Quark Matter 2018 is the XXVIIth 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.
Recent observations of long-range multi-particle azimuthal correlations in p-Pb and high multiplicity pp collisions provided new insights into collision dynamics and opened a possibility to study collective effects in these small systems.
In this talk, we present new measurements of $p_{\text{T}}$-differential elliptic flow coefficient $v_2$ for a variety of identified charged hadrons from pp and p-Pb collisions recorded by ALICE during the LHC Run 2 operation. The results for $v_2(p_{\text{T}})$ measured for $\pi^{\pm}$, $\text{K}^{\pm}$, $\text{K}^0_{\text{S}}$, p/$\bar{\text{p}}$, $\phi$ and $\Lambda/\bar{\Lambda}$ in p-Pb collisions at $\sqrt{s_{NN}}$ = 5.02 TeV are shown. Minimum-bias pp collisions are used to estimate and subtract the non-flow contribution in p-Pb measurements. Additionally, we present first measurements of $v_2$ of identified particles in pp collisions at $\sqrt{s}$ = 13 TeV.
The $p_{\text{T}}$ dependence, characteristic mass ordering and number of constituent quark scaling of $v_2$ allows us to test various theoretical models, constrain the initial conditions, and probe collective effects in small collision systems. Measurements of the $v_2$ of the $\phi$-meson, in particular, given it features a mass close to that of the proton, provide an opportunity to examine the particle production mechanism via quark recombination scenario.
Experimental evidence suggests that collectivity in asymmetric small systems ($p$, $d$, $^{3}$He+A) is directly related to the initial collision geometry. Therefore, a compelling question is whether the same argument can be extended to p+p, and even $e^+e^-$ collisions. We have modified A-Multiphase-Transport-Model (AMPT) to include the constituent quark structure of the proton. We find with very modest parton-parton cross sections a good description of the triangular and elliptic flow coefficients measured by ATLAS and CMS in p+p collisions, as a function of multiplicity and transverse momentum. We assess the non-flow separation techniques used by these experiments, by implementing their exact methods and comparing the result with the true flow relative to initial geometry in the model. The default AMPT model imposes formation times for partons which are much shorter than their respective de Broglie wavelength, at odds with a central assumption of semi-classical kinetic theory. This challenges the idea of modeling the QGP as well-defined quasi-particles undergoing Boltzmann evolution. We explore the impact on collectivity of enforcing a minimum de Broglie wavelength criterion, and find that it is significant for these smallest of systems. Lastly, we explore the minimal requirements for collectivity within this transport framework to determine whether the multiplicity and geometry of $e^+e^-$ collisions is sufficient to generate experimentally verifiable signals.
Proton-nucleus collisions play an important role as a control system for
interpreting hot nuclear matter effects in $A+A$ collisions. Yet, there is a large amount of data from both RHIC and the LHC that indicate that collective effects are also present in such small systems. Understanding the origin of these effects is still incomplete, since a variety of models with very different underlying physics have been shown to describe $p+A$ data.
We present a comprehensive set of measurements of soft-physics observables and
detailed comparisons with theoretical models. These measurements include
$v_2(p_T)$ and $v_3(p_T)$ for inclusive and identified particles at mid-rapidity, $v_2(p_T)$ for hadrons and heavy-flavor muons at forward rapidity, multi-particle correlations as a function of event multiplicity, the centrality dependence of $dN_{ch}/d\eta$, and identified particle spectra. The implications for the origin of collectivity in $p+Au$ collisions at RHIC will be discussed.
Measurements of long-range azimuthal correlations involving heavy-flavor quarks provide a powerful tool in unraveling the origin of collectivity observed in small collision systems. With data collected by the CMS experiment at the LHC in 2016, elliptic azimuthal anisotropy ($v_2$) of prompt $D^{0}$ meson at mid-rapidity in 8.16 TeV pPb collisions is presented from long-range two-particle correlations over a wide transverse momentum range. Results for light-flavor strange hadrons, including $K_{S}^{0}$, $\Lambda$, $\Xi^{-}$ and $\Omega^{-}$ are also presented in both pPb and PbPb collisions. Divided by the number of constituent quarks (NCQ), the NCQ scaling relation of $v_2$ among heavy and various light flavor species is tested. The result reveals key insights to heavy quark collectivity developed in high-multiplicity pPb systems.
The relevance of subnucleonic degrees of freedom and their fluctuations in the description of multiple experimental observations in small collision systems (flow harmonics, diffractive phenomena, hollowness effect...) has been recently established.
A representative example is the first measurement of symmetric cumulants, SC(n,m), performed by the CMS Collaboration in the three collision systems available at the LHC (p+p,p+Pb,Pb+Pb) [1]. In particular, SC(2,3), that provides a direct access to initial state fluctuations, shows a sign change with increasing centrality in p+p resembling the behavior of p+Pb and Pb+Pb interactions. This fact constitutes a powerful and stringent constraint on any realistic initial state model. We present a systematic study on the influence of spatial correlations between the proton constituents, in our case gluonic hot spots, their size and their number on SC(2,3) within a Monte Carlo Glauber framework [2]. When modeling the proton as composed by 3 gluonic hot spots, the most common assumption in the literature, we find that the inclusion of spatial correlations is indispensable to reproduce the negative sign of SC(2,3) in the highest centrality bins as dictated by data. Further, the subtle interplay between the different scales of the problem is discussed. To conclude, the possibility of feeding a 2+1D viscous hydrodynamic simulation with our entropy profiles and the theoretical uncertainties associated to this procedure are exposed.
[1] arXiv:1709.09189
[2] arXiv:1707.05592 (under review in PLB)
One of the key results of the LHC Run 1 was the observation of an enhanced production of strange particles in high multiplicity pp and p-Pb collisions at 7 and 5.02 TeV, respectively. A smooth increase of strange particles relative to the non-strange ones with event multiplicity has been observed in such systems. Results from Run 2 at the top LHC energy are extended exploiting a dedicated high multiplicity trigger. This offers the unique opportunity to study, in elementary collisions, the multiplicity range covered by semi-peripheral Pb-Pb collisions.
We present the latest results on multiplicity-dependent strangeness production at LHC energies with ALICE. The strangeness enhancement is investigated by measuring the evolution with multiplicity of single-strange and multi-strange baryon production relative to non-strange particles. We also present recent measurements of mesonic and baryonic resonances in small collision systems. We investigate the system size dependence in pp and p-Pb collisions to study how hadronic scattering processes affect measured resonance yields, as well as the interplay between canonical suppression and strangeness enhancement. The measurement of the $\phi(1020)$ meson as a function of multiplicity provides crucial constraints in this context. Energy and system-type invariance are discussed and an extensive comparison with statistical hadronization and QCD-inspired models is presented.
We study the critical properties of net-baryon-number fluctuations at the chiral restoration transition in a medium at finite temperature and net baryon density. The chiral dynamics of QCD is modeled by the Polykov-loop-extended Quark-Meson Lagrangian with the coupling of quarks to vector mesons. The Functional Renormalization Group is employed to properly account for the scaling properties of chiral observables at the phase boundary.
We focus on the properties of the net-baryon-number cumulants, $\chi_B^n$ for $n=1$ up to $n=8$, at and near the chiral phase boundary. We assess the influence of the location of the critical endpoint on the cumulant ratios and discuss the possibility to test non-equilibrium dynamics by comparing certain combination of baryon number susceptibilities.
The results are presented in the context of the recent experimental data of STAR and ALICE collaborations on fluctuation observables in heavy-ion collisions.
The fluctuations of conserved charges - such as electric charge, strangeness, or baryon number - in ultrarelativistic heavy-ion collisions provide insights into the properties of the hot and dense matter produced as well as the QCD phase diagram. They can be related to the moments of the multiplicity distributions of identified particles. We extend the previous and ongoing measurements of the cumulants of the net-pion, net-kaon, and net-proton distributions by investigating the correlated fluctuations of identified particles.
We present the first measurements with the ALICE detector of net-lambda fluctuations and of the off-diagonal cumulants between net-proton, net-pion and net-kaon distributions in Pb-Pb collisions. The results are obtained with the Identity Method, which, in particular, is applied in a novel way to account for the combinatoric background in the net-lambda analysis. The net-lambda fluctuations are compared with the corresponding net-proton and net-kaon results, previously measured by ALICE. Moreover, the off-diagonal cumulants are confronted with the lattice QCD predictions.
We present a lattice calculation on the cross-correlations of conserved charges
(baryon number, electric charge and strangeness) near the transition
temperature. We extrapolate to small baryo-chemical potentials, and thus we
cover typical STAR energies. We confront our finding to the latest STAR date
set on cross-correlations. In this work we use continuum lattice results with
resolution up to Nt=16.
We develop, within a canonical formulation of statistical mechanics, a systematic procedure to evaluate fluctuations of conserved quantities, such as baryon number, measured within an experimental acceptance. In nearly all experiments the baryon number fluctuations are approximated by the corresponding signals for net-proton measurements. We will discuss the validity and, in particular, the energy dependence of this approximation and provide quantitative estimates of differences between net-baryon number and net-proton fluctuations. Finally, we will compare our results up to the 4th cumulants with the corresponding measurements from the STAR and ALICE experiments.
We propose that rapidity dependent momentum correlations can be used to extract the shear relaxation time $\tau_\pi$ of the medium formed in high energy nuclear collisions. The stress-energy tensor in an equilibrium quark-gluon plasma is isotropic, but in nuclear collisions it is likely very far from this state. The relaxation time $\tau_\pi$ characterizes the rate of isotropization and is a transport coefficient as fundamental as the shear viscosity. We show that fluctuations emerging from the initial anisotropy survive to freeze-out, in excess of thermal fluctuations, influencing rapidity correlation patterns. We show that these correlations can be used to extract $\tau_\pi$. In [1] we describe a method for calculating the rapidity dependence of two-particle momentum correlations with a second order, causal, diffusion equation that includes Langevin noise as a source of thermal fluctuations. The causality requirement introduces the relaxation time and we link the shape of the rapidity correlation pattern to its presence. Here we examine how different equations of state and temperature dependent transport coefficients in the presence of realistic hydrodynamic flow influence the estimate of $\tau_\pi$. In comparison to RHIC data, we find that the ratio $\tau_\pi/\nu \approx 5-6$ where $\nu=\eta/sT$ is the kinematic viscosity. We further make predictions for Pb-Pb collisions at the LHC.
