transport properties than the other observables. This observation shows the necessity of accurate measurements of these observables in the future.
Based on:
[1] J.E. Parkkila, A. Onnerstad, D.J. Kim, Phys.Rev.C 104 (2021) 5, 054904, arXiv: 2106.05019 [hep-ph]
[2] J.E. Parkkila, A. Onnerstad, S. F. Taghavi, C. Mordasini, A. Bilandzic, D.J. Kim, arXiv: 2111.08145 [hep-ph]
Realistic modeling of nucleus-nucleus collisions at finite baryon chemical potential is necessary to extract the location of the critical point on the QCD phase diagram and to understand the findings of the recently concluded Beam Energy Scan (BES) program at RHIC and the future planned experiments at FAIR and NICA. We propose a hydrodynamic model with three new elements. Firstly, we present a new initial state model at non-zero chemical potential based on the Monte-Carlo sampling of the nucleon-nucleon scattering extrapolated to nucleus-nucleus collisions (LEXUS [1]). This model dynamically initializes hydro, which is evolved using MUSIC [2]. Secondly, we employed a new cross-over equation of state [3]. Finally, we calculated the departure functions at finite chemical-potential using a quasi-particle theory of transport [4] and used it in Cooper-Frye procedure. We present comparisons with STAR data for a wide range of collision energies - 7.7 GeV - 200 GeV.
We study the thermodynamic properties, such as the pressure and the entropy density, of a gas of glueballs by considering the contribution of the tower of various glueball states obtained by using recent lattice calculations as well as other model results. We also include, to our knowledge for the first time, the effect of glueball-glueball interaction on thermodynamic properties. The results are compared with the current Yang-Mills lattice data and to other theoretical approaches.
By using gravity/gauge correspondence, we employ an Einstein-Maxwell-Dilaton model to compute the equilibrium and out-of-equilibrium properties of a hot and baryon rich strongly coupled quark-gluon plasma. The family of 5-dimensional holographic black holes, which are constrained to mimic the lattice QCD equation of state at zero density, is used to investigate the temperature and baryon chemical potential dependence of the equation of state [1]. We also obtained the baryon charge transport coefficients, the bulk and shear viscosities as well as the drag force and Langevin diffusion coefficients associated with heavy quark jet propagation and the jet quenching parameter of light quarks in the baryon dense plasma, with a particular focus on the behavior of these observables on top of the critical end point and the line of first order phase transition predicted by the model.
[1] Grefa, J., Noronha, J., Noronha-Hostler, J., Portillo, I., Ratti, C., Rougemont, R. 10.1103/PhysRevD.104.034002
We study the thermal properties of scalar quantum field theories (QFTs) involving 3-leg and 4-leg interaction terms, with special attention on the role of bound states and resonances. Within a suitable unitarization scheme, for which the employed QFT is unitary, finite, and well defined for each value of the coupling constant, we calculate the scattering phase shifts, whose derivatives are used to infer the pressure of the system at nonzero $T$. A bound state emerges in each when the attraction is strong enough, but we show that it does not count as one state in the thermal gas, since a cancellation with the residual scattering interaction typically occurs. The amount of this cancellation depends on the details of the model and its parameters: a variety of possible scenarios is presented. Moreover, even when no bound state occurs, we estimate the role of the interaction in general and of resonances in particular.
In this talk I review recent progress in resummed perturbative calculations of the equation of state of QCD and N=4 supersymmetric Yang-Mills (SUSY) theory. In the case of QCD, I will review progress that has been made using hard-thermal-loop perturbation theory (HTLpt) at finite temperature and quark chemical potential(s), focussing on recent NNLO HTLpt predictions for the quadratic and quartic curvatures of the QCD phase transition line in different physics cases. The NNLO HTLpt predictions are found to agree well with available lattice data for the curvature coefficients where available, and provide predictions for these coefficients in cases where they have not been accurately determined on the lattice. In the second part of my talk, I will discuss recent results which extend the perturbative determination of N=4 SUSY thermodynamics through second-order in the 't Hooft coupling. The final result contains non-analytic terms which are not present in the strong-coupling limit and the resummed perturbative series shows signs of having a finite and large radius of convergence.
References:
[1] N. Haque and M. Strickland, Phys. Rev. C 103, 031901 (2021).
[2] Q. Du, M. Strickland, and U. Tantary, J. High Energ. Phys. 2021, 64 (2021).
[3] J.O. Andersen, Q. Du, M. Strickland, and U. Tantary, forthcoming.
Non-equilibrium Green’s functions provide an efficient way to describe the pre-equilibrium evolution of macroscopic quantities in early stages of heavy-ion collisions.
Within the kinetic theory framework we use moments of the distribution functions to calculate time dependent non-equilibrium Green’s functions describing the evolution of initial energy/momentum/charge perturbations [1]. Using kinetic theory in relaxation time approximation we will study the pre-equilibrium evolution of a Bjorken background and compute Green’s functions for the charge current and energy-momentum tensor for initial perturbations around this background. By calculating the Green’s functions, we show that only modes with long wavelength survive up into the hydrodynamic regime.
[1] [Kamata, Martinez, PP, Ochsenfeld, Schlichting, Phys. Rev. D (2020)]
Hydrodynamic models are a central component of nuclear collision phenomenology. In this talk, I show that relativistic causality is violated in the early stages of state-of-the-art heavy-ion hydrodynamic simulations of nuclear collisions. Up to 75% of the initial fluid cells violate nonlinear causality constraints, while superluminal propagation is observed by up to 15% the speed of light. Only after 2-3 fm$/c$ of evolution, do ∼50% of the fluid cells become definitely causal. Inclusion of pre-equilibrium evolution significantly reduces the number of acausal cells, but it does not eliminate them. These findings show that relativistic causality imposes constraints on the available model parameter space of heavy-ion collision simulations.
An advanced Hadron Resonance Gas Model (HRGM) based on the induced surface tension equation of state [1, 2] is developed which correctly accounts for weak decays. We report our results on fits of the ratios of particle yields measured in a wide range of centre-of-mass energies from a few GeV up to 2.76 TeV. In particular, our analysis of the STAR experiment data on hadronic multiplicities demonstrates that taking into account the weak decays is extremely important to have a model that can describe the data with high accuracy and in a physically correct way. Moreover, the inclusion of weak decays in the analysis of BES data leads to a decrease in the chemical freeze-out (CFO) temperature of hadrons by about $10-15$ MeV. For the first time, the results for the CFO parameters obtained by the fits to the BES program data in the collision energy range $\sqrt{s_{NN}}=7.7-200$ GeV are in complete agreement with the ones obtained earlier in different models for the ALICE energy $\sqrt{s_{NN}}=2.76$ TeV [2,3] as well as with the Lattice QCD results for the pseudocritical line [4]. Remarkably, it is shown that the CFO parameters of light (anti-, hyper-) nuclei obtained in [5] are not affected by these modifications. They are in agreement with the PHQMD simulations [6] and provide a solution to the so-called “snowballs in hell” problem.
References:
[1] K. A. Bugaev et al., Nucl. Phys. A 970 (2018) 133.
[2] K. A. Bugaev et al., Eur. Phys. J. A 56 (2020) 293.
[3] A. Andronic et al., Nature 561 (2018) 321 and references therein
[4] B. Szabolcs et al., Phys. Rev. Lett. 125 (2020) 052001
[5] O. Vitiuk et al., Eur. Phys. J. A 57 (2021) 74.
[6] S. Gläßel et al., arXiv:2106.14839
We study the interaction of leading jet partons in a strongly interacting quark-gluon plasma (sQGP) medium based on the effective dynamical quasi-particle model (DQPM). The DQPM describes the non-perturbative QCD nature of the sQGP at finite temperature $T$ and baryon chemical potential $\mu_B$ based on a propagator representation in terms of massive off-shell partons (quarks and gluons) which properties (characterized by complex self-energies, i.e. masses and widths) are adjusted to reproduce the lQCD EoS for the QGP in thermodynamic equilibrium. We present the results for the jet transport coefficients such as $\hat q$, the transverse momentum transfer per unit length, the drag coefficient $A$ as well as the energy loss per unit length $\Delta E =dE/dx$, in the QGP and investigate its dependence on QGP properties such as medium temperature $T$ and baryon chemical potential $\mu_B$ as well as on the jet properties such as leading jet parton momentum, mass, flavor, and the strong coupling constant. In this first study only elastic scattering processes of leading jet parton with the sQGP partons are explored discarding presently the radiative processes (such as gluon Bremsstrahlung) which are expected to be suppressed for the emission of massive gluons. We present a comparison of our results for the elastic energy loss in the sQGP medium with other theoretical approaches such as lattice QCD and the LO-HTL as well as with estimates of $\hat q$ by the JET and JETSCAPE collaborations based on a comparison of hydrodynamical calculations with the experimental heavy-ion data.
The azimuthal anisotropy of parton energy loss in non-central heavy-ion collisions can lead to jet anisotropy which in turn can provide insight into the path-length dependence of jet quenching. Jet anisotropy flow in this study is investigated within the Linear Boltzmann Transport model, in which the dynamical evolution of the QGP is simulated within the CLVisc hydrodynamic model with fully fluctuating event-by-event initial conditions. We quantify the colliding energy, centrality, jet transverse momentum dependence of jet anisotropy flow coefficients $v^\mathrm{jet}_2$ and $v^\mathrm{jet}_{3}$, with emphasis on their event-by-event correlations with the flow coefficients of the soft bulk hadrons. We find that the correlation between jet and bulk anisotropy is approximately linear and that the effect of the bulk $v_n$ fluctuation on the event-averaged jet $v^\mathrm{jet}_n$ is negligible. Other effects such as medium excitation with different jet cone sizes and viscosity of the QGP on jet anisotropy are investigated as well.
We perform a systematic data-driven extraction of the light parton transport properties in a quark-gluon plasma based on a hard-soft factorized parton energy loss model [1]. In this model, occasional hard interactions and frequent softer interactions are systematically factorized. The larger number of soft interactions makes possible an effective stochastic description of the parton-plasma interactions in terms of a small number of transport coefficients [2]. These soft transport coefficients can capture non-perturbative effects, agnostic to the strongly- or weakly-coupled nature of the underlying deconfined plasma.
We constrain the temperature dependence of these soft transport coefficients by performing a Bayesian model-to-data comparison with jet measurements from RHIC and LHC, allowing us to better understand the non-perturbative effects suffered by soft interactions in heavy ion collisions. We also study the dependence of the calibration results on the separation of the scale between soft and hard parton-plasma interactions in order to investigate the robustness of the approach. We discuss differences between this work's soft transport coefficients and the soft-hard ones extracted in other approaches, highlighting the strength of our factorized approach.
[1] Dai, Tianyu, Jean-François Paquet, Derek Teaney, and Steffen A. Bass. "Parton energy loss in a hard-soft factorized approach." arXiv preprint 2012.03441 (2020).
[2] Ghiglieri, Jacopo, Guy D. Moore, and Derek Teaney. "Jet-medium interactions at NLO in a weakly-coupled quark-gluon plasma." Journal of High Energy Physics 2016, no. 3 (2016): 1-58.
In heavy-ion collisions, large transverse momentum partons traverse the colored medium and lose energy via induced gluon radiation and elastic scattering, which modify jet structure relative to jets produced in vacuum. The semi-inclusive recoil jet measurement provides precise, data-driven suppression of the large uncorrelated background and uniquely enables the exploration of medium-induced modification of jet production over wide phase space, including low$p_\mathrm{T}$ for large jet resolution parameter $R$. Such measurement in pp and p--Pb collisions provides a good test for pQCD calculations, and sets as a reference for jet quenching and acoplanarity study in nucleus-nucleus collisions.