[1] S. Gavin, G. Moschelli, C. Zin, Phys. Rev. C 94, 024921 (2016).
In ultrarelativistic heavy-ion collisions, correlations of particles with opposite quantum numbers provide insight into charge creation mechanisms, the time scales of quark production, collective motion of the QGP, and re-scattering in the hadronic phase. The longitudinal and azimuthal widths of general charge balance functions for pions, kaons, and protons are used to examine the two-wave quark production model recently proposed to explain quark-antiquark production within the QGP, which predicts a large increase in up and down quark pairs relative to strange quark pairs around the time of hadronization. Furthermore, the magnitudes of the balance functions for different particle pairs provide quantitative differential information on pair production channels. In addition, a detailed study of balance functions for different identified hadrons probes the fragmentation mechanism of strings into different quark flavors. Balance functions are also analysed in small collision systems such as p-Pb and pp to study fragmentation effects and possible collective effects in high-multiplicity events.
We present a comprehensive set of measurements of general charge balance functions for pions, kaons, protons, and unidentified particle pairs in Pb-Pb ($\sqrt{s_{\rm{NN}}}=$ 2.76 and 5.02 TeV), p-Pb ($\sqrt{s_{\rm{NN}}}=$ 5.02 TeV) and pp ($\sqrt{s}=$ 5.02 and 7 TeV) collisions in ALICE. Theoretical expectations and Monte Carlo models are then compared with the experimental data. In Pb-Pb collisions, we observe that the $\Delta y$ and $\Delta\varphi$ widths of the charged-pion balance function are narrower in central collisions compared to peripheral ones, while the widths of the charged-kaon balance functions do not show a centrality dependence. These results are consistent with expectations based on the two-wave scenario and radial flow.
Direct photons are produced in various processes in pp, p-A and A-A collision and are sensitive to details of the space-time evolution the medium produced in heavy-ion collisions. The low $p_{\rm T}$ part of the direct photon spectrum is expected to be dominated by thermal direct photons - thermal radiation of hot matter, i.e., of the quark-gluon plasma and the hadron gas. At RHIC energies it is a challenge for hydrodynamical models to simultaneously describe the yield and the elliptic flow of direct photons in p-A and A-A collisions, which is often referred to as the direct-photon puzzle.
With ALICE, photons can be detected with either of the two electromagnetic calorimeters, EMCal and PHOS, and via reconstruction of $e^+e^-$ pairs from conversions in the ALICE detector material using the central tracking system. An additional hybrid method, combining the conversion information with that of the calorimeters has been developed. Where ever possible the results were combined to reflect our best estimate of the inclusive photon spectra and flow, $R_{\gamma}$, as well as the extraction of direct photon spectra or their upper limits and the flow.
In this talk, the first measurements on the direct photon production in pp collisions at $\sqrt{s} = 2.76$ and 8 TeV, as well as in p-Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV will be presented. Below 3 GeV/$c$, $R_\gamma$ was found to be consistent with unity in these collision systems. Furthermore, the direct photon spectra are in agreement with pQCD next-to-leading order calculations within the uncertainties. In addition, the final results on the direct photon elliptic flow ($v_{2}^{\gamma dir,SP}$) at $\sqrt{s_{\rm NN}} = 2.76$ TeV from the Pb-Pb run in 2010 in the $p_{\rm T}$ range of $0.9 < p_{\rm T} < 6.2$ GeV/$c$ will be presented in this talk. Comparisons to PHENIX results and to predictions of several theoretical models will be presented in order to shed light on the status of the direct photon puzzle at LHC energies.
In recent years data from small collision systems at LHC and RHIC have revealed
evidence for collective behavior of the produced hadrons. Collective behavior in
small systems clearly points towards a strongly coupled system being formed in
these collisions. If so, the matter formed must also radiate thermal or low
momentum direct photons. PHENIX is ideally positioned to search for any indications for thermal photon emission from small systems.
The versatility of RHIC allowed PHENIX to collect large data sets with a high
multiplicity trigger for p+p, p+A, d+Au and $^{3}$He$+$Au collisions at 200 GeV. These data sets are being analyzed with the methods already developed for the
measurement of low momentum direct photons from Au+Au collisions. Photons are measured through their conversions to electron-positron pairs in the material of the PHENIX vertex detector, and the fraction of direct photons is determined
after tagging photons from neutral pion decays. In this talk we will present
results from p+p and p+Au collisions.
In this talk, we present a systematic study of direct soft photon observables from the RHIC Beam Energy Scan (BES) to the LHC energies. We utilize the power of photons as clean and penetrating probes of the strongly-coupled nuclear matter created in relativistic heavy-ion collisions, together with the fact that the rapidly expanding Quark-Gluon Plasma (QGP) imprints its evolution on the photon spectrum and momentum anisotropies.
At higher energies, using the improved centrality selection in the recent hybrid approach [1], we first show that the tension in the direct photon elliptic flow between theory and experimental measurements is considerably reduced at both top RHIC and LHC energies. Then, predictions of the photon observables in Pb+Pb collisions at 5.02 TeV will be highlighted. At lower energies, a study of direct photon production in Au+Au collisions at 39 and 62.4 GeV will also be discussed. In particular, the role of finite baryon chemical potential in thermal photon emission will be quantified for the first time. The interplay between different collision energy scaling behavior for prompt and thermal photons in the final direct photon observables will be analyzed. This survey establishes photons as a powerful tool to elucidate the dynamics of QGP over a wide range of collision energies, to extract QCD transport coefficients, and to serve as a necessary complement to hadronic measurements.
[1] S. McDonald, C. Shen, F. Fillion-Gourdeau, S. Jeon and C. Gale, "Hydrodynamic predictions for Pb+Pb collisions at 5.02 TeV", Phys. Rev. C 95, no. 6, 064913 (2017)
The matter formed in central heavy-ion collisions at a few GeV per nucleon is commonly understood as resonance matter, a gas of nucleons and excited baryon states with a substantial contribution from mesonic, mostly pionic excitations. Yet, in the initial phase of the reaction the system is compressed to beyond nuclear ground state density and hence substantial modifications of the hadron properties are expected to occur.
The spectral distribution of virtual photons measured in Au+Au collisions at 2.42 GeV center of mass energy indicates strong medium effects beyond pure superposition of individual NN collisions. We present multi-differential distributions of low-mass electron pairs measured in Au+Au collisions at 2.42 GeV center of mass energy. The data is analyzed in terms of excess radiation above a conventional cocktail of contributions from meson decay after thermal freeze-out. This strong excess radiation is remarkably well described assuming emission from a thermalized system. To gain deeper understanding of the microscopic origin of the excess radiation, we extracted it centrality dependent true (not blue-shifted) temperature, its azimuthal distribution and polarization, as well as mass dependent effective slope parameter. Virtual photon spectra will be confronted with results of other experiments as well as with available model calculations.
We present an overview of recent results on in-medium spectral functions and transport coefficients of hadrons using the Functional Renormalization Group approach. Our method is based on a recently developed analytic continuation procedure that allows to calculate real-time quantities like spectral functions at finite temperature and chemical potential. Results for the quark, the sigma and the pion spectral function as well as for the shear viscosity over entropy density ratio are shown using the quark-meson model [1]. These quantities are studied in different regimes of the phase diagram, in particular near the chiral critical endpoint. Moreover, recent results for in-medium vector and axial-vector meson spectral functions are presented which are based on an extended linear-sigma model including quarks [2]. It is shown how the rho and the a1 spectral functions become degenerate at high temperatures and chemical potentials due to the restoration of chiral symmetry. Future applications of these recent developments are discussed, which include the calculation of dilepton spectra and the identification of experimental signatures of the chiral phase transition in the QCD phase diagram.
[1] R.-A. Tripolt, L. von Smekal, J. Wambach, Int. J. Mod. Phys. E26 (2017) 1740028
[2] C. Jung, F. Rennecke, R.-A. Tripolt, L. von Smekal, J. Wambach, Phys. Rev. D 95 (2017) 036020
The production of low-mass dielectrons is one of the most promising tools for the understanding of the chiral symmetry restoration and of the thermodynamical properties of the Quark-Gluon plasma (QGP) created in heavy-ion collisions. At low invariant mass, the dielectron production is sensitive to the properties of vector mesons in the medium and modifications related to the chiral symmetry restoration. In the intermediate mass region ($1.2 < m_{\rm ee} < 2.8$GeV/$c^2$) dielectrons are dominated by correlated electron pairs from heavy-flavour hadron decays, which carry information on the heavy-quark energy loss and collectivity. Thermal radiation from the medium contributes to the dielectron yield over a broad mass range and give insight into the temperature of the medium.
To single out the signal characteristics of the QGP, it is crucial to understand the primordial e$^{+}$e$^{-}$ pair production in vacuum, i.e. in minimum-bias proton-proton collisions, and to disentangle hot from cold-nuclear matter effects with p-Pb collisions. Moreover, observations of collective effects in high-multiplicity pp and p-Pb collisions show surprising similarities with those in heavy-ion collisions. The underlying physics processes in such events can be further studied with the measurements of correlated e$^{+}$e$^{-}$ pairs.
In this talk, we will give an overview of the latest measurements of e$^{+}$e$^{-}$ pair production in pp collisions at $\sqrt{s}$ = 7 TeV and 13 TeV, in p-Pb collisions at $\sqrt{s_{{\rm{NN}}}}$ = 5.02 TeV, and in Pb-Pb collisions at $\sqrt{s_{{\rm{NN}}}}$ = 2.76 TeV and 5.02 TeV with ALICE. Its implications for the production of heavy quarks and virtual photons will be presented, as well as the dependence of the dielectron spectra with the charged-particle multiplicity in the event, or the centrality of the collision. In Pb-Pb collisions, the comparison of the measured dielectron yield with the expectation from known hadronic sources will be discussed.
Exclusive photoproduction of Upsilon(nS) meson states off protons is measured in ultraperipheral pPb collisions at a center-of-mass energy per nucleon pair of 5.02 TeV. The measurement is carried out in the $\Upsilon (nS)\to\mu^+\mu^-$ decay modes, with data collected by the CMS experiment corresponding to an integrated luminosity of $32.6$~nb$^{-1}$. Differential cross sections, as a function of the $\Upsilon$(nS) transverse momentum squared $p_{T}^2$, and rapidity $y$, are presented. The $\Upsilon$(1S) photoproduction cross section is extracted as a function of the photon-proton center-of-mass energy over the $91 < W_{\gamma p}<826$~GeV range. The data are compared to theoretical perturbative quantum chromodynamics predictions and to previous measurements.
We present a novel formulation of the IP-Glasma initial state model in 3+1D, where the 2D boost invariant IP-Glasma is generalized to 3D through JIMWLK rapidity evolution of the pre-collision Wilson lines [1]. By breaking boost invariance, the 3D model no longer trivially satisfies Gauss' law at the initial time, and we now enforce it locally. We compare the time evolution of the chromo-electric and chromo-magnetic fields in the 3D case with the boost invariant result.
As the longitudinal dynamics of heavy ion collisions are measured to greater levels of precision, it is imperative that theoretical models describe the 3-dimensional nature of the Quark Gluon Plasma. We couple our 3D IP-Glasma model to MUSIC+UrQMD, for a fully 3-dimensional simulation of heavy ion collisions, and study the rapidity dependence of the second Fourier harmonic v2(η) and the charged hadron multiplicity dNch /dη.
[1]Bjoern Schenke and Soeren Schlichting. “3D glasma initial state for relativistic heavy ion collisions”. In: Phys. Rev. C94.4 (2016).
In the high-energy limit of heavy-ion collisions, the system right after a collision is described as an over-occupied gluon plasma expanding in the beam direction. Its space-time evolution can be studied by means of real-time lattice gauge theory simulation techniques with dynamical quarks. To find observable consequences of such nonequilibrium evolution, the understanding of quark dynamics is crucial since they couple to electromagnetic probes. We present results for nonequilibrium quark production in the longitudinally expanding QCD plasma. We find that the quark number density per unit transverse area and rapidity shows almost linear growth in time, and its growth rate is consistent with a simple kinetic theory estimate involving only two-to-two scattering processes in small-angle approximation. We also show that the quark transverse momentum spectra for a wide range of quark masses exhibit an exponential shape that resembles a thermal Boltzmann distribution.
Reference: N. Tanji and J. Berges, arXiv:1711.03445
At the LHC, in ultra-peripheral heavy-ion collisions the highly boosted electromagnetic field of the Pb ions represents a source of quasi-real photon. Vector meson photo-production measurements in p-Pb (Pb-Pb) collisions are sensitive to the gluon parton distribution functions in the proton
(nucleus). LHCb results on charmonium production in ultra-peripheral p-Pb and Pb-Pb collisions will be presented.
Measurements of dijet production and photo-nuclear interactions in heavy-ion collisions probe several nuclear mechanisms. In particular, dijet measurements in pPb collisions have been shown to be one of the most important tools for constraining the gluon nuclear parton distribution functions (PDFs) at large Bjorken-x. Dijet production in pp and pPb collisions at a nucleon-nucleon centre-of- mass energy of 5.02 TeV is reported with the data samples collected with the Compact Muon Solenoid detector at the Large Hadron Collider. The dijet pseudorapidity distributions are measured as a function of dijet average transverse momentum in order to study the nuclear modifications of PDFs at various factorization scales. The final results from pp and pPb data samples are compared with next-to-leading- order perturbative QCD predictions obtained from both nucleon and nuclear PDFs. A significant modification of dijet pseudorapidity distributions in pPb collisions with respect to the measured pp reference is observed which indicates that the gluon PDF in lead ions is modified and the results are incompatible with predictions with DSSZ PDF without gluon EMC effects. Photo-nuclear jets are also measured in pp and pPb collision systems. The yield and angular correlation of low-pT jets at forward rapidity, $5.0<|\eta|<6.5$, are studied using the CASTOR calorimeter, which is sensitive to PDFs at low values of $x$ and $Q^2$. The prospects of future measurements of forward and ultra-peripheral jets in various collision systems as well as dijet production in pPb at 8.16 TeV and in Run III will be discussed.
The electromagnetic field of relativistic heavy ions can be described by a flux of virtual photons. In ultraperipheral collisions (UPC), where the impact parameter is larger than the sum of nuclei radii, the interaction of these photons with the nucleus can provide insight into its structure and allow us to probe nuclear shadowing via photoproduction of charmonia.
Extensive efforts on this subject have been made by the ALICE collaboration and led to published measurements of J/Psi and Psi(2S) photoproduction in LHC Run 1 at forward (J/Psi) and at mid-rapidity. In addition, ALICE has reported a large excess of J/Psi at very low transverse momentum in peripheral Pb-Pb collisions, which is suggestive of coherent J/Psi photoproduction in collisions with nuclear overlap.
A substantially larger data set was recorded in LHC Run 2, allowing differential measurements in rapidity and in transverse momentum. In particular, the increased energy in Run 2 means that the Bjorken-x value probed in Pb-Pb UPC at midrapidity decreases from 10^-3 to 5*10^-4, and in collisions with nuclear overlap the measurement of the coherent J/Psi component becomes sensitive to its polarization and to the shape of its transverse momentum spectrum. At forward rapidity, the Run-2 dataset extends the measurement of photoproduced J/Psi beyond 50% in centrality, imposing strong constraints on initial-state models.
In this talk we will present the latest results on charmonium production in UPCs, final results on the J/Psi coherent photoproduction cross-section in Pb-Pb collisions at $\sqrt{s_{NN}}$ = 5.02 TeV with nuclear overlap at mid rapidity and new results at forward rapidity in the same system. The discussion will include comparisons to the ultraperipheral measurements and to theoretical model calculations. Perspectives for these measurements in LHC Run-3 and Run-4 will also be shown.
Quarkonia, i.e. bound states of $b\bar{b}$ and $c\bar{c}$ quarks, are powerful observables to study the properties of nuclear matter under extreme conditions. The formation of a Quark-Gluon Plasma (QGP), which is predicted by lattice calculations at high temperatures as reached at LHC energies, has a strong influence on the production and behavior of quarkonia. A suppression, due to the color screening effect, with respect to the proton-proton results scaled by the number of binary collisions is expected. However, charmonium measurements from Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ = 2.76 and 5.02 TeV revealed a smaller suppression than what was observed at lower energies at the SPS and RHIC. Concurrently, the produced J/$\psi$ present a significant elliptic flow ($v_{2}$) in semi-central collisions. These measurements point to a competition between charmonium suppression and (re)generation at LHC energies, with a participation of the charm quarks to the collectivity of the medium. Thus quarkonium measurements offer great possibilities to gain further knowledge about the QGP.
In this presentation, latest ALICE results on the bottomonium and charmonium production in nucleus-nucleus collisions will be presented. This includes measurements of $\Upsilon$(1S) and $\Upsilon$(2S) nuclear modification factors ($R_{\rm AA}$) at forward rapidity and the charmonium $R_{\rm AA}$ and $v_{2}$ as a function of centrality, $p_{\rm T}$ and rapidity in Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV. Also, first results from $J/\psi$ measurements in Xe-Xe collisions at $\sqrt{s_{\rm NN}}$ = 5.44 TeV will be presented. Further on, the experimental results will be compared to various calculations from theoretical models.
The experimentally observed dissociation and regeneration of bound quarkonium states in heavy-ion collisions provide a powerful tool to probe the dynamics of the hot, dense plasma. These measurements are sensitive to the effects of color screening, color recombination, or other, new suppression mechanisms. In the large-statistics Run 2 lead-lead and proton-lead collision data, these phenomena can be probed with unprecedented precision. Measurements of the ground and excited quarkonia states, as well as their separation into prompt and non-prompt components, provide further opportunities to study the dynamics of heavy parton energy loss in these large systems. In addition, quarkonium production rates, and their excited to ground states ratios, in small, asymmetric systems are an interesting probe of cold nuclear matter effects. In this talk, the latest ATLAS results on quarkonia production will be presented, including new, differential measurements of charmonium suppression and azimuthal modulation in lead-lead collisions, and a broad measurement of the production of five quarkonium states, differential in quarkonium kinematics, in proton-lead collisions.
Heavy quarkonium related observables are very useful to obtain information about the medium created in relativistic heavy ion collisions. In recent years the theoretical description of quarkonium in a medium has moved towards a more dynamical picture in which decay and recombination processes are very important. In this talk we will discuss the equations that describe the evolution of the heavy quarks reduced density matrix in different approximations, highlighting the color dynamics that is absent in the Abelian case, and we will study their semi-classical limit. This will allow us to obtain stochastic equations (similar to Langevin or Boltzmann equations) that can be useful to obtain phenomenological predictions. We will observe that the region of validity of the Langevin-like or Boltzmann-like equations in QCD is much smaller than in the corresponding QED case. The reason for this can be understood by studying how differently the free energy evolves in these two theories. This observation will allow us to propose an equation with a small computational cost that captures many of the essential features of quarkonium evolution in a QCD plasma.
These results are based on [1] and on work in preparation.
[1]-Quantum and Classical Dynamics of Heavy Quarks in a Quark-Gluon Plasma. ArXiv:1711.10812. J-P. Blaizot and M. A. Escobedo.
In this study, we investigate the real-time evolution of quarkonium bound states
in a quark-gluon plasma in an improved QCD based stochastic potential model. This
model describes the quarkonium dynamics in terms of a Schrödinger equation with
an in-medium potential and two noise terms encoding the residual interaction
between the heavy quarks and the medium. The time evolution described by this
equation is unitary, since the effective potential term is real-valued. At a glance this is at odds with lattice results, but we explain why this it is
actually not the case.
We discuss the the time evolution of the existence probabilities of bound states in a static medium and in a boost-invariantly expanding quark-gluon plasma. We draw two conclusions from our results: One is that the outcome of the stochastic potential model is qualitatively consistent with the experimental data in relativistic heavy-ion collisions. The other is that the noise plays an important role in order to describe quarkonium dynamics in medium, in particular it causes decoherence of the quarkonium wave function. The effectiveness of decoherence is controlled by a new length scale, correlation length of the noise. Its effect has not been included in existing phenomenological studies, and we discuss its importance in detail.
Furthermore, if time allows, we also discuss strategies to take account of dissipation effects in addition to diffusion effects caused by the residual interactions between the heavy quarks and the medium.
[1] S. Kajimoto, Y. Akamatsu, M. Asakawa, and A. Rothkopf, arXiv:1705.03365, Phys. Rev. D in press.
[2] S. Kajimoto, Y. Akamatsu, M. Asakawa, and A. Rothkopf, in preparation.