In this contribution, we report the semi-inclusive distribution of charged jets recoiling from a high-$p_\mathrm{T}$ charged hadron trigger in pp collisions at $5.02 ~\mathrm{TeV}$, with emphasis on the region of low recoil jet $p_\mathrm{T}$ and large $R$. The semi-inclusive recoil jet distribution as a function of $p_\mathrm{T}$ and $\Delta\varphi$ will be presented, where $\Delta\varphi$ is the relative azimuthal angle between trigger track and recoil jets. The results, including the $R$-dependence, will be compared to models.
The modification of the substructure of jets due to interactions with a hot QCD medium, the quark-gluon plasma, can be used to study the properties of this medium. Due to the nature of a jet, as a composite object of multiple particles, there are many observables one could construct and study. There is no indication that a single observable will be sufficient to understand the interaction of the parton shower with the hot QCD medium. We investigate how the correlation of jet observables is affected by jet quenching. The medium effect on this correlation is quantified using the Kullback-Leibler divergence and a principle component analysis. We also consider the experimental constraints and the influence of the large uncorrelated background present in heavy-ion collisions on the measurement of these observables. We present a framework in which all these ingredients are combined to determine which correlation of observables can be best used to constrain the medium-jet modification while being robust against the large underlying event. As the framework is fully data driven it can easily be deployed on heavy-ion data from RIHC and LHC experiments.
In this poster, measurements of the azimuthal opening angle and transverse momentum correlations between isolated photons and their associated jets, which are sensitive to medium induced parton momentum broadening, are reported for the first time with the high statistics pp and PbPb data recorded in 2017 and 2018. Isolated photon production and their detection techniques will also be summarized.
We study the thermalization of highly energetic partons in a high-temperature QCD plasma. We investigate the non-equilibrium dynamics using an effective kinetic description of QCD, following the evolution of a highly energetic parton from the hard momentum scales all the way to the medium scales, while keeping track of the recoil onto the medium [1-2]. We find that successive radiative emissions are important to develop a turbulent energy cascade which drives the fragmentation of energy to the medium scales and dominates the collinear region. Elastic interactions with the medium are more significant near medium scales, and primarily responsible for out-of-cone energy loss and the equilibration of the energy. We discuss the implications of our findings for phenomenological descriptions of jet quenching physics and studies of jet thermalization in heavy ion collisions.
[1]- S. Schlichting, I. Soudi, Fragmentation and equilibration of jets in a QCD plasma (8 2020).
[2] Y.Methar-Tani, S.Schlichting, I.Soudi, in preparation
Hard partonic scatterings serve as an important probe of quark-gluon-plasma (QGP) properties. The properties of jets and their constituents can provide a tool for understanding the partonic energy loss mechanisms. Low momentum jets offer a unique window into partonic energy loss because they reconstruct the partons which have lost a significant amount of energy to the QGP medium. The main difficulty in studying low momentum jets in heavy ion collisions is the presence of a significant uncorrelated background of low momentum hadrons from soft processes. One way to deal with this background is to use jet-hadron correlations to fit and subtract the soft, flow-modulated background. This technique allows measurements of the near and away side yields. We present constituent yields for Pb--Pb collisions at $\sqrt{s_{NN}}$= 5.02 TeV. These yields are a measurement of the raw fragmentation function. We discuss prospects for unfolding the distributions of yields to get a corrected fragmentation function for low jet momenta.
The measurement of jet deflection in heavy-ion collisions promises to provide unique and incisive insight into the physics of jet quenching and the quasi-particle nature of the QGP. However, observation of large-angle jet deflection favors using low transverse momenta ($p_{\rm T}$) jets, which is challenging in the high-background environment of heavy-ion collisions. The semi-inclusive approach to coincidence measurements, with data-driven background removal, is the only established analysis technique that can carry out such measurements with precision estimation of systematic uncertainties. In this poster, the STAR experiment at RHIC reports the first measurement of semi-inclusive $\gamma_{\rm dir}$+jet and $\pi^{0}$+jet azimuthal correlations in $p+p$ and central Au+Au collisions at $\sqrt s_{\rm NN}$=200 GeV. Charged-particle recoil jets are reconstructed using the anti-$k_{\rm T}$ algorithm with R = 0.2 and 0.5, and uncorrelated recoil jet contributions are corrected using a Mixed Event technique. Azimuthal distributions are reported for recoil jets with $p_{\rm T,jet} >$5 GeV/${\it c}$. The distributions in $p+p$ collisions are compared to NLO pQCD calculations including Sudakov broadening, and those in Au+Au collisions are compared to theoretical model calculations incorporating jet quenching.
Diffusion wake is a unique signal of the medium response which provides rich information of quark-gluon plasma in high-energy heavy-ion collisions. It can be characterized by a depletion of the azimuthal angle distribution of hadrons in the trigger direction in $\gamma$/Z jet events. However, this signal, if integrated over a large range of rapidity, can be overwhelmed by an enhancement of soft hadrons from multiple parton interaction (MPI) at the LHC energy, which is uniform in azimuthal angle. A recent 2D jet tomography can be applied to the event selection to enhance the path length of jet propagation and hence the signal of jet-induced diffusion wake. In this work, it is found that diffusion wake has a unique structure in the longitudinal direction and can be measured even without the assistance of 2D jet tomography at the LHC energy. We use an azimuthal cut to isolate hadrons affected by the diffusion wake and reduce contribution from the jet side. We find an unambiguous signal of diffusion wake. We further use a Gaussian fitting method to extract diffusion wake component and study its sensitivity to the properties of the dense QGP medium like flow and shear viscosity.
Jets are excellent probes for the study of the deconfined matter formed in heavy ion collisions. In particular, jet substructure measurements can help us understand the interaction dynamics of high-energy partons with the quark-gluon plasma. We introduce a new infrared and collinear safe observable: jet energy flow measurements using jets reconstructed with different resolution parameters $R$. These measurements can help us gauge the competition between the dependence of energy loss on the opening angle of the shower on the one hand and the generation of large-angle fragments by radiative energy loss which give opposite trends for the R-dependence of the nuclear modification factor. In this poster we present a first measurement of jet energy flow in JEWEL simulations, and highlight its sensitivity to jet energy loss and medium recoil effects.
Precision measurements of jet substructure are used as a probe of fundamental QCD processes. The primary Lund jet plane density is a two-dimensional visual representation of the radiation off the primary emitter within the jet that can be used to isolate different regions of the QCD phase space. We present a new measurement with the ALICE detector of the primary Lund plane density for inclusive charged-particle jets in pp collisions at $\sqrt{s} = $ 13 TeV, in the transverse momentum range [20,120] GeV/$c$.This is the first measurement of the Lund plane density in an intermediate jet $p_{\rm T}$ range where hadronization and underlying event effects play a dominant role. The projections of the Lund plane density onto the splitting scale $k_{\rm T}$ and splitting angle $\Delta{R}$ axis are shown, highlighting the perturbative/non-perturbative and wide/narrow angle regions of the splitting phase space. Through a 3D unfolding procedure, the Lund plane density is corrected for detector effects which allows for quantitative comparisons to MC generators to provide insight into how well generators describe different features of the parton shower and hadronization.
Jets are algorithmic proxies of hard scattered quarks/gluons created in collisions of high energy particles. In the last few years, there has seen an explosion of jet substructure results from all experiments derived from exploiting clustering algorithms. Jet quenching via parton energy loss in heavy ion collisions is an established probe for exploring the properties of the quark-gluon plasma. Since jets are multi-scale objects, there is a need to characterize different likely mechanisms of medium interaction leading to energy loss for jets of varying shower topologies. In this talk, we present novel differential measurements of the jet shower in $pp$ collisions at $\sqrt{s_{\rm{NN}}} = 200$ GeV and discuss their connection to parton evolution. We then proceed to tag specific jet populations in Au$+$Au collisions based on jet substructure observables, such as opening angle and the splitting formation time calculated using the leading and subleading subjets or charged particles within the jet. These observables are shown to be experimentally robust to the heavy ion underlying event. With multiple jet classes based on their shower topology in central Au+Au collisions, we compare and contrast their energy loss via jet quenching observables such as dijet momentum asymmetry and recoil jet yield. With the topologically selected jet populations in central Au$+$Au collisions, we compare and contrast the jet energy via traditional jet quenching observables. Such measurements, for the first time, point towards a space-time study of energy loss phenomenon via selections on jet formation time and opening angle.
We tried to locate the initial jet production positions in QGP, using the jet energy loss along the path length direction, the asymmetry perpendicular to the path length from gradient-tomography and the energy momentum distribution inside the jet with deep learning. These machine learning assisted Jet tomography help to locate the jet production positions with reasonable precision that helps us to look for Mach cones whose opening angles are direct measures of the QGP equation of state.
We present the scale dependence of the jet-medium interactions seen in the modification of jet substructure observables in high-energy heavy-ion collisions by systematic studies with JETSCAPE 3, a publicly available software package of a framework for Monte Carlo event generators [1]. In high-energy heavy-ion collisions, jet partons interact with the quark-gluon plasma medium while changing their energy and virtuality via their shower evolution. Measured jets are reconstructed from the final state particles in the shower and thus carry information about the interactions with the medium at the various scales of the jet partons. The multi-stage framework for the jet evolution of JETSCPE is designed to cover a broader range of the scale in the in-medium parton shower evolution by stitching multiple models together; Each model becomes active depending on the virtuality or energy of a parton.
Recently, we found that the explicit virtuality dependence in the jet quenching strength qhat [2] at the early high-virtuality phase is essential for the simultaneous description of the experimental data for the reconstructed jet suppression and single-particle suppression. In this study, we perform numerical simulations with a model incorporating the virtuality-dependent formulation with MATTER+LBT setup within the JETSCAPE framework. We systematically study the observables characterizing internal structures of jets to explore the details of the strength of the interaction with the medium at each scale. In particular, we examine the splitting function, which displays the effect of the medium interaction with a parton with large virtuality at the very early stage, and the jet fragmentation function, which clearly shows the medium effect on partons throughout a wide range of scales.
[1] JETSCAPE Collaboration (A. Kumar et al.), Jet quenching in a multi-stage Monte Carlo approach, Nucl. Phys. A 1005, 122009 (2021); JETSCAPE Collaboration (A. Kumar et al.), JETSCAPE framework: p+p results, Phys. Rev. C 102, no.5, 054906 (2020).
[2] Amit Kumar, Abhijit Majumder, and Chun Shen, Energy and scale dependence of qhat and the JET puzzle, Phys. Rev. C, 101(3):034908, 2020.
The evolution of leading partons and jets through deconfined QCD matter is a multi-scale phenomenon and remains as one of the challenging problems in heavy-ion physics. To address this, we use the JETSCAPE framework [1] in which the production of the hard parton is factorized from the evolution of the produced QGP. To incorporate various scales involved in the jet-medium interaction during the different epochs of the parton shower, a multi-stage energy loss model is constructed [2]. We propose a new functional form of the transport coefficient $\hat{q}$ that weakens as the parton’s virtuality becomes larger and reduces to the traditional hard-thermal-loop (HTL) $\hat{q}$ at smaller virtuality [3]. In this talk, we demonstrate that a multi-stage jet quenching model with modified HTL $\hat{q}$ and recoil-hole based medium response are crucial for a simultaneous description of the nuclear modification factor for inclusive jets and leading hadrons at RHIC and LHC collision energies.
The study carried out also highlights one of the major successes of the JETSCAPE framework [1,2] in providing a tool to set up an effective parton evolution using a multi-stage energy-loss scheme. In this approach, the space-time information of the QGP is embedded in the parton shower during the high virtuality phase modeled using the MATTER event generator, followed by the low virtuality phase modeled by the LBT event generator. The switching between the jet energy loss stages is carried out on parton-by-parton basis based on the off-shellness or energy of the parton. The jet-medium response is incorporated through a weakly-coupled transport description with recoil particles excited from the QCD medium. The recoil-hole formalism shows sensitivity to the distribution of energy-momentum of particles inside the jet and hence puts further constraints on the jet quenching mechanism. The study presented demonstrates that the jet transport coefficient $\hat{q}$ indeed has a resolution scale dependence in addition to the conventional temperature and energy dependence encoded in hard-thermal-loop formula.
[1] JETSCAPE Collaboration (J. H. Putschke (Wayne State U.) et al.), The JETSCAPE framework, arXiv:1903.07706 [nucl-th] (2019).
[2] JETSCAPE Collaboration (A. Kumar et al.), Jet quenching in a multi-stage Monte Carlo approach, Nucl. Phys. A 1005, 122009 (2021); JETSCAPE Collaboration (A. Kumar et al.), JETSCAPE framework: p+p results,' Phys. Rev. C 102, no.5, 054906 (2020).
[3] Amit Kumar, Abhijit Majumder, and Chun Shen, Energy and scale dependence of q-hat and the JET puzzle. Phys. Rev. C, 101(3):034908, 2020.
Jets are collimated sprays of hadrons and serve as an experimental tool for studying the dynamics of quarks and gluons. In particular, differential measurements of jet substructure enable a systematic exploration of the parton shower evolution. The SoftDrop grooming technique utilizes the angular ordered Cambridge/Aachen reclustering tree and provides a correspondence between the experimental observables such as the shared momentum fraction $(z_{\rm{g}})$, groomed jet radius or split opening angle $(R_{\rm{g}})$ and the QCD splitting functions in vacuum. In this poster, we present fully corrected correlations between $z_{\rm{g}}$ and $R_{\rm{g}}$ at the first split for jets of varying momenta and radii in $pp$ collisions at $\sqrt{s} = 200$ GeV. To study the evolution along the jet shower, we also present the splitting observables at the first, second and third splits along the jet shower for various jet and initiator prong momenta. As these novel measurements are presented in three dimensions, we outline the correction procedure so that it can be used as a template for future multi-differential measurements across all experiments.
So far, analytical jet quenching formalisms have either assumed that modifications to the jets’ structure are dominated by a few hard in-medium scatterings or by multiple soft interactions. However, it is known that neither of these regimes corresponds to any currently available experimental set-up, and thus bridging the gap between these limits under a single framework is a crucial step towards a complete description of jet quenching. In this talk we discuss a novel strategy, dubbed the Improved Opacity Expansion (IOE), which provides a self consistent way of interpolating between the single hard and multiple soft regimes for dense media populated by a few hard scattering centers. We compute the momentum broadening distribution and the medium induced radiation rate in the IOE, showing that the two previous limits are recovered. As an application, we show how the IOE allows to take into account Moliere scattering in the computation of jet substructure observables
Properties of dijets may provide sensitive probes of jet quenching in Quark-Gluon Plasma. Dijet invariant mass measurements in small systems provide an essential baseline for such studies in Pb-Pb collisions. In this poster, we present the first measurements of dijet invariant mass in minimum bias pp and p-Pb collisions at $\sqrt{s_\mathrm{NN}}=5.02$ TeV by ALICE. Jets are clustered using the anti-$k_\mathrm{T}$ algorithm with $R=0.4$, and an azimuthal angle of $\pi/2$ at mininum between the two jets. The dijet invariant mass is measured in the low mass range from $80$ to $150$ GeV/$c^2$.
The different modification of quark- and gluon-initiated jets in the quark-gluon plasma produced in heavy-ion collisions is a long-standing question that has not yet received a definitive answer from experiments. In particular, the relative sizes of the modification of quark and gluon jets differ between theoretical models. Therefore a fully data-driven technique is crucial for an unbiased extraction of the quark and gluon jet spectra and substructure. We perform a proof-of-concept study based on proton-proton and heavy-ion collision events from the PYQUEN generator with statistics accessible in Run 4 of the Large Hadron Collider. We use a statistical technique called topic modeling to separate quark and gluon contributions to jet observables. We demonstrate that jet substructure observables, such as the jet shape and jet fragmentation function, can be extracted using this data-driven method. These results suggest the potential for an experimental determination of quark and gluon jet spectra and their substructure.
Different than for inclusive jets, leading jet cross sections constitute normalized probability distributions for the leading jet to carry a longitudinal momentum fraction relative to the initial fragmenting parton. The formation and evolution of leading jets can be described by jet functions that satisfy non-linear DGLAP-type evolution equations. We present a parton shower algorithm that allows for the systematic calculation of leading-jet cross sections where logarithms of the jet radius and threshold logarithms are resummed to next-to-leading logarithmic (NLL) accuracy. By calculating the mean of the leading jet distribution, we are able to quantify the average out-of-jet radiation, the so-called jet energy loss. When an additional reference scale is measured, we are able to determine the energy loss of leading jets at the cross section level which is identical to parton energy loss at leading-logarithmic accuracy. We present comparisons to the first direct measurements of vacuum and medium-induced energy loss at LEP and the LHC in proton-proton and heavy-ion collisions.
[1] Neill, Ringer, Sato: JHEP 07 (2021) 041, arXiv 2103.16573
Heavy-flavour (charm and beauty) jets are excellent probes to study Quantum Chromodynamics (QCD). Their precise measurements in proton-proton collisions are used to verify perturbative QCD calculations and improve our modelling capabilities by constraining Monte Carlo generators. They also serve as the reference measurements for more complex systems such as Pb–Pb collisions, helping to disentangle various energy loss mechanisms and their dependence on the quark mass.
In this contribution, we present a measurement of $b$-jets in pp collisions at $\sqrt{s} = 5.02$ TeV using a machine-learning-based method. This method, which utilizes the long lifetime of beauty hadrons, was chosen to optimize $b$-jet tagging performance in terms of efficiency and purity. It also provides flexibility in the working point selection, enabling smooth switching from high efficiency to high purity settings. The projected performance gain, with enhanced $b$-jet purity relative to other approaches, will enable methodologically consistent measurements in Pb–Pb collisions, and more differential jet studies in pp collisions.
The phase transition from hadronic matter to quark-gluon plasma (QGP) is a phenomenon that occurs under extreme conditions of high temperature and high density, as achieved at the relativistic heavy ion collider (RHIC). The QGP causes energy loss of high momentum particles which is observed as suppression of high momentum hadron production in A+A collisions relative to p+p collisions. The study presented in this poster uses PHENIX data to evaluate the energy loss of partons in the QGP in the various collision systems provided by RHIC.
We measure the fractional momentum loss with two quantities, Sloss and S’loss. Sloss is obtained by comparing the inclusive $p_\textrm{T}$ spectra in the A+A and p+p collisions. In contrast, S’loss is obtained by comparing in-plane and out-of-plane spectra using the azimuthal anisotropy $v_2$. These quantities are extracted from PHENIX data in Au+Au, Cu+Au, and Cu+Cu collisions at $\sqrt{s_{\textrm{NN}}}$ = 200 GeV for $\pi^0$s, and the Au+Au collisions at $\sqrt{s_{\textrm{NN}}}$ = 200 GeV for charged hadrons. The interpretation of these results and their impact on our understanding of the path length dependence of energy loss in the QGP will be discussed.
The jet transverse momentum diffusion coefficient $\hat{q}$ is an important transport coefficient governing the radiative energy loss of a parton propagating the Quark-Gluon Plasma (QGP) created in Heavy-Ion Collisions. Based on perturbative arguments [1], which were recently extended to next-to-leading order [2], the dimensionless ratio $\hat{q}/T^3$ was shown to be connected with the dimensionless specific shear viscosity $\eta/s$. This connection requires two assumptions: (a) that the medium is describable in terms of quasiparticle excitations, and (b) that the mean-free path of the parton is related to the average transport cross section of a quasiparticle in the medium implying that the interaction with the medium constituents is of the same form and strength as the interaction among the quasiparticles themselves.
Based on this idea, results of a microscopic calculation of $\hat{q}$ are presented by applying a quasiparticle model that was shown to provide a successful, effective description for the strongly coupled QGP [3]. This approach allows the determination of $\hat{q}$ in dependence of the identity of the traversing highly energetic parton and the composition of the hot strongly interacting matter. The behaviour of the jet quenching parameter as a function of temperature and parton momentum is discussed also in comparison with a recent Bayesian parameter estimation by the JETSCAPE Collaboration [4].
[1] A. Majumder, B. Müller and X. N. Wang, Phys. Rev. Lett. 99 (2007), 192301.
[2] B. Müller, Phys. Rev. D 104 (2021) no.7, L071501.
[3] V. Mykhaylova, M. Bluhm, K. Redlich and C. Sasaki, Phys. Rev. D 100 (2019) no.3, 034002.
[4] J. Mulligan et al. [JETSCAPE], arXiv:2106.11348 [nucl-th].
Motivated by experimental results implying the possible QGP (quark-gluon plasma) formation in small colliding systems, we extend the hydro-based framework incorporating non-equilibrated components which play an essential role in small colliding systems. It has been widely accepted that relativistic hydrodynamics well describes the dynamics of the QGP at low $p_{\mathrm{T}}$ regimes in large colliding systems. Hence hydro-based frameworks are used to tease out properties of the QGP in high-energy heavy-ion collisions. In contrast, particle productions in small colliding systems have been studied through QCD-motivated phenomenological models such as perturbative QCD (semi-)hard processes followed by string fragmentation. As keeping these pictures in each regime, the ``marriage" of relativistic hydrodynamics and QCD-motivated phenomenological framework is indispensable to explore the dynamics over wide ranges of colliding systems.
We realize this as the dynamical core--corona initialization framework (DCCI) [1-3]. In DCCI, QGP fluids are generated from initial partons obtained from PYTHIA/PYTHIA Angantyr [4-5] which reflects the total energy-momentum of incoming nuclei. We phenomenologically describe the fluidization of the initial partons with the dynamical aspects of the core--corona picture. Partons with sufficient secondary scatterings tend to generate QGP fluids (core) as equilibrated matter.
While partons with insufficient secondary scatterings tend to survive as non-equilibrated matter (corona). This framework is, so to speak, the hydrodynamic afterburner for PYTHIA. By treating both locally equilibrated QGP fluids and non-equilibrated matter, the DCCI, as a hydro-based Monte Carlo event generator, is capable of describing from low to high $p_{\mathrm{T}}$, from backward to forward rapidity, and from small to large colliding systems.
In this talk, we investigate the interplay between core and corona components in high-energy nuclear collisions using DCCI. We reveal that the particle production from the core becomes dominant above $\langle dN_{\mathrm{ch}}/d\eta \rangle \sim 18$ regardless of the system size and the collision energy. Remarkably, the corona components turn out to dilute $\langle p_T \rangle$ and $v_2\{2\}$ obtained from the core components even in Pb--Pb collisions in which the entire system is often assumed to be locally equilibrated. These results suggest the importance of both equilibrated and non-equilibrated contributions in both small and large colliding systems towards an accurate understanding of the QGP properties.
[1] Y. Kanakubo, M. Okai, Y. Tachibana, and T.~Hirano, PTEP 2018, 121D01 (2018).
[2] Y. Kanakubo, Y. Tachibana, and T. Hirano, Phys.~Rev.~C101, 024912 (2020).
[3] Y. Kanakubo, Y. Tachibana, and T. Hirano, arXiv:2108.07943 [nucl-th].