Understanding experimental results on nuclear modification factors of heavy quarkonia as well as open heavy mesons is complicated due to the interplay between the cold and hot medium effects. In order to disentangle these two effects it is crucial to have a good understanding of thermal behavior of quarkonia and heavy quarks in the hot medium. The quarkonium spectral function is of the most interest, as the deformation of its resonance peak structure relates to the dissociation temperatures of quarkonia and its slope at the vanishing frequency in the vector channel is connected to the heavy quark diffusion coefficient.
We will present our recent results on quarkonia spectral functions obtained from quenched lattice QCD simulations at $T\in [0.35, 2.25]T_c$. The simulations have been performed on very large and fine lattices where both charm and bottom quarks can be treated relativistically. Using multiple random sources we have computed charmonia and bottomonia correlators being 2 times more precise compared to our previous study[1,2]. In order to gain more robust information on the quark mass dependences of the thermal modifications we also computed the hadron correlators with additionally 4 different values of heavy quark masses ranging in between those of charm and bottom quarks.
We show reconstructed spectral functions from another two stochastic methods [3] besides the Maximum Entropy Method. This allows us to study systematic uncertainties of the dissociation of quarkonium states from temperature and quark mass dependence of the spectral functions. We also estimate heavy quark diffusion coefficients using the low-frequency behavior of vector spectral functions.
[1]H. Ohno, Quark Matter 2017
[2]H. Ohno, PoS LATTICE 2015 (2016) 175
[3]H.-T. Ding, O. Kaczmarek, S. Mukherjee, H. Ohno, H.-T. Shu, in preparation
Quarkonia breaking in nucleus-nucleus collisions is a powerful tool to probe
density and temperature of the medium created in heavy ion collisions. Forward rapidity measurements in $p(d)$+Au collisions are essential to understand how quarkonia states are affected by initial state effects, formation time, and local particle multiplicity. Earlier measurements in Au+Au collisions showed a stronger suppression of forward J/$\psi$s compared to mid-rapidity results, indicating the
possibility of a smaller contribution of regenerated quarkonia states at forward
rapidity.
This presentation will report on the latest quarkonia studies performed by the
PHENIX collaboration in the rapidity range 1.2$<|y|<$2.2, including (i) the
nucleus size dependence of the J/$\psi$ nuclear modification factor in $p$+Au, $p$+Al and $^3$He+Au collisions; (ii) the status of recent analyses of $\Upsilon$ states in $p$+$p$, $p$+Au, and in the large statistics 2014 Au+Au data set, the largest data set obtained by PHENIX.
The matter created in non-central heavy-ion collisions is expected to possess a significant fraction of the initial angular momentum carried by the two colliding nuclei. This angular momentum can lead to vorticity of the system and be partially transferred to the spin of produced particles due to the spin-orbit coupling, leading to the phenomenon of global polarization. The STAR Collaboration observed finite signals in Au+Au collisions at $\sqrt{s_{NN}}$ = 7.7-39 GeV, indicating non-zero vorticity of the system, where the polarization decreases with increasing energy. The energy dependence can be understood by a shear flow structure in the initial state and/or the initial tilt of the source in combination with baryon transparency. Such a tilt and vorticity are closely related to the directed flow, elliptic flow, and the chiral anomalous phenomena. According to model calculations, the vorticity is expected to have a strong dependence on the hyperon emission angle relative to the reaction plane, but the models differ in their predictions. More precise and detailed experimental results are needed for further understanding of vorticity in heavy-ion collisions.
We present new results on the global polarization of Lambda hyperons in Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV. Recent high-statistics data by a factor of hundred larger compared to previous results allow one to study differential dependencies of the polarization on the collision centrality, hyperon's transverse momentum, pseudorapidity, and emission angle relative to the reaction plane. The polarization dependence on the event-by-event charge asymmetry is presented and its possible relation to the axial current induced by the initial magnetic field is discussed. Furthermore, a possible local vortical structure along the beam direction caused by azimuthal anisotropic flow will be discussed.
Large magnetic field and large angular momentum are expected to be present in the initial stages of high-energy heavy-ion collisions. One of the physics interests of the heavy-ion program using the ALICE detector at the LHC is to look for signatures of these effects. This can be achieved by studying the angular distributions of the decay daughters of hyperons and vector mesons.
We present new measurements related to spin alignment of K$^{*0}$ vector mesons at mid-rapidity for Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 2.76 and 5.02 TeV. The zeroth element of the spin density matrix element, $\rho_{00}$, is found to have values slightly below 1/3 at low transverse momentum ($p_{\mathrm{T}}$) for K$^{*0}$ mesons, while it is consistent with 1/3 (no spin alignment) at higher $p_{\mathrm{T}}$. No spin alignment is observed for K$^{*0}$ in pp collisions at $\sqrt{s}$ = 13 TeV and for the spin zero hadron K$^{0}_{S}$ in 20-40$\%$ Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 2.76 TeV.
The $\rho_{00}$ values are not only sensitive to the angular momentum of the system but also to
the production mechanism of the vector meson. The centrality dependence of the $\rho_{00}$ results
with production plane and event plane in Pb--Pb collisions at LHC energies will be discussed in detail.
In hydrodynamic picture of heavy ion collisions, local thermal vorticity of the QGP fluid leads to polarization of nonzero spin hadrons produced out of it [1]. The nonzero polarization of $\Lambda$ hyperons has been recently discovered by STAR in non-central Au-Au collisions in RHIC Beam Energy Scan program [2].
We further study the predictions of the hydrodynamic model for different components of the $\Lambda$ polarization in the framework of 3-dimensional viscous hydrodynamic model with UrQMD initial state, and demonstrate that:
at RHIC Beam Energy Scan energies, the global polarization $P$ of produced Lambda is directed along the total angular momentum of the fireball (perpendicular to reaction plane). $P$ decreases with collision energy from 1.7 to 0.2% [3], in agreement with STAR results [2]. The global $\Lambda$ polarization further decreases towards LHC energies,
at full RHIC and LHC energies, the dominant component of polarization is the one in the beam direction. This component has a quadrupole structure in transverse momentum plane. Its amplitude $f_2$ shows a mild decrease with collision energy, and it is thus detectable at LHC enegries.
We show that different components of polarization are driven by different properties of the hydrodynamic expansion: whereas the global polarization $P$ is a result of initial shear flow in the reaction plane, $f_2$ is driven by anisotropy of the transverse expansion (which is also responsible for elliptic flow $v_2$). Polarization component in the beam direction is a generic effect present even in a simple 2-dimensional hydrodynamic calculation with longitudinal boost invariance [4].
[1] F. Becattini, V. Chandra, L. Del Zanna, E. Grossi, Ann. Phys. 338 (2013) 32.
[2] STAR collaboration, Nature 548 (2017), 62–65
[3] I. Karpenko, F. Becattini, Eur. Phys. J. C (2017) 77: 213.
[4] F. Becattini, Iu. Karpenko, arXiv:1707.07984, accepted to Phys.Rev.Lett.
Relativistic thermodynamics with spin provided the polarization 4-vector to characterize the spin alignment in rotating systems. Based on a Yang-Mills flux-tube initial state and a high-resolution, (3+1)D particle-in-cell relativistic (PICR) hydrodynamics simulation, we numerically obtain the polarization vector for $\Lambda$ hyperons at NICA and FAIR energies, and find that the $y$ component of the polarization vector is dominant, while $x$ and $z$ components are anti-symmetric in the transverse momentum space, implying a vanishing contribution to the global polarization (at collider frame). Besides, we analyze the dependence of $\Lambda$ polarization effect on centrality, energy and freeze-out time, in our model. The linear dependence of $\Lambda$ polarization on impact parameter reveals that the polarization stems from the initial orbital angular momentum; the polarization effect is found to decrease with increasing energy, which is in line with the recent results from RHIC BES program, and is attributed to the more intensive thermal motion of particles at higher energies. The time evolution of the $\Lambda$ polarization in our calculation agrees with the time evolution of vorticity predicted previously, and indicates the limit of applicability of hydrodynamic model at late stages of the expansion.
[1] Y.L. Xie, D.J. Wang, and L. P. Csernai, Phys. Rev. C 95, 031901(R) (2017).
[2] Y. L. Xie, M. Bleicher, H. Stöcker, D. J. Wang, and L. P. Csernai, Phys. Rev. C 94, 054907 (2016).
A new framework for relativistic hydrodynamics with spin is proposed. It is based on the conservation laws for charge, energy, momentum, and angular momentum. The conservation laws lead to hydrodynamic equations for the charge density, local temperature, and fluid velocity, as well as for the spin polarization tensor. The resulting set of differential equations extends the standard picture of perfect-fluid hydrodynamics, with a conserved entropy current, in a minimal way.
In addition, the properties of the relativistic spin density matrices for spin-1/2 particles, which have been used recently in works on the polarization of Lambda hyperons, are discussed. Their relations to the Pauli-Lubański four-vector and different forms of the spin tensor are elucidated.
The proposed framework forms a basis for hydrodynamic interpretation of polarization measurements in heavy-ion collisions.
Based on the recent work by WF, B. Friman, A. Jaiswal, and E. Speranza, "Relativistic fluid dynamics with spin", arXiv:1705.00587.
Anisotropic flow is a key observable to characterize the system created in heavy-ion collisions, as it is sensitive to its initial state, transport properties, the equation of state and freeze-out conditions. In this presentation, we present the anisotropic flow coefficients of inclusive charged particles in Pb-Pb collisions at $\sqrt{s_{NN}}$ = 2.76 and 5.02 TeV, and Xe-Xe collisions at $\sqrt{s_{NN}}$ = 5.44 TeV. The results are reported for a wide range of particle transverse momenta within the pseudo-rapidity range $|\eta| < 0.8$ at different collision centralities. The energy and system dependence are found to place strong constraints on the temperature dependence of $\eta/s$ and the modeling of the initial state respectively. We also present detailed studies of flow fluctuations in heavy-ion collisions, in order to precisely characterize the underlying flow probability distribution function. We find evidence of non-Bessel-Gaussian fluctuations and we discuss the origin of this observation.
Valuable information about the behavior of a heavy-ion collision system can be obtained by changing the species of colliding nuclei, and in particular using species of different size. This change in system size can probe characteristic behavior in a way that is not possible with a single collision system. Already, results of small collisions systems such as p-p, p-A, d-A, and $^3$He have received much attention. Recently, the LHC performed $^{129}$Xe-$^{129}$Xe collisions, a system with a size that is intermediate between small systems like p-p and large systems like Pb-Pb, at almost the same collision energy.