[4] T. Sj\"{o}strand, S. Mrenna, and P. Z. Skands, Comput. Phys. Commun. 178, 852 (2008).
[5] C. Bierlich, G. Gustafson, L. L\"{o}nnblad, H. Shah, JHEP 10 134 (2018).
Short-lived resonances can probe strongly interacting matter produced in high-energy heavy ion collisions. K$^{*}(892)^{\pm}$ resonance is particularly interesting because of its very short lifetime (∼ 4 fm/c), comparable to the one of the hadronic phase. Therefore, it may be sensitive to the competing rescattering and regeneration mechanisms which modify the particle’s momentum distributions after hadronization. In this poster, recent measurements of resonance production in proton proton (pp) collisions as a function of event multiplicity and transverse spherocity will be presented, exploiting the large sample of pp collisions at $\sqrt{s}$ = 13 TeV collected by ALICE. These measurements show the onset of phenomena typical of heavy-ion collisions, like collective behaviour and suppression of the yield ratios of short-lived resonances to stable particles with increasing multiplicity. In addition, new preliminary results for charged-particle multiplicity dependent studies of K*(892)$^{\pm}$ production in pp collisions will also be presented.
Hadronic resonances are effective tools for studying the hadronic phase in ultra-relativistic heavy-ion collisions. In fact, their lifetime is comparable to the hadronic phase and resonances are sensitive to the hadronic phase effects such as re-scattering and regeneration processes which might affect the resonance yields and shape of the transverse momentum spectra. $\Lambda(1520)$ has a lifetime of around 13 fm/$\it{c}$, which lies in between the lifetimes of $K^*$ and $\Phi$ resonance. The resonance to stable particle yield ratios can be used to study the properties of the hadronic phase. Recently, ALICE observed the suppression of the $\Lambda(1520)/\Lambda$ ratio in Pb-Pb collisions at $\sqrt{s_{NN}}$ = 2.76 TeV as a function of centrality. It is therefore interesting to investigate the multiplicity dependent study of $\Lambda(1520)/\Lambda$ ratio for pp collisions, since this can serve as a baseline for heavy-ion collisions.
In this contribution, we present new results on the measurement of the baryonic resonance $\Lambda(1520)$ as a function of charged-particle multiplicity in pp collisions at $\sqrt{s}$ = 5.02 and 13 TeV. The transverse momentum spectrum, the integrated yield $< dN/dy >$, the average ${\textit{p}_{\rm T}}$ $<{\textit{p}_{\rm T}}>$ and the $ \Lambda(1520)/\Lambda$ yield ratio will be presented as a function of charged-particle multiplicity.
Recent measurements reveal that J/ѱ yields increase with increasing charged-particle multiplicity in pp and p—Pb collisions at the LHC. Different mechanisms have been proposed to explain this observation. One of them is the influence of multiple parton interactions (MPI) in the initial state of the collision. Measurements of the excited charmonia, as the ѱ(2S) state, state as a function of charged-particle multiplicity are important to disentangle the impact of possible final-state effects.
This poster presents the measurement of charmonium yields in pp collisions at √s = 13 TeV and p—Pb collisions at √sNN = 8.16 TeV as a function of charged-particle multiplicity. J/ѱ and ѱ(2S) are reconstructed in their dimuon decays within rapidity window -4.0 < ylab < -2.5. Charmonia yields are normalised to their respective average values. The charged-particle multiplicity is measured at central rapidity and also normalised to its average value. The excited-to-ground state ratio is also shown. Results are compared with model calculations.
Previous ALICE publications have shown, in pp collisions at the LHC, an increase of the inclusive J/$\psi$ yields as a function of charged-particle multiplicity. Such an increase was found to be stronger than linear, and both J/$\psi$ and multiplicity were measured at midrapidity. The causes for this behavior have been investigated in previous studies with PYTHIA8 and attributed to possible auto-correlation effects. Insight on this effect could be gained by measuring the charged-particle multiplicity in three azimuth regions relative to the direction of the J/$\psi$.
Data collected with ALICE at the LHC during Run 2 is used to investigate the relative J/$\psi$ yield, measured at mid-rapidity (|y|<0.9) in its di-electron decay channel and as a function of the charged-particle multiplicity, in various regions of the azimuthal angle with respect to the emission of the J/$\psi$ meson.
In this contribution, new measurements of this correlation performed in pp collisions at $\sqrt{s}$=13 TeV TeV will be shown.
Heavy-flavor hadrons, containing charm and beauty flavors are believed to be vital probes for the understanding of Quantum Chromodynamics (QCD) in high-energy hadronic collisions: right from the study of production mechanisms in proton-proton ($pp$) collisions to the investigation of Cold Nuclear Matter (CNM) effects in proton-nucleus (p--A) collisions and their suppression in the search of Quark Gluon Plasma (QGP) in nucleus-nucleus (A--A) collisions. Recently, the observation of heavy-ion-like features in small systems ($pp$ and p$-$A) like collective-like phenomena, strangeness enhancement etc, continues to generate considerable interest in the scientific community. In this regards, a cruial question arises, whether the QGP-like phenomena involve all the particles in the system or it is the effect of contributions from the processes like resonance decays, jets, underlying events (UE) etc. Therefore, small systems need to be re-investigated properly including the light and heavy-flavor sectors. In the present work, using transverse spherocity, one of the event-topology variables used to separate jetty and isotropic events from the pool of event samples, we aim to understand the production dynamics of heavy-flavors through the transverse momentum spectra, double differential yield and mean transverse momentum of J/$\psi$, $\rm D^{0}$ and $\Lambda_{c}^{+}$ as a function of charged-particle multiplicity. For the current analysis, the events are generated by using 4C tuned PYTHIA8 for $pp$ at $\sqrt{s}$ = 13 TeV, which is quite successful in explaining the heavy-flavor particle production at the LHC energies. We observe a clear dependence of spherocity on the production of J/$\psi$, $\rm D^{0}$ and $\Lambda_{c}^{+}$.
Collective behaviour of final-state hadrons, and multiparton interactions are studied in high-multiplicity 𝑒𝑝 scattering at a centre-of-mass energy 𝑠√=318 GeV with the ZEUS detector at HERA. Two- and four-particle azimuthal correlations, as well as multiplicity, transverse momentum, and pseudorapidity distributions for charged-particle multiplicities 𝑁ch≥20 are measured. The dependence of two-particle correlations on the virtuality of the exchanged photon shows a clear transition from photoproduction to neutral current deep inelastic scattering. For the multiplicities studied, neither the measurements in photoproduction processes nor those in neutral current deep inelastic scattering indicate significant collective behaviour of the kind observed in high-multiplicity hadronic collisions at RHIC and the LHC. Comparisons of PYTHIA predictions with the measurements in photoproduction strongly indicate the presence of multiparton interactions from hadronic fluctuations of the exchanged photon.
High statistics data sets from experiments at RHIC and the LHC with small and large collision species have enabled a wealth of new flow measurements, including the event-by-event correlation between observables. One exciting such observable $\rho$($v^2_n$,[$p_T$]) gauges the correlation between the mean transverse momentum ($p_T$) of particles in an event and the various flow coefficients ($v_n$) in the same event. Recently it has been proposed that very low multiplicity events may be sensitive to initial-state glasma correlations rather than flow-related dynamics. We find utilizing the IP-JAZMA framework that the color domain explanation for the glasma results are incomplete. We then explore predictions from PYTHIA-ANGANTYR having only non-flow correlations and AMPT having both non-flow and flow-type correlations. We find that PYTHIA-ANGANTYR has non-flow contributions to $\rho$($v^2_n$,[$p_T$]) in $p$+O, $p$+Pb, O+O collisions that are positive at low multiplicity and comparable to the glasma correlations. It is striking that in PYTHIA-8 in $pp$ collisions there is actually a sign-change from positive to negative $\rho$($v^2_n$,[$p_T$]) as a function of multiplicity. The AMPT results match the experimental data general trends in Pb+Pb collisions at the LHC, except at low multiplicity where AMPT has the opposite sign. In $p$+Pb collisions, AMPT has the opposite sign from experimental data and we explore this within the context of parton geometry. In this presentation, we will discuss the detailed model study on the $v_n$-$p_T$ correlation in [Phys. Rev. C 103, 064906 (2021)]
Recently the PHENIX Collaboration has made available two-particle correlation Fourier coefficients for multiple detector combinations in minimum bias p+p and 0-5% central $p$+Au, $d$+Au, $^3$He+Au collisions at 200 GeV (arXiv:2107.06634). Using these coefficients for three sets of two-particle correlations, azimuthal anisotropy coefficients $v_2$ and $v_3$ are extracted for midrapidity charged hadrons as a function of transverse momentum. As discussed in arXiv:2107.07287 and in this talk, we use the available coefficients to explore various non-flow hypotheses as well as compare the results with theoretical model calculations. The non-flow methods fail basic closure tests with AMPT and PYTHIA/ANGANTYR, particularly when including correlations with particles in the low multiplicity light-projectile going direction. In data, the non-flow adjusted $v_2$ results are modestly lower in $p$+Au and the adjusted $v_3$ results are more significantly higher in $p$+Au and $d$+Au. However, the resulting higher values for the ratio $v_3/v_2$ in p+Au at RHIC compared to p+Pb at the LHC is additional evidence for a significant over-correction. Incorporating these additional checks, the conclusion that these flow coefficients are dominated by initial geometry coupled with final-state interactions (e.g.~hydrodynamic expansion of quark-gluon plasma) remains true, and explanations based on initial-state glasma are ruled out. The detailed balance between intrinsic and fluctuation-driven geometry and the exact role of weakly versus strongly-coupled pre-hydrodynamic evolution remains an open question for triangular flow, requiring further theoretical and experimental investigation.
The main goal of the ALICE experiment is to study the physics of strongly interacting matter, including the properties of the Quark-Gluon Plasma (QGP). The relative production of strange hadrons with respect to non-strange hadrons in heavy-ion collisions was historically considered one of the signatures of QGP formation. However, recent measurements in proton-proton (pp) and proton-lead (p-Pb) collisions have shown features that are reminiscent of those observed in lead-lead (Pb-Pb) collisions, exhibiting an increase in the production of strange hadrons relative to pions with the charged particle multiplicity in the event.
We report the new preliminary mid-rapidity measurement of the transverse momentum spectra and yields of ${\rm K^{0}_{s}}$, $\Lambda$ and $\bar \Lambda$ in the p-Pb collision system at $\sqrt{s_{\rm NN}}$ = 8.16 TeV. Results have been obtained in several multiplicity bins, so that a comparison to lower energy p-Pb results and to similar measurements in pp and Pb-Pb collisions can be performed. Finally, the comparison to phenomenological models will be discussed.
Jets are collimated sprays of particles produced from the fragmentation and hadronization of hard-scattered partons in high energy hadronic and nuclear collisions. Jet properties are sensitive to details of parton showering processes and expected to get modified in the presence of a dense partonic medium. Recently features similar to those in heavy-ion collisions have been observed in high multiplicity events in small collision systems, however, the suppression of inclusive jet production cross section is found to be absent. Measurements of jet properties can shed light on the current understanding of the observed behavior of high multiplicity events in such systems. In this work, we will present the centrality dependence of charged jet properties, viz. mean charged particle multiplicity, transverse momentum profile and fragmentation functions for leading jets in the range of jet $p_{\rm T}$ from 10 – 120 GeV/c at midrapidity in p-Pb collisions at 5.02 TeV with ALICE.
Recent results in high-multiplicity pp collisions show interesting features similar to those that are associated to the formation of a quark-gluon plasma in heavy-ion collisions [1]. Investigating the modification of the intra jet properties as a function of event multiplicity in pp collisions can provide deeper insight into the nature of these effects.