We perform hydrodynamic simulations of Xe-Xe and Pb-Pb collisions and argue that hydrodynamic behavior dictates definite relations between the results, regardless of the details of the simulations, due to scaling laws inherent to fluid dynamics. This can be used to test the hydrodynamic framework in general and search for a breakdown of hydrodynamics with decreasing system size.
Conversely, one can extract detailed information about system properties by studying carefully-selected observables. For example, the relative elliptic flow of the two systems in very central collisions is sensitive to the expected prolate deformation of the Xenon nucleus. Additionally, we describe a procedure to determine the average viscosity in a model-independent way by comparing selected ratios of flow coefficients in the two systems.
References: arXiv:1711.08499 and work in progress
The elliptic and higher-order azimuthal anisotropy Fourier harmonics ($v_n$) are obtained for pPb collisions at $\sqrt{s_{NN}} = 8.16$ TeV over a wide range of event multiplicities based on multiparticle correlations. The data were collected by the CMS experiment during the 2016 LHC run. A sample of peripheral PbPb collisions at $\sqrt{s_{NN}} = 5.02$ TeV covering a similar range of event multiplicities to the pPb results is also analyzed for comparison. The ratios of different harmonic moments are obtained for both $v_2$ and $v_3$ with high precision, which allows a direct comparison to theoretical predictions assuming a hydrodynamic evolution of the created medium with initial-state density fluctuations, particularly probing the non-Gaussian nature of initial-state fluctuations in small collision systems. The presented results provide crucial insights into the origin of collective long-range correlations observed in small collision systems.
Precise measurements of higher-order coefficients of anisotropic flow ($v_n$, $n>3$) are now available in Pb+Pb data collected at the Large Hadron Collider. Higher-order coefficients are interesting because they do not simply originate as a response of the medium to its initial geometry, but rather from an intricate nonlinear coupling of harmonics of lower order. Hence, they serve as a powerful tool for the investigation of those properties of the quark-gluon plasma which are independent of the (so far very uncertain) initial conditions of the hydrodynamic evolution, such as its viscosity and freeze-out conditions.
I review the state-of-the-art framework describing higher-order harmonics, and I show that it is incomplete: It does not allow for a consistent characterization of flow coefficients defined with more than one nonlinear contribution, such as hexagonal flow, $v_6$. More specifically, I show that the present determinations of the nonlinear response coefficients of hexagonal flow, reported in both theoretical and experimental analyses, have been carried out under hidden underlying assumptions, which spoil the physical interpretation of the final results.
With the aim of curing these issues, I present an improved framework which encompasses the existing one, and allows for an exact characterization of any flow coefficient defined with an arbitrary number of nonlinear contributions. I perform an explicit application of the new framework to recent experimental data: I derive exact formulas for the nonlinear response coefficients of $v_6$, which I then extract from ALICE data. Doing so, I obtain the first experimental determination of the nonlinear coefficient coupling $v_6$ to $v_2$ and $v_4$. This quantity turns out to present a very specific centrality dependence, that is not captured by existing viscous hydrodynamic calculations. Promising applications of the presented formalism to upcoming high statistics Run2 data will be emphasized.
The recently proposed symmetric cumulants and non-linear flow mode coefficients provide new observational probes to study initial conditions and microscopic transport properties of the quark-qluon plasma (QGP) formed in heavy-ion collisions. Comparison of such measurements with viscous hydrodynamic calculations should, in particular, enable the study of the temperature dependence of the shear viscosity to entropy density ratio ($\eta/s$).
We present the measurements of symmetric cumulants and non-linear flow modes of charged hadrons up to the 8th harmonic in Pb-Pb collisions at $\sqrt{s_{\rm NN}}$=5.02 TeV. The results will be compared to those at lower energies and to calculations from hydrodynamic models. Together they provide better constraints on the initial conditions, $\eta/s(T)$ and freeze-out conditions. In addition, we present the first results of $p_{\rm{T}}$-differential non-linear flow modes for charged pions, kaons and (anti-)protons measured in Pb-Pb collisions at $\sqrt{s_{\rm NN}}$=5.02 TeV. These results cover a wide centrality range from ultra-central up to very peripheral collisions. These new results allow us to identify the contribution of the linear and non-linear terms to the observed mass ordering and particle-type grouping in different flow harmonics, thus providing increased discriminatory power in the study of initial conditions as well as a new stringent constraint to hydrodynamical calculations.
Since its startup in 2009, the Large Hadron Collider at CERN has spent about 3 months of its operating time providing nucleus-nucleus (Pb-Pb) collisions. Peak Pb-Pb luminosity is now over 3 times design and integrated luminosity is expected to attain the initial design goal of 1 nb-1 in the 4th Pb-Pb run in late 2018. Following the demonstration of their feasibility in 2012, two one-month runs have been devoted to proton-nucleus (p-Pb) collisions in multiple conditions, with luminosity far beyond expectations. Recently, Xe-Xe collisions have also been demonstrated in a short run. All the LHC experiments now participate fully in the heavy-ion programme.
With this experience in hand, strategies to overcome physical performance limits established, and upgrades to the LHC and its injector chain in the pipeline, it is timely to take stock of the prospects and challenges for future performance of the LHC with nuclear beams.
The ALICE Collaboration will undertake a major upgrade of the detector apparatus during the second LHC long shutdown (LS2, 2019-20) in view of the Runs 3 and 4 (2021 to 2029). The objective of the upgrade is two-fold: i) an improvement of the tracking precision and efficiency, in particular in the low-momentum range; ii) an improvement of the readout capabilities of the experiment, in order to fully exploit the luminosity for heavy ions envisaged after LS2.
The first goal will be achieved by replacing the Inner Tracking System (ITS) with a new tracker, composed of seven layers of silicon pixel detectors. The ITS will be made up of about 25000 Monolithic Active Pixel Sensors with fast readout, resulting in a material thickness reduced to 0.3% (inner layers) – 1% (outer layers) of the radiation length and a granularity of $28\times28$ $\mu m^2$. The resolution of the track position will improve by about a factor of three in the direction transverse to beams and by a factor of five along the beams, reaching for example 20 $\mu m$ in both directions at a transverse momentum of 1 GeV/c.
The second goal will be achieved, among other measures, by replacing the readout chambers of the 90 $m^3$ Time Projection Chamber (TPC) with Micro Pattern Gas Detectors. In particular, the new readout chambers will consist of stacks of 4 Gas Electron Multiplier (GEM) foils combining different hole pitches. The upgraded detector will operate continuously without the use of a triggered gating grid. It will thus be able to record all Pb-Pb collisions at the anticipated LHC interaction rate of 50 kHz. New readout electronics will send the continuous data stream to a new online farm at a rate of 3 TByte/s.
The presentation will review the results of the extensive R&D programs, which are now concluded, the final technology and design choices, and the status of the production of the two detectors. Highlights of the physics programme with the upgraded ALICE central barrel will also be presented.
ALICE is the experiment specifically designed for the study of the
Quark-Gluon Plasma in heavy-ion collisions at the CERN LHC. The ALICE detector
will be upgraded during the LHC Long Shutdown 2, planned for 2019-2020,
in order to cope with the maximum interaction rate of 50 kHz of Pb-Pb foreseen
for Runs 3 and 4.
The ambitious programme of high-precision measurements, expected for the
muon physics after 2020, requires an upgrade of the front-end and readout electronics
of the existing Muon Spectrometer. This concerns the Cathode Pad Chambers
(CPC) used for the tracking and the Resistive Plate Chambers (RPC) used
for triggering purposes and muon identification. The RPC will be operated with
amplification, contrary to what is currently done, with a new FEERIC front-end
chip. Regarding the CPC, a new all-in-one SAMPA chip will be used to equip the
1.1 million readout channels. For both systems the data transmission will use the
GBT chip, developed at CERN, and a Common Readout Unit (CRU) which will
send the data to the acquisition.
The Muon Forward Tracker (MFT), an internal tracker added in front of the
front absorber of the existing Muon Spectrometer, is also part of the ALICE detector
upgrade programme. It is based on an assembly of circular planes made
of Monolithic Active Pixel Sensors (MAPS), covering the pseudorapidity range
2.5 < $\eta$ < 3.6. The MFT will improve present measurements and enable new ones.
In particular, the precise measurement of the offset to the primary vertex for the
muon tracks will permit, for the first time in ALICE, the statistical separation of
open charm and beauty production at forward rapidity, rejecting at the same time
a large fraction of background muons coming from pion and kaon decays.
A selection of results from the physics performance studies will be presented,
together with an overview of the technical aspects of the upgrade project.
The second phase of the Beam Energy Scan at RHIC, BES-II, is scheduled for 2019-2020 and will explore with precision measurements the high baryon density region of the QCD phase diagram. Some of the key measurements at center-of-mass energies at 19.6 GeV to 7.7 GeV in collider mode and 7.7 GeV to 3.0 GeV are: the kurtosis of net-protons that could pinpoint the position of a critical point, the directed flow of baryons vs. energy that might prove a softening of the equation of state, and the chiral restoration in the di-lepton channel. The measurements will be enhanced by the detector upgrades to extend STAR's experimental reach. The upgrades currently under way comprise: the replacement of the inner TPC sectors that increases the rapidity coverage of identified particles, the Event Plane Detector that improves the triggering and event plane resolution, and the end-cap TOF that extends the PID capabilities to larger rapidities in one hemisphere of STAR. Building on these upgrades STAR is planning to further enhance its detector capabilities by installing a Forward Calorimeter System integrating an electromagnetic and hadronic calorimeter and a Forward Tracking System combining 3 Silicon mini-strip disks and 4 Small-Strip Thin Gap Chamber (sTGC) wheels ala ATLAS. The upgrade is motivated by studying the initial state of nucleons and nuclei and the exploration of cold QCD physics in the very high and low regions of Bjorken $x$. The talk will highlight the physics opportunities enabled by these upgrades.
The sPHENIX experiment at RHIC will collect high statistics proton-proton, proton-nucleus and nucleus-nucleus data, starting in the early 2020's. The sPHENIX capabilities enable state-of-the-art studies of jet modification, upsilon suppression and open heavy flavor production to probe the microscopic nature of the strongly-coupled Quark Gluon Plasma, and will allow a broad range of cold QCD studies.
The sPHENIX detector will provide precision vertexing, tracking and electromagnetic and hadronic calorimetry in the central pseudorapidity region $|\eta| < 1.1$, with full azimuth coverage, at the full RHIC collision rate, delivering unprecedented data sets for hard probe tomography measurements at RHIC.
In this talk we will present a brief overview of the sPHENIX detector design with emphasis on calorimetry. The novel design of the sPHENIX calorimeters includes a tungsten/scintillating fiber electromagnetic calorimeter and two steel/scintillating tile hadronic calorimeter sections. The design is optimized for high jet energy resolution, while special attention to possible biases resulting from the combination of a relatively thin hadronic calorimeter and the large fluctuations in hadronic shower composition and shower development that usually limit calorimeter performance in large systems. The solution we have chosen – deep longitudinal segmentation with towers in each longitudinal section overlapping in azimuth and rapidity – was extensively simulated within the GEANT4 simulation framework and repeatedly tested in particle beams in the T1044 test beam facility at FNAL. Both simulation data and test beam data, and the resulting jet physics performance, will be presented in this talk.
We report recent ALICE results on primary charged particle and neutral meson production in pp (2.76, 5.02, 7 and 8 TeV), p-Pb (5.02 TeV), Pb-Pb (2.76 and 5.02 TeV) and Xe-Xe (5.44 TeV) collisions. The transverse momentum ($p_{\rm T}$) spectra of charged hadrons used in the analysis were measured in the kinematic range of $0.15 < p_{\rm T} < 50$ GeV/$c$ and $ |\eta|< 0.8$. The charged hadron spectra from Pb-Pb and Xe-Xe collisions are divided in nine centrality intervals in the range of 0-80 %. As we achieved significantly smaller systematic uncertainties in the current analysis, the previously published results from p-Pb and Pb-Pb (2.76 TeV) collisions were reanalyzed.
Neutral mesons were reconstructed through their two-photons decays. The photons were measured via several complementary methods, using eighter the central tracking system identifying photons converted to $e^{+} e^{-}$ pairs in the material of the inner barrel detectors or the electromagnetic calorimeters. Thus we used the respective advantages of the detectors, i.e. the excellent momentum resolution of the conversion photons down to very low transverse momenta and the high reconstruction efficiency and triggering capability of calorimeters. This approach allowed to measure the neutral meson spectra in wide range of transverse momenta.
In this talk we will report a measurement of the nuclear modification factors of primary charged particles and of light neutral mesons in Pb--Pb (2.76 TeV and 5.02 TeV), in Xe-Xe (5.44 TeV) and in p-Pb (5.02 TeV) collisions with ALICE at the LHC. We compare the nuclear modification factors obtained for different collision systems as a function of transverse momentum, collision centrality as well as charged particle multiplicity (${\rm d}N_{{\rm ch}}/\rm{d}\eta$). We will present comparison to results from other experiments and to model calculations and review several scaling properties such as transverse mass scaling and $x_{\rm T}$ scaling in pp collisions.
The spectra of charged particles in XeXe and PbPb collisions at $\sqrt{s_{NN}} = 5.44$ TeV and $\sqrt{s_{NN}} = 5.02$ TeV, respectively, are presented in six ranges of collision centrality. The PbPb nuclear modification factor is constructed with a measured pp reference, and the XeXe nuclear modification factor is formed with an extrapolated pp reference. Both are found to be heavily suppressed in the most central collisions. The path-length and collision-energy dependence of parton energy loss are probed by comparing these two systems of differing size. The data are also compared to various theoretical models, as well as previous measurements at lower collision energies. The pPb nuclear modification factor is constructed using a measured pp reference and is seen to be slightly above unity for the highest transverse momentum probed by the measurement. This illustrates contributions due to initial-state effects, such as anti-shadowing in the nuclear parton distribution functions.
The measurement of charge particle production in heavy ion collisions, when compared with $pp$ data, provides insight into the properties of the hot and dense quark-gluon plasma. The ATLAS detector at the LHC recorded 0.49 nb$^{-1}$ of Pb+Pb collisions, 25 nb$^{-1}$ of $p$+Pb collisions and 4.2 pb$^{-1}$ of $pp$ collisions, all at the center-of-mass energy $\sqrt{s_{NN}}=\sqrt{s}=5.02$ TeV. Recently, ATLAS also recorded 3 $\mu$b$^{-1}$ of Xe+Xe collisions at $\sqrt{s_{NN}}=5.44$ TeV, which offers a new opportunity to study the system size dependence of the parton energy loss. The large acceptance of the ATLAS detector allows measurements of charged hadron spectra in a wide range of both pseudorapidity and transverse momentum, differentially in collision centrality. The charged hadron spectra measured in Pb+Pb, Xe+Xe, and $p$+Pb collisions are compared to the analogous spectra measured in $pp$ collisions, and the resulting nuclear modification factors $R_\mathrm{AA}$ and $R_\mathrm{pPb}$ are studied.
Recent measurements of hadron and jet RAA at very high energies provide in combination crucial new input to our understanding of jet quenching. The increased precision of these observables has shown how hadrons and jets with comparable energies are suppressed differently. This is natural, since triggering on a high energy hadron constitutes selecting an unusual jet whose fragmentation pattern is unusually hard and unusually narrow in angle. By using the hybrid strong/weak coupling model including finite resolution, we study the various different physical effects that contribute to the observed results in data, and provide a simultaneous description of jet and hadron RAA data across the full kinematic range available in heavy ion collisions at the LHC. We also compare to hadron data from RHIC.
We present charged particle spectra at midrapidity measured in lead-lead collisions at a center-of-mass energy per nucleon pair of 5.02 TeV with ALICE, in twenty centrality classes ranging from most central (0-5%) to very peripheral (95-100%) collisions. At high transverse momentum ($8 < p_{\rm T} <30$ GeV/$c$), the average nuclear modification factor ($R_{\rm AA}$) is found to increase from 0-5% central to 75-85% peripheral collisions, beyond which it strongly falls to very low values for the most peripheral collisions (95-100%). Our findings support the idea that peripheral collisions are affected by biases caused by the event selection and collision geometry, which can lead to an apparent nuclear modification in peripheral collisions even in the absence of jet quenching. The results in peripheral collisions are consistent with a PYTHIA-based model without nuclear modification. Our study provides an explanation of the observation that $R_{\rm AA}$ is lower than unity in peripheral Pb-Pb collisions, but equal to one in p-Pb collisions at similar charged particle multiplicity.
The equation of state (EoS) in $2+1$ flavor QCD has recently been established in the continuum limit at the physical quark masses in ab initio lattice QCD calculations. The HotQCD collaboration result provides the EoS in the temperature range from $130$ to $400$ MeV. We extend the HotQCD equation of state to higher temperatures. We utilize the Highly Improved Staggered Quarks (HISQ) action. We perform computations at the pion mass of about $300$ MeV since the effects of heavier than physical light quark masses are negligible above $400$ MeV. To control the cutoff effects and approach to the continuum limit, computations are done on the lattices with temporal extent $N_\tau=4$, $6$, $8$, $10$ and $12$. We provide a continuum estimate up to temperatures of 2 GeV.
We study spatial isovector meson correlators in $n_f=2$ QCD with dynamical domain-wall fermions on $32^3\times 8$ lattices at temperatures $T=220-380$ MeV. We measure the correlators of all spin-one ($J=1$) isovector operators. We observe an approximate degeneracy of all considered correlators with increasing temperature. This approximate degeneracy suggests emergent $SU(2)_{CS}$ and $SU(2*n_f)$ symmetries at high temperatures, that mix left- and right-handed quarks.
Understanding the properties of strong interaction matter with its physical
spectrum of light and strange quarks near the pseudo-critical temperature
of (2+1)-flavor QCD is one of the central goals of high energy nuclear
physics. It generally is expected that the analytic crossover transition in
QCD is sensitive to properties of the true chiral PHASE transition at
vanishing quark masses [1,2]. This sensitivity is increased in higher order
cumulants of net charge fluctuations which currently are being measured
by STAR and PHENIX at RHIC and by ALICE at the LHC.
In order to connect these experimental findings to predictions arising from
QCD in the chiral limit it is mandatory to establish the properties of QCD in
this limit. While there are many indications that the chiral PHASE
transition is a second order transition in the universality class of O(4)
sigma models [3], this is by no means established in lattice QCD
calculations as in none of these cases continuum extrapolated results
exist [4].
In this talk we will present the status of our calculations for (2+1)-flavor
QCD with the Highly Improved Staggered Quarks (HISQ) on three different
lattice sizes ($N_\tau = 6, 8, 12$) and with 5 values of light quark masses
that are up to a factor 6 smaller than in nature. This allows us to
systematically control the chiral and continuum limit of QCD. We show
that our results are consistent with O(N) scaling in the chiral limit,
supporting the existence of a second order phase transition. We will also
present first continuum extrapolated results for the PHASE transition
temperature in the chiral limit and discuss the relevance of corrections to
scaling that need to be controlled when extrapolating to QCD with its
physical quark mass spectrum.
[1] A. Bazavov et al., PRD 85 (2012) 054503
[2] A. Bazavov et al., PRD95 (2017) 074505
[3] S.-T. Li and H.-T. Ding, PoS LATTICE2016 (2017) 372
[4] H.-T. Ding, F. Karsch, and S. Mukherjee, IJMPE24 (2015) 1530007
In this talk we will study the relaxation coefficients $\tau_\pi$ and $\tau_J$ of the shear stress tensor $\pi^{\mu\nu}$ and the light quark current $J$ respectively. These are second-order transport coefficients which can be determined in perturbation theory. After reviewing the perturbative kinetic theory framework that has been recently used to determine their respective first-order coefficients $\eta$ and $D$ (shear viscosity and quark diffusion) at NLO in pQCD we will apply it to these second-order coefficients. While $\eta$ and $D$ get reduced by a factor of 5 at NLO for $\alpha_\mathrm{s}\sim0.3$, the dimensionless ratios $T \tau_\pi/(\eta/s)$ and $\tau_J/D$ show a mild increase ($<50\%$) at NLO. We further argue that, through the properties of the collision operator, lower bounds can be obtained in kinetic theory for these coefficients. After presenting the bounds, we compare our results with the strong-coupling AdS/CFT ones: we argue that, while (NLO) kinetic theory can yield first-order transport coefficients in the ballpark of the strong-coupling ones, the lower bounds imply widely different behaviours at second order.