We will present the latest results of multiplicity dependence of charged-particle jet properties (average charged particle multiplicity, radial transverse momentum density and fragmentation functions) for leading charged-particle jets reconstructed using anti-$k_{\rm T}$ jet finding algorithm with radius parameter $R$ = 0.4 in the jet $p_{\rm T}$ range from 10 - 120 GeV/$c$ at midrapidity in pp collisions at $\sqrt{s}$ = 13 TeV with ALICE.
[1] Vardan Khachatryan et al. Phys. Lett. B 765 (2017), JHEP 09 (2010).
Internal properties of jets and their production in small collision systems (pp and p--Pb) are tightly connected to perturbative and non-perturbative aspects of quantum chromodynamics (QCD), such as cold nuclear matter effects. Recent studies of high-multiplicity final states of small collision systems also exhibit signatures of collective effects that are thought to be associated with hot and dense, color-deconfined QCD matter, which is known to be formed in collisions of heavier nuclei. The absence to date of jet quenching signals raises a question about the origin of the observed collectivity and calls for more accurate jet quenching measurements in small collision systems. ALICE is uniquely positioned to do precise charged-particle jet measurements due to its high efficiency in the reconstruction of charged particles.
In this contribution, we will report new results on charged-particle jet production in p-Pb and pp collisions at $\sqrt{s_{\rm{ NN}}}=5.02$ TeV measured by ALICE in LHC Run 2. The data extend the transverse momentum range and jet cone radius span of previous measurements of $p_{\rm T}$ differential cross section spectra and nuclear modification factor $R_{\rm{pPb}}$ published by ALICE. In addition, the multiplicity dependence of charged-particle jet properties (mean charged-constituent multiplicity, transverse momentum profile and fragmentation functions) in p-Pb and pp collisions at $\sqrt{s_{\rm{NN}}}=5.02~\rm{TeV}$ and $\sqrt{s_{\rm{ NN}}}=13~\rm{TeV}$ respectively will also be presented and compared to predictions of theoretical models.
Two-particle differential correlators of particle numbers ($R_2$) and particle transverse momenta ($P_2$ and $G_2$), recently measured in Pb-Pb collisions, emerged as powerful tools to gain insights into particle production mechanisms and infer transport properties such as the ratio of shear viscosity to entropy density of the medium created in Pb-Pb collisions. In this talk, recent ALICE measurements of these correlators in pp collisions at $\sqrt{s}$ = 7 and 13 TeV and p-Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV are presented to provide baseline references to measurements in Pb-Pb collisions and seek evidence, in particular, for viscous effects expected to arise in fluid-like systems produced in these collisions. Additionally, these measurements in small systems also probe particle correlations associated with jets as well as low-$p_{\rm T}$ processes and their change with system size. The strength and shape of the correlators are studied as a function of produced particle multiplicity to identify evidence for longitudinal broadening that might reveal the presence of viscous effects in these smaller systems. The measured correlators and their evolution from pp and p-Pb to Pb-Pb are additionally compared to predictions from Monte Carlo models, and the potential presence of viscous effects is discussed.
The ALICE Collaboration presents results of a search for jet quenching effects in high multiplicity pp collisions at $\sqrt{s} = 13~\rm{TeV}$ by measuring the semi-inclusive acoplanarity distribution of charged-particle jets that recoil from a high transverse momentum trigger-track. The search for jet quenching is performed by comparing the acoplanarity distributions measured in high multiplicity and minimum bias events. High multiplicity events are selected by online trigger based on a signal amplitude measured in the forward V0 scintillator detectors. Removal of background jet yield uncorrelated with a trigger-track is performed utilizing a data-driven statistical approach.
In this poster, we will show that the acoplanarity distributions, measured in high multiplicity events, exhibit a marked suppression and broadening when compared to the corresponding distributions obtained from minimum bias events. The distributions are corrected for momentum smearing due to instrumental effects. The observed features are not caused by jet quenching, since they can be reproduced by PYTHIA 8 event generator, which does not account for jet quenching. Analysis of the PYTHIA events reveals that the suppression and broadening of the hadron-jet acoplanarity distributions are the consequence of a bias induced by the ALICE high multiplicity trigger.
In heavy-ion collisions at relativistic energies, a hot and dense medium called quark-gluon plasma (QGP) is created. Intriguingly, the collective motion of produced particles, which is thought to be a strong evidence of the formation of QGP, is also seen in small systems like pp and p–Pb collisions. Such a study can be done in the ALICE experiment at the LHC via long-range two-particle correlations. In this poster, we discuss how to determine the flow coefficients in pp and p–Pb collisions using the template fit method to subtract non-flow contributions based on examination of the method with event generators. A model study to understand the flow of identified particles like $\pi$, $K$, and $p$ is discussed as well.
Recently, many results suggesting the production of Quark-Gluon Plasma (QGP) in high multiplicity events in small collision systems have been presented. These results were reported with surprise and have been a topic of intense discussion.
The purpose of the analysis presented in this talk is to look for the possibility of QGP formation in small collision systems through the measurement of low-mass vector mesons ($\rho$, $\omega$, $\phi$) decaying in the lepton pair channel. The analysis focuses on forward-rapidity muons instead of mid-rapidity electrons since the ALICE muon tracking system at forward rapidity allows for high-purity muon identification and momentum measurement to low transverse momentum. In addition, thanks to a dedicated muon pair trigger, it has been possible to enhance efficiently the sample of muon pairs.
This presentation will report the multiplicity and transverse momentum dependence of the low-mass vector meson production in pp collisions at √s = 13 TeV with full LHC-Run2 (2015-2018) data.
Jet observables, including jet fragmentation transverse momentum $j_{\rm T}$, parallel momentum $p_{\left|\right|}$, jet fragmentation function $\xi$ and jet constituent transverse momentum $p_{\rm T, track}$ distributions, have been investigated in p $+$ p and p $+$ Pb collisions at $\sqrt{s_{\rm NN}} = 5.02~{\rm TeV}$ via a multiphase transport model (AMPT) [1,2] with new hadronization model which contains both dynamical quark coalescence and fragmentation schemes [3]. With the new hadronization model, the recent ALICE measurements of $j_{\rm T}$ distributions can be quantitatively described, especially for low and intermediate $j_{\rm T}$ regions. We observe that high-energy jets have more large-$j_{\rm T}$ particles than low-energy jets, which are consistent with the experimental measurements. Importantly, the predicted ratio of $j_{\rm T}$ distributions between p $+$ Pb and p $+$ p shows a sizeable enhancement above unity of low-$j_{\rm T}$ particles and a suppression of intermediate-$j_{\rm T}$ particles, which indicates the possible effects from jet-medium interactions in small systems. This jet observable is suggested to probe the QGP medium effects in small systems in this talk. On the other hand, the $p_{\left|\right|}$ ratio of jet distribution is proposed as a complementary observable to probe jet-medium response in small systems.
We also implement jet fragmentation $\xi$ and $p_{\rm T, track}$ distributions and compare to CMS measurements. The similar enhancement of soft jet particles and suppression of hard jet particles appear in both new hadronization model and original AMPT hadronization model, which demonstrate that this enhancement (suppression) is model independent. We also systematically study the difference between two hadronization models, and find out that it is important for studying jet observables in small systems with a proper hadronization scheme.
[1] X.-P. Duan, W. Zhao, G.-L. Ma, "Probing QGP medium effect on jet observables in small systems with AMPT $+$ new hadronization model", arXiv:2021.xxxx.
[2] Z.-W. Lin, C. M. Ko, B.-A. Li, B. Zhang, and S. Pal, "Multiphase transport model for relativistic heavy ion collisions", Phys. Rev. C 72, 064901 (2005).
[3] W. Zhao, C. M. Ko, Y.-X. Liu, G.-Y. Qin, and H. Song, "Probing the Partonic Degrees of Freedom in High-Multiplicity $p$-Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV", Phys. Rev. Lett. 125, 072301 (2020).
ALICE is the experiment at the LHC specifically designed to study the properties of the quark-gluon plasma (QGP), a deconfined state of matter created in ultrarelativistic heavy-ion collisions. In this context, light-flavour particle production measurements play a key role, as they can probe statistical hadronization and partonic collectivity. Recent measurements in small colliding systems (pp and p-Pb) highlight an enhancement of produced strange hadrons with increasing multiplicities.
How do we further probe whether the multiplicity in itself is the driving force behind this effect? In this contribution we will study the production of strange light-flavour hadrons at low-multiplicity in pp collisions at $\sqrt(s)=5.02$ TeV, extending at low multiplicity the observations reported in pp, p-Pb and A-A interactions, in order to probe the onset of strangeness enhancement.
In contrast, this contribution we will discuss light-flavour particle production in pp collisions at $\sqrt(s)=13$ TeV, constrained to high multiplicities as a function of event topology. The event topology in this analysis is estimated through the transverse spherocity, which categorizes events based on the azimuthal distribution of tracks. The transverse spherocity is sensitive to the hard and soft processes and is a useful tool to distinguish the isotropic and jet dominated events in pp collisions. The interplay between multiplicity and transverse spherocity classes on light-flavour production can be understood by comparing the results obtained from the extreme selection of multiplicity and/or transverse spherocity.
These measurements will be compared with the Monte Carlo (MC) predictions obtained from models such as PYTHIA 8 and Herwig7.
Anisotropic flow is a key probe of the existence of the quark-gluon plasma. Small collision systems, such as proton-proton (pp) and proton-lead (p--Pb), are usually used to provide the reference data for collisions of heavy nuclei. However, inspection of high-multiplicity p--Pb and pp collisions revealed surprising features, usually attributed to collective effects in heavy-ion collisions.
In this poster, recent results on flow harmonics, nonlinear flow response and symmetric cumulants with the ALICE detector at the LHC will be presented. The observables characterize the properties of the strongly interacting matter. The properties include nonlinear response to initial geometry, event plane correlations and flow fluctuations. The observables are measured in both large systems (Pb--Pb, Xe--Xe) and small systems (p--Pb, pp) to show the asymptotic behavior of the flow-like effects from large to small systems.
We postulate that non-perturbative QCD evolution of a single parton in the vacuum will develop the long-range QCD collective effects of a multi-parton system, reminiscent of those observed in high-energy hadronic or nuclear interactions with large final-state particle multiplicity final-state particles [1]. Proton-Proton collisions at the Large Hadron Collider showed surprising signatures of a strongly interacting, thermalized quark-gluon plasma (QGP), which was thought only to form in collisions of large nuclear systems. Another puzzle observed earlier in electron-position collisions is that production yields of various hadron species appear to follow a thermal-like distribution with a common temperature. We propose searches for thermal and collective properties of a single parton propagating in (or “colliding into”) the vacuum using high multiplicity jets in high-energy elementary collisions, using a new frame with the jet direction defined as the beam z axis. In this single jet frame, a series of observables relevant to signatures of QGP are studied using the PYTHIA 8 Monte Carlo event generator in pp collisions at LHC energies. Experimental observation of collective and thermal effects in such single parton systems will offer a new view of non-perturbative QCD dynamics of multi-parton systems at the smallest scales. Absence of any collective effect may offer new insights into the role of quantum entanglement in the observed thermal behavior of particle production in high energy collisions. Opportunities for additional studies at future facilities, such as the EIC or a proposed muon-ion collider, future circular collider (FCC) are also discussed..