Understanding the microscopic properties of the hot and dense QCD matter produced at RHIC and the LHC is a critical task for heavy-ion physics. Toward this end, we have developed a non-perturbative microscopic approach to study the bulk, transport and spectral properties of the quark-gluon plasma (QGP) [1-2], treating light, heavy and static partons in a unified framework. Starting from a relativistic effective Hamiltonian with a universal color force, we employ a many-body T-matrix approach, solved self-consistently and constrained by lattice QCD (lQCD) data for the equation of state (EoS), heavy-quark (HQ) free energy and quarkonium correlator ratios. The predictive power resides in the emerging spectral functions and transport properties. In particular, we find a strongly coupled solution where the low-momentum low-temperature parton spectral functions dissolve due to collision widths in excess of 0.5 GeV and give a way to dynamically formed hadronic resonance/bound states which take over in the EoS. The calculations of the HQ diffusion coefficients and the shear viscosity to entropy density ratio yield values (and ratios) near the conjectured lower quantum bounds, corroborating the liquid-like structure of the QGP near Tc. An extension of the approach to finite baryon density and its implications for QCD phase structure will be discussed using benchmarks from various lQCD susceptibilities.
[1]SYF Liu, R Rapp, arXiv:1612.09138
[2]SYF Liu, R Rapp, arXiv:1711.03282
We establish a set of equations for moments of the distribution function. On the one hand, these equations generalize the fluid dynamics to the out-of-equilibrium evolution of boost invariant plasmas. On the other hand, they systematically generalize the theoretical framework of viscous hydrodynamics to arbitrary orders. These moments quantify details of the momentum anisotropies of out-of-eqilibrium phase space distributions. The evolution of these moments measures the evolution of systems towards thermalization to a finer level, beyond the commonly used ratio of longitudinal to transverse pressures. In the hydrodynamical regime, these moments are found to correspond to viscous corrections. In the relaxation time approximations, these moments obey a coupled set of equations that can be truncated order-by-order. Truncations at the lowest orders give rise to the exact form of the second order and third order viscous hydrodynamic equations of motion for the Bjorken flow. Solving the equations of the moments, we are able to identify an attractor solution that controls a transition from a free streaming fixed point to a hydrodynamic fixed point. In particular, this attractor solution provides a renormalization of the effective value of the shear viscosity to entropy density ratio, η/s, taking into account off-equilibrium effects.
[1] Jean-Paul Blaizot, Li Yan, JHEP 1711(2017) 161, arXiv:1703.10694
[2] Jean-Paul Blaizot, Li Yan, arXiv:1712.03856
The experimental data collected by the ATLAS experiment during the 2015 Pb+Pb and 2017 Xe+Xe LHC runs offer new opportunities to study charged particle azimuthal anisotropy. The high-statistics Pb+Pb sample allows for a detailed study of the azimuthal anisotropy of produced particles. This should improve the understanding of initial conditions of nuclear collisions, hydrodynamical behavior of quark-gluon plasma and parton energy loss. New ATLAS measurements of differential and global Fourier harmonics of charged particles ($v_n$) in 5.02 TeV Pb+Pb and 5.44 TeV Xe+Xe collisions in a wide range of transverse momenta, pseudorapidity ($|\eta|<2.5$) and collision centrality are presented. The higher order harmonics, sensitive to fluctuations in the initial state, are measured up to $n=7$ using the two-particle correlation, cumulant and scalar product methods. The dynamic properties of QGP are studied using a recently-proposed modified Pearson's correlation coefficient, $\rho(v_n^2,p_\mathrm{T})$, between the eventwise mean transverse momentum and the magnitude of the flow vector in 5.02 TeV Pb+Pb and $p$+Pb collisions. Several important observations are made. The elliptic and triangular flow harmonics show an interesting universal $p_\mathrm{T}$-scaling. A linear correlation between the $v_2$ and $v_3$ coefficients at low and high $p_\mathrm{T}$ ranges is observed and quantified. The Pearson's correlation coefficient $\rho(v_2^2)$ is found to be negative in peripheral and positive in central Pb+Pb collisions. The value of $\rho(v_3^2)$ is found to be much smaller than $\rho(v_2^2)$ and to have similar centrality behavior as $\rho(v_2^2)$.
The azimuthal anisotropies of particle spectra measured in proton-nucleus (pA) and nucleus-nucleus (AA) collisions play a key role in constraining QCD matter properties like the shear viscosity over entropy density ratio eta/s. We compare calculations of v_n’s from viscous fluid dynamics and from kinetic transport which start both from the same initial conditions and which implement the same matter properties. We observe that both approaches lead to parametrically different eta/s-dependencies of the elliptic anisotropy v_2, and they may thus lead to quantitatively different results for the phenomenologically inferred value of eta/s. The parametric differences can be traced to the boost-invariant longitudinal expansion of pA and AA collisions which induces in fluid dynamic results of the eta/s-dependence of v2 a dominant sensitivity on the initial conditions. Transport theory is free of this problem and it accounts for the order of magnitude of experimentally observed signal strengths v_n with sizeable mean free path.
New measurements of collective flow in XeXe collisions at a center-of-mass energy of 5.44 TeV per nucleon pair, collected by the CMS experiment at the LHC, are presented. The $v_{2}$, $v_{3}$ and $v_{4}$ Fourier coefficients of the anisotropic azimuthal distribution are obtained employing three different analysis techniques: two-particle correlations, scalar product method and multiparticle cumulants, which have different sensitivities to non-flow and flow fluctuation effects. The results are shown as a function of transverse momentum ($p_{T}$) for various centrality selections, and compared with corresponding results from PbPb collisions. These new measurements in a smaller nucleus-nucleus system than PbPb provide additional insights into the system-size dependence of the collective flow induced by the dominant collision geometry and its fluctuations. In particular, these results, compared to theoretical predictions and Monte Carlo generators, will provide important details on the system size dependence of the medium response in heavy ion collisions and provide a unique opportunity to study the onset of flow from small to large systems.
Microscopic transport approaches are the tool to describe the non-equilibrium evolution in low energy collisions as well as in the late dilute stages of high energy collisions. In this talk, a newly developed hadronic transport approach, SMASH (Simulating Many Accelerated Strongly-interacting Hadrons) is introduced. After explaining all the components of this approach, e.g. initial conditions and resonance properties, the approach is validated by a comparison to an analytic solution of the Boltzmann equation. Light and strange particle production and collective flow are compared to experimental data from elementary and nucleus-nucleus collisions in the low energy regime accessible at GSI. The implications of this new approach for dilepton production are discussed including an outlook on the non-equilibrium hadronic production of electromagnetic probes at high beam energies at RHIC and LHC. In addition, the impact of resonance properties on transport coefficients of hadronic matter is pointed out. A detailed understanding of a hadron gas with vacuum properties is required to establish the baseline for the exploration of the transition to the quark-gluon plasma in heavy ion collisions at high net baryon densities.
References:
J. Weil et al, „Particle production and equilibrium properties within a new hadron transport approach for heavy-ion collisions“, Phys. Rev C 94 (2016) no. 5, 054905
J. Tindall et al, „Equilibration and freeze-out of an expanding gas in a transport approach in a Friedmann–Robertson–Walker metric“, Phys.Lett. B770 (2017) 532-538
The measurement of the decorrelation of flow harmonics, $v_n$, and event plane angles, $\Psi_n$, (or flow vector, $V_n\equiv v_ne^{in\Psi_n}$) in the longitudinal direction explores the non-boost-invariant nature of the initial collision geometry and final state collective dynamics. The decorrelations were first observed at the LHC, but are predicted by several (3+1)D hydrodynamic models to be stronger for lower $\sqrt{s_{NN}}$ at RHIC due to the smaller number of initial partons and shorter string length at lower $\sqrt{s_{NN}}$. We report the results from large minimum-bias Au+Au datasets at $\sqrt{s_{NN}}=$ 200 GeV (1.2 billion events) and 54 GeV (1 billion events) with the STAR detector. The factorization ratio, $r_n(\eta)=3D\langle V_n(-\eta)V_n^*(\eta_{\mathrm{ref}}) \rangle/\langle V_n(\eta)V_n^*(\eta_{\mathrm{ref}}) \rangle$, is used to measure the decorrelation between $\eta$ and $-\eta$ relative to a common reference $\eta_{\mathrm{ref}}$. Non-flow correlations are suppressed by a large rapidity gap between $\eta$ from the TPC ($|\eta|<1$) and the $\eta_{\mathrm{ref}}$ from the Forward Meson Spectrometer ($2.5<\eta_{\mathrm{ref}}<4$). The results are obtained for $v_2$ and $v_3$ as a function of transverse momentum and centrality for the two collision energies. They are compared with results from the LHC and calculations from different models. The decorrelations do not scale trivially with the beam rapidity $y_{beam}$, i.e. $r_n(\eta/y_{beam})$ from different beam energies do not overlap. Hydrodynamic models tuned to the Pb+Pb data at 2760 GeV fail to describe the strength of the decorrelation at 54 and 200 GeV. These results will help to constrain the initial condition along longitudinal direction and help to understand the longitudinal evolution of the fireball.
Using the extraordinary versatility of RHIC in selecting different colliding
species, the PHENIX experiment has collected data in p+Al, p+Au, d+Au, and $^{3}$He$+$Au at 200 GeV center-of-mass energy and conducted a comprehensive set of anisotropic flow measurements. These geometry-controlled experiments provide a unique testing ground for theoretical models that produce azimuthal particle correlations based on initial and/or final state effects.
New results that will be presented at this conference include a complete set of
triangular anisotropies of inclusive charged particles and final results on
identified pion, kaon and proton $v_2(p_T)$. The $v_3$ measurements are particularly sensitive to the initial-state fluctuations and the duration of the hot matter stage; the mass-ordered splitting in $v_2(p_T)$ provides information about the role of early-stage collective flow and late-stage hadronic rescattering. Detailed model comparisons with all observables will be discussed.