[1] https://arxiv.org/abs/2104.11735
There is strong evidence for the formation of small droplets of quark-gluon plasma in $p/d/^{3}$He+Au collisions at the Relativistic Heavy Ion Collider (RHIC) and in $p$+$p$/Pb collisions at the Large Hadron Collider. In particular, the analysis of data at RHIC for different geometries obtained by varying the projectile size and shape has proven insightful. In this talk, we present a new analysis that confirms the previous results and extends the measurements of $v_2$ and $v_3$ towards larger pseudorapidity acceptances. The $v_2$ measurements are further extended to non-central collisions and minimum bias $p$+$p$ collisions.
The thermodynamical properties of the high-temperature and high-density system produced in relativistic heavy-ion collisions can be understood with a systematic study of the produced hadrons' transverse momentum ($p_{\rm T}$ ) spectra. The $p_{\rm T}$ spectra of these hadrons can be described well by a distribution using the Tsallis statistics. The Tsallis parameters $q$ and $T$ measure the degree of deviation of the system from an equilibrium state and the effective temperature at freeze-out conditions, respectively. The Tsallis formalism with the inclusion of flow velocity can describe the $p_{\rm T}$ spectra from low to high $p_{\rm T}$ ranges. This formalism overcomes the drawback of the limited pT range description through the blast-wave fits of the $p_{\rm T}$ spectra.
In this work, a detailed study of the $p_{\rm T}$ spectra of the identified charged particles (pions, kaons, protons) as well as all charged particles in the heavy-ion collisions at the Relativistic Heavy Ion Collider (RHIC) energies (from $\sqrt{s_{\rm NN}} = $ 7.7 GeV to 200 GeV) and at the Large Hadron Collider (LHC) energies ($\sqrt{s_{\rm NN}} =$ 2.76 TeV to 5.44 TeV) are performed using the non-extensive Tsallis statistics. The extracted Tsallis parameters are found to be dependent on the particle species, collision energy, centrality, and fitting ranges of the $p_{\rm T}$. With increases of the collision energies, $q$ increases in a systematic manner whereas $T$ has a decreasing trend. It is observed that the parameters $q, T$, changes with increasing $p_{\rm T}$ fitting ranges and at mid $p_{\rm T}$ region the parameter are found to be unchanged, which can describe the physics of the systems. The Tsallis parameters and the quality of fitting are found to follow a mass ordering. The contribution of the flow velocity of the particles are considered with the Tsallis statistics through Tsallis blast-wave (TBW) model, which is found to have a better description of the $p_{\rm T}$ spectra of different particle species. The thermodynamic parameters and extracted energy density at the kinetic freeze-out will be presented as a function of collision energy.
Jet fragmentation can be studied using the transverse momentum ($j_T$) and longitudinal momentum fraction ($z$) of constituent particles. The $j_T$ distributions of jet fragments have been measured in pp and p—Pb collisions at $\sqrt{s_{NN}}$ = 5.02 TeV with ALICE, and various parton-shower models reasonably describe the pp results. In this analysis we carry out more detailed measurements of $j_T$ distributions for charged jets in pp collisions, in several z ranges. The $z$-dependent $j_T$ distributions will be compared with various models to test our current understanding of jet fragmentation and hadronisation. In this poster, the current status and future plans of the data analysis will be presented.
Proton-ion collisions at the LHC and RHIC have yielded unexpected trends, notably in measurements of jet nuclear modification factors at different collision centralities. Recent preliminary measurements from STAR in p+Au collisions at $\sqrt{s_{{\rm NN}}}=200$ GeV demonstrate inherent correlations between high-$Q^{2}$ parton scatterings and event activity (EA), measured using either detectors at backward (Au-going) rapidities or underlying event (UE) at mid-rapidity. The measurements at STAR disfavor jet quenching as an explanation for the suppression of jet yield observed in high-EA collisions. This leads to an opportunity to probe the early stages of the collisions and cold nuclear matter (CNM) effects. In this talk, we show correlations of backward-rapidity EA with mid-rapidity UE, as well as measurements of EA-dependent modifications to charged hadron spectra and jets. In particular, we present measurements of the UE for various EA selections and discuss its kinematic dependence on jet pseudorapidity and transverse momentum, $p_{{\rm T}}$, as a means of examining the initial hard scatterings. We also investigate the EA dependence of high-$p_{{\rm T}}$ hadron and jet properties -- including fully corrected ungroomed and SoftDrop groomed jet substructure observables as a function of EA -- to study the impact of initial and final state effects.
Collective flow-like signals including the ridge structure observed in small collision systems at high energies that are similar to those in large collision systems have led to questions about the onset of collectivity in nuclear collisions. Multiparticle cumulant methods are better in extracting the flow signals as they can suppress nonflow effects that are especially significant in small systems. For example, a negative four-particle cumulant $c_{2}\{4\}$ is expected when the correlation comes from the collective flow. A previous study from a hydrodynamics-based hybrid model could not produce negative $c_{2}\{4\}$ for $p+p$ collisions at 13 TeV [1], regardless of the analysis method such as the standard, two-subevent and three-subevent cumulants.
In this study [2], we use the string melting version of a multi-phase transport (AMPT) model without or with the sub-nucleon geometry for the proton to study multiparticle cumulants in $p+p$ collisions at 13 TeV. We have found that both versions of the model can produce $c_{2}\{4\}<0$ for high-multiplicity events. The relation between $c_{2}\{4 \}$ and the parton scattering cross section is non-monotonic, where only a finite range of parton cross sections can lead to negative $c_{2} \{4 \}$ for high-multiplicity $p+p$ events. In addition, the AMPT version with the proton sub-nucleon geometry describes the multiplicity dependence of $c_{2} \{4 \}$ much better than the version without. This demonstrates the importance of incorporating the sub-nucleon geometry and the potential of using multiparticle cumulants to probe the detailed sub-nucleon geometry in studies of small collision systems.
[1] W. B. Zhao, Y. Zhou, H. J. Xu, W. T. Deng and H. C. Song, Phys. Lett. B 780, 495-500 (2018).
[2] X. L. Zhao, Z. W. Lin, L. Zheng and G. L Ma, arXiv: 2112.01232.
Relativistic hydrodynamics has been successful in describing the evolution of Quark-Gluon Plasma, as well as understanding and predicting experimental measurements highlighting the collective behavior of the observed hadrons created in relativistic heavy-ion collisions. In parallel with the remarkable progress made in numerical fluid dynamics, the study of analytical solutions remains helpful in capturing intuitive pictures and important features.
In this work, we report a new exact and explicit analytical solution for relativistic ideal hydrodynamic equations. In this solution, the fluid expands in the longitudinal direction, and contains the plateau structure for finite rapidity range, and can be either symmetric or asymmetric (with respect to mid-rapidity). Both these features are controlled by two parameters in the solution, which allows flexibility in covering the longitudinal shape of p-A and A-A collisions. We further calculate the corresponding pseudorapidity distribution of hadron yields, and find good agreement with the experimental measurements in high-energy Pb-Pb, Au-Au, p-Pb, and d-Au collisions. We also stress the importance of exact solutions as a baseline for the assessment of non-equilibrium features, and for the validation of numerical simulations. Considering that this exact solution can be rapidity-asymmetric, this last point appears especially valuable.
Direct photons are widely used probe to study the properties and evolution of the hot and dense medium (e.g. QGP) produced in high energy heavy-ion collisions. Being color neutral, they do not interact strongly with the medium and are produced at all stages of the collision.
A universal scaling of the direct photon yield with charged particle multiplicity has been observed for a wide range of collision systems at different center of mass energies (Au+Au at 200 GeV, 62.4 GeV, 39GeV, Cu+Cu at 200 GeV, d+Au at 200 GeV and p+Au at 200 GeV). The measurement also hints that the QGP turn off/on transition region may exist between large and small system collisions.
In this poster, the analysis status of the low transverse momentum direct photon production in Cu+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV using external conversion method with the PHENIX detector is presented. The present study will help bridge the gap between small and large systems, and hence will provide more information about the transition region.
Previous ALICE results indicate a stronger than linear increase of the inclusive normalized J/$\psi$ yield with charged-particle multiplicity, both measured at mid-rapidity, in proton-lead collisions at $\sqrt{s_{NN}}=5.02$ TeV. The corresponding ALICE results on proton-proton collisions at$\sqrt{s}=13$ TeV provide a clearer picture of a stronger than linear increase.
In PYTHIA8, this behavior has been associated with auto-correlation effects in proton-proton collisions. This has been achieved by investigating the multiplicity dependence of J/$\psi$ production in different regions of the azimuthal angle, which is the difference between the J/$\psi$ meson and the charged particle emission angle. For proton-lead collisions, no results on these distributions for the J/$\psi$ meson are available yet.
This poster will present first results on the multiplicity dependence of the normalized J/$\psi$ yield for proton-lead collision in regions of the azimuthal angle, using ALICE data at $\sqrt{s_{NN}}=5.02$ TeV recorded during the LHC data taking Run 2 in 2016.
With current and future heavy-ion experiments focusing on understanding the baryon-rich QCD (Quantum Chromodynamic) matter produced at low collisional energies, first-principle knowledge of the equation of state in such regions is essential for analyzing experimental data in terms of transport simulations and to constrain effective models of QCD.
We construct a novel equation of state (EoS) describing QCD matter at finite temperature $T$ and baryon $B$, electric $Q$ and strangeness $S$ chemical potentials by utilizing the alternate expansion scheme from recent lattice QCD results [1]. This procedure allows to reliably estimate the baryon and strangeness densities at larger values of the baryon chemical potential $\mu_B$ compared to the usual Taylor expansion in terms of susceptibilities. We use the latter only to incorporate the $\mu_Q$ and $\mu_S$ dependence into thermodynamic quantities, which is sufficient for studying the matter as produced in relativistic heavy-ion collisions. Our simple parametrization provides reliable results for all temperatures since it interpolates between the confined phase at low $T$ described by the Hadron-Resonance Gas (HRG) model, lattice QCD results around and above the transition temperature $T_c(\mu_B)$, and $\mathcal{O}(g^5)$ perturbative QCD results in the high-$T$ regime.
[1] S. Borsányi, Z. Fodor, J.N. Guenther, R. Kara, S.D. Katz, P. Parotto, A. Pásztor, C. Ratti, K.K. Szabó. Lattice QCD equation of state at finite chemical potential from an alternative expansion scheme. Phys.Rev.Lett. 126 (2021) 23, 232001.
Heavy quarkonia are important probes of the matter created in heavy ion-collisions. The complex
heavy-quark potential is an essential ingredient of dynamical models of quarkonium production in
heavy-ion collisions, e.g. in models based on open quantum system approach.
We calculate the complex heavy-quark potential in (2+1)-flavor QCD with physical quark masses on
the lattice using large temporal extent. The heavy-quark potential is extracted from the Wilson
line correlators in Coulomb gauge. Then we extract the underlying spectral functions using multiple
conceptually different analysis methods -- spectral function fits, an HTL inspired fit for the
correlation function, Padé rational approximation and the Bayesian BR spectral reconstruction and
compare the implications of each for the existence and properties of a well defined dominant
spectral peak.
The peak position corresponds to the real part of the potential, while the width corresponds to
the imaginary part of the potential. While all the methods roboustly point toward a significant
imaginary part of the potential that increases with increasing separation between quark and
antiquark, the expected screening of the real part of the potential is not evident in our
calculations.
References:
[1] D. Bala et al, e-Print: 2110.11659 [hep-lat], submitted to PRD
[2] D. Hoying et al, Contribution to: Lattice 2021, e-Print: 2110.00565 [hep-lat]
Above the chiral restoration crossover some cumulants
of quark (baryon) number and charge fluctuations approach
a free quark gas value already at T ~ 200-250 MeV and
are considered sometimes as evidence of deconfinement.
At the same time at these temperatures very clear patterns
of chiral spin symmetry, which is a symmetry of the color
charge and electric interactions, and which is not a symmetry
of free quark gas, are observed. This symmetry suggests that
degrees of freedom are chirally symmetric quarks connected
by electric field into color singlet objects ("long strings").
The cumulants of conserved charge fluctuations are given
by integrals of spatial correlators of conserved charge.
If quarks are free these correlators demonstrate on a
finite lattice remarkable diffractive patterns that are
induced by quarks that are separated by a large distance.
In full QCD these diffractive patterns are absent which
indicates that a confining interaction does not allow
quarks to be separated by a large distance. These effects
are clearly visible in the correlators in a region where
the correlators are suppressed by a few orders of magnitude.
Hence the cumulants of conserved charge fluctuations are
simply insensitive to the deep infrared region where
confinement is manifest.
Bulk properties of nuclear matter can be extracted by employing femtoscopic methods to study the high-energy systems emerging from relativistic heavy-ion collisions. The space-time structure of the particle-emitting source can be examined by observing the effects of quantum-statistics and final-state-interactions on the pair correlations of particles, with data collected by the STAR experiment from $\sqrt{s_{_{NN}}}=200$ GeV Au+Au collisions created at RHIC. On account of being less susceptible to resonance decays and having a smaller reaction-cross-section while interacting with hadrons, kaons provide a complementary probe of the particle-emitter as compared to pion analyses. Results from Bose-Einstein correlations between pairs of charged kaons will be presented in this study and compared to descriptions based on a Levy-shaped source distribution.
The breakthroughs in computer vision and image recognition of the past decade using convolutional neural networks (CNNs) have shown that adapting neural network architectures to the symmetries associated with a particular machine learning problem leads to models that perform better and are easier to train and to interpret. These successes have led to applications in lattice gauge theory, such as detecting phase transitions or improving the performance of Monte Carlo methods. In this talk we present lattice gauge equivariant convolutional neural networks (L-CNNs) [1], a general framework for formulating neural networks that are equivariant under lattice gauge symmetry, and demonstrate that L-CNNs can outperform non-equivariant CNNs in non-linear regression tasks. Moreover, we prove that L-CNNs are able to generate arbitrarily shaped Wilson loops from just a few gauge equivariant network layers.
[1] M. Favoni, A. Ipp, D. I. Müller, D. Schuh, arXiv:2012.12901
State-of-the-art lattice QCD studies of hot and dense strongly interacting matter currently rely on extrapolation from zero or imaginary chemical potentials. The ill-posedness of numerical analytic continuation puts severe limitations on the reliability of such methods. Here we use the more direct sign reweighting method to perform lattice QCD simulation of the QCD chiral transition at finite real baryon density on phenomenologically relevant lattices. This method does not require analytic continuation and avoids the overlap problem associated with generic reweighting schemes, so has only statistical but no uncontrolled systematic uncertainties for a fixed lattice setup. This opens up a new window to study hot and dense strongly interacting matter from first principles. We demonstrate that the method can penetrate the region where extrapolation methods stop being predictive, by performing simulations up to a baryochemical potential-temperature ratio of $\mu_B/T=2.7$ - thus covering the range of the RHIC Beam Energy Scan in the baryochemical potential.
Direct photons are an important probe into the thermal and collective properties of Quark Gluon Plasma (QGP). Precise measurement of the direct photon anisotropy is necessary to provide additional insight into the photon production mechanisms in QGP which helps constrain theoretical models and thus solve the so-called direct photon puzzle. In this poster, analysis status of the elliptic and triangular flow is presented from the high statistics Au+Au data taken in 2014 by the PHENIX experiment. Two different event plane detectors are used to take into account possible non-flow effects in the data.
The magnetic fields generated in non-central heavy-ion collisions are among the strongest fields produced in the Universe, reaching magnitudes comparable to the scale of the strong interactions. Backed by model simulations, the resulting field is expected to be spatially modulated, deviating significantly from the commonly considered uniform profile. In this work, we present the next step to improve our understanding of the physics of quarks and gluons under extreme conditions by using lattice QCD simulations with $2+1$ staggered fermions with physical quark masses and an inhomogeneous magnetic background for a range of temperatures covering the QCD phase transition. We apply a field with strength given by a $1/\cosh^2$ function and analyze the impact on the computed observables and on the transition. We study the physics of the QCD medium by calculating local chiral condensates, Polyakov loops, electric currents and perform the continuum limit extrapolation. We find that the observables show non-trivial spatial features due to the interplay between the sea and the valence effects, especially around the critical temperature.
The hadron resonance gas (HRG) model is often believed to correctly describe the confined phase of QCD. This assumption is the basis of many phenomenological works on QCD thermodynamics and of the analysis of hadron yields in relativistic heavy ion collisions. We use first principle lattice simulations to calculate corrections to the ideal HRG model. Namely, we determine the subleading fugacity expansion coefficients of the grand canonical free energy, receiving contributions from processes like kaon-kaon or baryon-baryon scattering. We achieve this goal by performing a two dimensional scan on the imaginary baryon number chemical potential (μB)—strangeness chemical potential (μS) plane, where the fugacity expansion coefficients become Fourier coefficients. We carry out a continuum limit estimation of these coefficients by performing lattice simulations with temporal extents of
Nτ=8, 10, 12 using the 4stout-improved staggered action. We then use the truncated fugacity expansion to extrapolate ratios of baryon number and strangeness fluctuations and correlations to finite chemical potentials. Evaluating the fugacity expansion along the crossover line, we reproduce the trend seen in the experimental data on net-proton fluctuations by the STAR collaboration.
The quark model has proven successful in describing the basic building blocks of strongly interacting particles in the Standard Model, where hadronic states consist of quarks and gluons. At the same time, Lattice QCD predicts the possibility of glueball candidates in the mass range 1550-1750 MeV/$c^{2}$, which have never been observed.
The experimental search for the existence of mesons with no quark content is both interesting and challenging as the glueball is very likely to mix with surrounding quark-antiquark scalar meson states with the same quantum numbers.
The large statistics data sample collected by ALICE in pp collisions at the highest LHC center of mass energy provides an opportunity to measure high mass resonances, whose characteristics and internal structure are still unknown. Measurements help us understanding the nature of particles as well as their formation mechanism.
We report on the measurements of invariant mass distributions of K$^{0}_\mathrm{S}$K$^{0}_\mathrm{S}$ and K$^{+}$K$^{-}$ pairs in pp collisions at $\sqrt{s}$ = 13 TeV using the ALICE detector at central rapidity. We will discuss the structure of the invariant mass distributions and perspectives for the search of glueball states.
In ultra-central heavy-ion collisions, the effects of event-by-event fluctuations on anisotropic flow are relatively more pronounced due to less geometrical anisotropy of initial transverse profiles. The magnitudes of elliptic flow $v_2$ and triangular flow $v_3$ were reported to be almost the same value in ultra-central collisions [1]. Dynamical models based on relativistic viscous hydrodynamics describe anisotropic flow in non-central collisions well, however, failed to reproduce these $v_2$ and $v_3$ data in ultra-central collisions simultaneously [2,3]. Since the hydrodynamic description is supposed to be better in larger systems, the failure of the viscous hydrodynamic models in ultra-central collisions implies the existence of overlooked phenomena. This problem is known as "ultra-central puzzle" and has not been resolved yet.
In this talk, we investigate the effects of hydrodynamic fluctuations on anisotropic flow in ultra-central collisions. We employ an integrated dynamical model [4] with relativistic fluctuating hydrodynamics [5,6] to describe the dynamics of heavy-ion collisions at the LHC energy and compare the results among ideal, viscous, and fluctuating hydrodynamics. In this framework, hydrodynamic fluctuations are introduced through the fluctuation-dissipation relation [5]. Since the anisotropic flow is driven mainly by fluctuations in ultra-central collisions, hydrodynamic fluctuations are expected to play an important role in understanding anisotropic flow.
First, we employ smooth and azimuthally symmetric initial conditions at impact parameter $b=0$ fm from the optical Glauber model to investigate the effects of genuine hydrodynamic fluctuations on anisotropic flow coefficients. We show that $v_2$ and $v_3$ caused by hydrodynamic fluctuations alone are almost the same value which is, however, almost half of the experimental flow coefficients. Second, we introduce a weight function of impact parameters to simulate ultra-central collisions efficiently and compare the results from Monte-Carlo Glauber initial conditions with hydrodynamic fluctuations to experimental data. Even with hydrodynamic fluctuations, we cannot reproduce $v_2$ and $v_3$ quantitatively at the same time. Nevertheless, we find the $v_2/v_3$ ratio becomes closer to the experimental data due to hydrodynamic fluctuations.
[1] S. Chatrchyan et al., (CMS Collaboration), JHEP 02, 088 (2014).
[2] C. Shen, Z. Qiu, and U. Heinz, Phys. Rev. C 92, 014901 (2015).
[3] P. Carzon, S. Rao, M. Luzum, M. Sievert, and J. Noronha-Hostler, Phys. Rev. C 102, 054905 (2020).
[4] T. Hirano, P. Huovinen, K. Murase, and Y. Nara, Prog. Part. Nucl. Phys. 70, 108 (2013).
[5] K. Murase, Ph. D thesis, The University of Tokyo (2015).
[6] K. Murase and T. Hirano, Nucl. Phys. A 956, 276 (2016).
Very recently, a non-equilibrium effective field theory framework has been formulated for fluctuating hydrodynamics [1]. In this talk, we present examples of applying this novel formalism to study the properties of QCD-like systems. In the first example, we study the dependence of the conductivity/resistivity on the external magnetic field in a chiral medium (the constituent of which includes chiral fermions). While it is widely believed that chiral magnetic effect (CME) would lead to a negative magneto-resistivity, we find that CME together with hydrodynamic fluctuations gives rise to a positive magneto-resistance [2]. Second, in the view that non-Gaussian fluctuations of baryon density are important for the QCD critical point search, we derive evolution equations for the critical non-Gaussian fluctuations of a conserved density and obtain closed-form solutions based on field theory techniques [3]. Those results can be readily implemented for simulations in realistic situations of heavy-ion collisions. In addition, we find that nonlinear interactions among noise fields, which are missing in traditional stochastic hydrodynamics, could potentially contribute to the quartic (fourth-order) fluctuations in the scaling regime in off-equilibrium situations.
[1] Michael Crossley, Paolo Glorioso and Hong Liu, “Effective field theory of dissipative fluids,” JHEP 09 (2017) 095.
[2] Noriyuki Sogabe, Naoki Yamamoto and Yi Yin, “Positive magnetoresistance induced by hydrodynamic fluctuations in chiral media,” JHEP 09 (2021) 131.
[3] Noriyuki Sogabe and Yi Yin, “Off-equilibrium non-Gaussian fluctuations near the QCD critical point: an effective field theory perspective,” [arXiv:2111.14667 [nucl-th]].
We develop a general decomposition of an ensemble of initial density profiles in terms of an average state and an orthonormal basis of modes that represent the event-by-event fluctuations of the initial state. The basis is determined such that the probability distributions of the amplitudes of different modes are uncorrelated. Based on this decomposition, we quantify the different types and probabilities of event-by-event fluctuations in Glauber and Saturation models and investigate how the various modes affect different characteristics of the initial state. We perform simulations of the dynamical evolution with KøMPøST and MUSIC to investigate the impact of the various modes on final-state observables and their correlations. By comparing results for the mode-by-mode linear response of $v_n$ and $\langle p_t\rangle$ with event-by-event simulations, we further quantify the accuracy of mode-by-mode approaches.
Exploring the shape of the pair-source function for particles such as pions or kaons has been an important goal of heavy-ion physics, and substantial effort has been made in order to understand the underlying physics behind the experimental observations of non-Gaussian behavior. In experiments, since no direct measurement is possible, femtoscopic (momentum) correlations are utilized to gain information about the space-time geometry of the particle emitting source. Event generators such as the EPOS model, however, provide direct access to the freeze-out coordinates of particles, and thus the source function can be constructed and investigated. The EPOS model has already proven to be successful in describing many different experimental observations for systems characterized by baryon chemical potential close to zero, but so far the source shape has not been explored in detail. On this poster an event-by-event analysis will be presented, focusing on the two-particle source function measured in $\sqrt{s_{NN}}$ = 200 GeV Au+Au collisions generated by the EPOS model. The emergence of the non-Gaussian behavior at different phases of the model as well as a detailed centrality and average transverse momentum dependence of the extracted source parameters will be discussed.
The study of nuclear matter over a wide range of collision energy is provided by the RHIC Beam Energy Scan (BES). One focus of the program, namely to locate the critical point (CP) in the QCD phase diagram, is closely tied to the measurement of kurtosis in net-proton multiplicity distribution as a function of $\sqrt{s_{NN}}$.
Previous results from BES-I obtained with $3.1 \sigma$ significance motivated us to increase the statistics and to extend the collision energy down to $\sqrt{s_{NN}} = 3.0$ GeV in the BES-II. The event-by-event fluctuations in net-lambda multiplicity distribution for the first BES showed that the cumulant ratios have a similar energy and multiplicity dependence compared to those for protons, and the observed deviation from Poisson baseline can be attributed to baryon number and strangeness conservations. It is also known from the previous work that the derived freeze-out parameters show sensitivity to the quark content of the hadrons, implying a quark mass dependence in the process of hadronization. We present in this poster, the lambda fluctuation analysis in Au+Au collisions at the lowest center of mass energy ($\sqrt{s_{NN}}= 3.0$ GeV), where we continue the comparison with proton fluctuations and analyze the behaviour of both baryons, specifically in terms of their difference in quark content and applicable conservation laws.
One of methods to study the properties of hot and dense nuclear matter created in high-energy nuclear collisions is femtoscopic measurements. This method provides information about space-time characteristics of the particle emission region, which has a size and lifetime of the order of $10^{-15}$ m and $10^{-23}$ s, respectively. From non-identical particle correlations, one can obtain information about asymmetry in the emission process between those two kinds of particles. Such an emission asymmetry gives knowledge of which type of particles, on average, are emitted earlier and from which region of the source. Using different combinations of pion, kaon, and proton pairs, one can obtain complete knowledge on geometric and dynamic (times of emission) properties of the particle emitting source. Such investigation could provide information about differences between the emission of light mesons (pions), strange mesons (kaons), and baryons (protons).
In this poster, the STAR results on femtoscopic observables of various particle combinations of pions, kaons, and protons from Au+Au collisions at Beam Energy Scan program will be presented.
Understanding the phase diagram of QCD by measuring fluctuations of conserved charges in heavy-ion collision is one of the main goals of the beam energy scan program at RHIC. For a precise measurement of the cumulants it is necessary to grasp the role of charge conservation in heavy-ion collision measurements. Within this work, we calculate the role of hadronic interactions and momentum cuts on cumulants of conserved charges up to fourth order in a system in equilibrium within a hadronic transport approach (SMASH). In our model the net-baryon, net-charge and net-strangeness is perfectly conserved on an event-by-event basis and the cumulants are calculated as a function of subvolume sizes and compared to analytic expectations. This reflects the experimental situation in which e.g. the net-baryon number is conserved in a heavy-ion collision and the results depend on the rapidity window. We find a modification of the kurtosis due to charge annihilation processes in systems with simplified degrees of freedom. Furthermore the result of the full SMASH hadron gas for the net-baryon and net-proton number fluctuations is presented for systems with zero and finite values of baryochemical potential. Additionally the problem of mapping between the net-proton and net-baryon fluctuations is addressed and we find that due to dynamical correlations the cumulants of the net-baryon number cannot be recovered from the net-protons. Finally the influence of deuteron cluster formation on the net-proton and net-baryon fluctuations in simplified system is shown. This analysis is important to better understand the relation between measurements of fluctuations in heavy-ion collisions and theoretical calculation which are often performed in a grand canonical ensemble.
Higher-order cumulants of net-proton distributions are sensitive to the details of the phase structure of the QCD phase diagram. Lattice QCD and QCD-based model calculations indicate that the signs of sixth and eighth-order cumulants have different combinations in the hadronic phase, partonic phase, and near the transition temperature.
In this poster, we report the first measurements of seventh and eighth-order cumulants of net-proton distributions in the high statistics Au+Au collisions at $\sqrt{s_{NN}}$ = 27, 54.4, and 200 GeV. The measurements are done at mid-rapidity $|y|<0.5$ within 0.4 $< p_{T} < 2.0$ GeV/$c$ using the Time Projection Chamber and Time-of-Flight detector. The measurements in Au+Au collisions at 200 GeV will be compared to those from Zr+Zr and Ru+Ru collisions to understand the system size dependence. The signs of the measured sixth, seventh, and eighth order cumulants will be contrasted to those expected from the lattice QCD and QCD-based models. The ratios of the measured cumulants will also be compared with those obtained from the transport and thermal models to understand the role of baryon number conservation and the validity of models.
Investigation of the femtoscopic correlation functions in heavy ion collisions is an important tool to access the space-time structure of the hadron production of the sQGP. The description of the measured correlation functions is often assumed to be Gaussian or exponential, but a detailed analysis reveals that the statistically correct assumption is a generalized Gaussian, the symmetric alpha-stable Lévy distribution. One of the resulting source parameters, the Lévy stability parameter $\alpha$, describing the shape of the source, may be related to anomalous diffusion in the final state. In this poster we present measurements of two-particle, Bose-Einstein correlation functions in $\sqrt{s_{_{\mathrm{NN}}}} =$ 5.02 TeV PbPb collisions at CMS. We investigate the centrality and transverse mass dependence of the parameters of the correlation functions: the strength or the intercept parameter $\lambda$, the HBT scale parameter $R$ and the stability parameter $\alpha$.
In this talk, I will review the basics of 3+1d quasiparticle anisotropic hydrodynamics (aHydroQP) and highlight some phenomenological comparisons with experimental data at different energies. Then, I will present comparisons of the kaon femtoscopic HBT radii using aHydroQP at 200 GeV where our model shows very good agreement with the experimental data.
Next, I will show predictions of the kaon’s HBT radii and their ratios at 5.02 TeV.
We discuss the quantum fluctuations of energy in subsystems of hot relativistic gas for both scalar and spin half particles. For small subsystem sizes, we find a substantial increase of fluctuations compared to those known from standard thermodynamic considerations. However, if the size of the subsystem is sufficiently large, we reproduce the result for energy fluctuations in the canonical ensemble. Interestingly for spin half particles, the results for quantum fluctuation depend on the form of the energy-momentum tensor used in the calculations, which is a feature described as pseudo-gauge dependence. However, for sufficiently large subsystems the results obtained in different pseudo-gauges converge and agree with the canonical-ensemble formula known from statistical physics. Although different forms of the energy-momentum tensor of gas are a priori equivalent, our finding suggests that the concept of quantum fluctuations of energy in very small thermodynamic systems is pseudo-gauge dependent. On a practical side, our results can be used in the context of relativistic heavy-ion collisions to introduce limitations of the concepts such as classical energy density or fluid element. Also, the results of our calculations determine a scale of coarse-graining for which the choice of the pseudo-gauge becomes irrelevant. In a straightforward way, our formula for quantum fluctuation can be applied in other fields of physics, wherever one deals with the hot and relativistic matter.
Further using the formalism developed to obtain the fluctuation of energy we also estimated the quantum features of the baryon number fluctuations in subsystems of a hot and dense relativistic gas of fermions. Our results for the baryon number fluctuation also suggest that for small system size quantum mechanical effects can be significant. Such a system size dependence of quantum statistical fluctuation can be helpful to shed new light on the experimental data, particularly for a small system probed in the heavy-ion collision experiments.
Femtoscopic correlations are measured over a broad multiplicity range using data from the LHC Run II collected by the CMS experiment for small colliding systems. Studies are performed for correlations of charged hadrons produced in proton-proton (pp) collisions at $\sqrt{s} = $ 13 TeV and for correlations with all pair combinations of $\text{K}^{0}_{\text{S}}$, $\Lambda$ and $\overline{\Lambda}$ in proton-lead (pPb) collisions at $\sqrt{s_{_{\mathrm{NN}}}} =$ 8.16 TeV. Results from pp collisions are compared to data from lower energies and from the ATLAS experiment, as well as with theoretical expectations from the color glass condensate and hydrodynamical models. In addition, identified particles are employed to perform the first femtoscopic correlation measurements of $\text{K}^{0}_{\text{S}}\text{K}^{0}_{\text{S}}$ and of $\Lambda~\!\overline{\Lambda}$ and $\text{K}^{0}_{\text{S}}\Lambda\oplus\text{K}^{0}_{\text{S}}\overline{\Lambda}$ correlations in pPb colliding systems. In these cases, the shape of the correlation function varies considerably for the different particle pairs, unveiling the effect of the strong final state interaction in each case. Intriguingly, the invariant radii and the correlation intensity results for $\text{K}^{0}_{\text{S}}\text{K}^{0}_{\text{S}}$ in pPb collisions share similarities with those from charged hadrons in pp collisions. On the other hand, the scattering parameters obtained for baryon-baryon and baryon-antibaryon strong interactions in pPb collisions behave as found in previous results for larger colliding systems.
The study of correlation and fluctuation of event-by-event mean transverse momentum ($p_\mathrm{T}$) is a useful tool to understand the dynamics of the system produced in ultrarelativistic heavy-ion collisions. The measurement of higher-order fluctuations of mean-$p_\mathrm{T}$ can help in probing the hydrodynamic behavior of the system and is considered to be a direct way of observing initial-state fluctuations. It can also be sensitive to the early-time evolution of the produced quark-gluon plasma.
We present the first measurement of three- and four-particle $p_\mathrm{T}$ correlators and their intensive ratios, related to the skewness and kurtosis of event-by-event mean-$p_\mathrm{T}$ distribution, as a function of average charged-particle density in Pb--Pb collisions at $\sqrt{s_\mathrm{NN}}$ = 5.02 TeV and Xe--Xe collisions at $\sqrt{s_\mathrm{NN}}$ = 5.44 TeV using the data recorded by the ALICE detector. For the baseline study, the analysis is performed also in pp collisions at $\sqrt{s}$ = 5.02 TeV. The measurements are compared to corresponding results from the STAR experiment at lower collision energies and to different theoretical model predictions.
We investigate extensions of the Hadron Resonance Gas (HRG) Model beyond the ideal case by implementing both attractive and repulsive additions to the model [1]. When considering additional states exceeding those measured with high confidence by the Particle Data Group, attractive corrections to the overall pressure in the HRG model are imposed. On the other hand, we also apply excluded-volume corrections, which ensure there is no overlap of baryons by turning on repulsive (anti)baryon-(anti)baryon interactions. We see that these two extensions are complementary and focus on the agreement with first-principles lattice QCD results on fluctuations of conserved charges. We note that these results are interesting for heavy-ion-collision systems at both the LHC and RHIC. In particular, we find interesting ratios of susceptibilities that are sensitive to one correction and not the other. This allows us to constrain the excluded volume and particle spectrum effects separately. Additionally, we see that strangeness susceptibilities indicate a smaller excluded volume for hyperons than non-strange baryons.