Recently, near-side azimuthal angular correlations across a large pesudorapidity gap, commonly called as long-range ridge-like correlations, have been observed in small collision systems. It opens up opportunities to explore the multiparton dynamics of QCD and the limitation of fluid dynamics description of the matter created in these collisions. We report the STAR measurement of azimuthal harmonics $v_2$ and $v_3$ in the p+Au and d+Au data collected in various energies such as 19.6, 39, 62.4 and 200 GeV. The non-flow contributions, which are suppressed by requiring a large $\Delta\eta$ gap, are estimated in most peripheral collisions and subtracted. After non-flow subtraction at each beam energy, $v_2$ and $v_3$ are obtained as a function of centrality and transverse momentum. The $v_2$ signals are also extracted using four-particle azimuthal correlations, where the influence of non-flow is quantified by comparing to the standard cumulant method, as well as the two-subevent and three-subevent cumulant methods. It is found that both the influence of non-flow and the strength of the long-range $v_2$ and $v_3$ have a strong beam energy dependence. The results are compared to similar studies in peripheral Au+Au collisions and calculations from different models. This measurement provides new constraints on theoretical models of long-range collectivity and its energy dependence in small collision systems.
Measurements of four-particle flow cumulants $c_{n}\{4\}=\langle v_n^4\rangle -2\langle v_n^2\rangle^2$ for $n=2$ and 3, and symmetric cumulants $SC(n,m)=\langle v_n^2 v_m^2\rangle-\langle v_n^2\rangle\langle v_m^2\rangle$ for $(n,m)=(2,3)$ and $(2,4)$ are presented in $pp$, $p$+Pb and peripheral Pb+Pb collisions at various collision energies, aiming to probe the long-range collective nature of multi-particle production in small systems. Results are obtained using the standard cumulant method, as well as the two-subevent and three-subevent cumulant methods. Results from the standard method are found to be strongly biased by non-flow correlations as indicated by strong sensitivity to the chosen event class definition. A systematic reduction of non-flow effects is observed when using the two-subevent method and the results become independent of event class definition when the three-subevent method is used. The values of $v_n\{4\}=\sqrt[4]{-c_n\{4\}}$ are found to be constant over the range 40<$N_{ch}$<200 in $pp$ collisions, providing direct evidence that multi-particle collectivity persists to low multiplicity. The measured $SC(n,m)$ shows an anti-correlation between $v_2$ and $v_3$, and a positive correlation between $v_2$ and $v_4$. The magnitude of $SC(n,m)$ is constant with $N_{ch}$ in $pp$ collisions, but increases with $N_{ch}$ in $p$+Pb and Pb+Pb collisions. The normalized symmetric cumulants $SC(n,m)/\langle v_n^2\rangle\langle v_m^2\rangle$ are found to be independent of $p_\mathrm{T}$, suggesting $v_n$-$v_m$ correlations reflect the global properties of the event. These measurements provide further evidence for long-range multi-particle collectivity, and quantify the nature of its event-by-event fluctuations.
First results on two-particle angular correlations for charged particles emitted in $e^+e^-$ collisions using 730 $pb^{-1}$ of data collected between 91 and 209 GeV with the ALEPH detector at LEP are presented. With the archived data, the correlation functions are studied over a broad range of pseudorapidity $\eta$ (rapidity $y$) and azimuthal angle $\phi$ with respect to the electron-positron beam axis and the event thrust axis. Short-range correlations in $\Delta\eta$ ($\Delta y$), which are studied with $e^+e^-$ annihilations which reveal jet-like correlations. Long-range azimuthal correlations are studied differentially as a function of charged particle multiplicity. Those results are compared to event generators and are complementary to the studies of the ridge signals in high multiplicity pp, pA and AA collisions at the RHIC and the LHC.
Multi-particle correlations in hadronic colliding systems at both RHIC and the LHC are under detailed investigation in recent years. A wealth of experimental evidence suggests the presence of collective phenomena and the formation of a quark-gluon plasma (QGP) also in high-multiplicity pp and pPb collisions. In particular, multi-particle cumulant analyses have established the collective nature of these correlations. Nevertheless, despite the fact that a common paradigm seems to emerge for all hadronic systems, the exact underlying mechanism still needs to be understood. In particular, the measurement of long-range collective azimuthal correlations down to low multiplicities is still challenging experimentally due to contamination from jet-like correlations. New methods of multiparticle correlations using subevents were developed to suppress jet-like correlations at low multiplicities. Based on data collected by CMS experiment in pp and pPb collisions, correlated azimuthal anisotropies ($v_n$) between different orders are studies using the symmetric cumulants without sub-event and also using N subevents (N = 2, 3 and 4). Furthermore, the subevent cumulant method is also applied to extract $v_n$ harmonics using six- and eight-particle correlations. These results provide crucial insights on the physical origin of observed long-range correlations in small colliding systems down to very low multiplicities.
Many observables which are used as a signature of collective effects in heavy-ion collisions when measured in high multiplicity pp and pA interactions reveal a very similar behaviour. We present first measurements of different order flow coefficients and their magnitude correlations using the Symmetric Cumulants for data collected by ALICE during the LHC Run 2 operation. The data sample includes pp collisions at $\sqrt{s}$ = 13 TeV, p-Pb at $\sqrt{s_{NN}}$ = 5.02 TeV, Xe-Xe at $\sqrt{s_{NN}}$ = 5.44 TeV and Pb-Pb collisions at $\sqrt{s_{NN}}$ = 5.02 TeV. Such a broad spectrum of colliding systems with different energies and wide range of multiplicity allow for detailed investigation of their collision dynamics. The measurements are based on the newly developed pseudorapidity-subevent technique, which was proven to be particularly important for studies in small systems. The results are compared to various theoretical models providing an important insight into initial conditions and the nature of collective phenomena in different collision systems.
At very small fractional momentum of the nucleon, the increase of the gluon density is expected to saturate, but no experimental measurements have yet shown this saturation effect without ambiguity. Measurements of prompt photon production at forward rapidity in p-Pb collisions represent suitable tests for the onset of the gluon saturation.
The LHCb experiment is well suited for prompt photon measurement as it is equipped with tracking, particle identification and calorimetry detectors that cover the same pseudo-rapidity acceptance 2 < eta < 5. New results on prompt photon measurements in p-Pb collisions at $\sqrt{s_{NN}}=5$TeV will be presented.
We present new symmetric cumulant measurements, as well as two-, four- and six-particle $v_n$ measurements (and their ratios) for charged and particle identified hadrons. These measurements will be presented for a broad range of transverse momenta and centrality intervals in U+U collisions at $\sqrt{s_{NN}}$= 193 GeV and Au+Au, Cu+Au, Cu+Cu, d+Au and p+Au collisions at $\sqrt{s_{NN}}$=200 GeV. The measurements indicate the expected trends for hydrodynamic-like viscous attenuation in the medium produced in the different systems, the influence of initial-state fluctuations, system shape ($\varepsilon$), system-size and asymmetry, and the transport coefficients ($\eta/s$, $\zeta/s$, ... ) on the flow coefficients ($v_n$). The measurements are also compared to viscous hydrodynamic calculations to pin down the roles of initial-state fluctuations, mixed harmonic correlations and system size and shape ($\varepsilon$). The implication of these measurements for understanding the medium properties of these systems will be discussed.
Ultra-peripheral heavy ion collisions occur when the nuclei have large impact parameter and interact through photon-induced reactions. These include processes in which an energetic photon emitted by one nucleus resolves the partonic structure of the other and stimulates jet production. Much like deep inelastic scattering, such processes provide a clean probe of the nuclear parton distributions. This is in contrast to other observables in ion-ion and proton-ion collisions which typically involve the convolution of parton distributions in both incident particles. Thus jet photo-production represents the most direct opportunity to study nuclear parton distributions until a future electron-ion collider is constructed. This talk presents new measurements of ultra-peripheral jet photo-production in Pb+Pb collisions with the ATLAS detector at the LHC. Events are selected using a combination of forward neutron and rapidity gap requirements. Final states with two or more jets are used to construct event level observables $H_{\mathrm{T}}$, $x_{\mathrm{A}}$ and $z_{\gamma}$, which characterize the hard scattering process. Measurements of differential cross sections in these three quantities, after unfolding for detector response, are presented. The results are compared with theoretical calculations using different nPDF parameterizatinos, which highlight the potential of this data in future global analysis for precision nPDF determination.
We present calculations of hadron production from gluon dominated non-equilibrium matter in various small collision systems using the IP-Glasma model combined with a state-of-the-art fragmentation prescription based on the Lund model. We study bulk observables such as particle spectra, nuclear modification factors (R_{pA}), proton-to-pion ratios and multi-particle azimuthal angular correlations. We demonstrate that characteristic features of hadronic observables such as the baryon to meson ratio, mass ordering of v2(pT) and <pT>, are naturally reproduced within the initial state framework [1]. We also present first results on a systematic comparison of such observables across different systems, including p+p and p+Pb collisions at the LHC as well as p/d/He3+Au at RHIC.
[1] B. Schenke, S. Schlichting, P. Tribedy, R. Venugopalan, Phys.Rev.Lett. 117 (2016) no.16, 162301
We calculate isolated photon production at forward rapidities in proton-nucleus collisions in the Color Glass Condensate framework [1]. Our calculation uses dipole cross sections solved from the running coupling Balitsky-Kovchegov equation with an initial condition fit to deep inelastic scattering data and extended to nuclei with an optical Glauber procedure that introduces no additional parameters beyond the basic nuclear geometry. We present predictions for future forward RHIC and LHC measurements. The predictions are also compared to updated results for the nuclear modification factors for pion production, Drell-Yan dileptons and J/Psi mesons in the same forward kinematics, consistently calculated in the same theoretical framework. We find that leading order, running coupling high energy evolution in the CGC picture leads to a significant nuclear suppression at forward rapidities. This nuclear suppression is stronger for photons than for pions. We also discuss how this might change with next-to-leading order high energy evolution.
[1] Isolated photon production in proton-nucleus collisions at forward rapidity, B. Ducloué, T. Lappi, H. Mäntysaari, arXiv:1710.02206 [hep-ph]
Reaching next-to-leading order (NLO) accuracy in perturbative calculations of particle production in QCD at high energy is essential for reliable phenomenological applications. In recent years, the Color Glass Condensate effective theory (the natural framework for such calculations) has indeed been promoted to NLO accuracy. However, the first NLO calculations met with unexpected difficulties, among which a huge scheme-dependence with respect to the scale choice in the running of the coupling. The NLO correction to the cross-section for single inclusive particle production in pA collisions at forward rapidities was found to vary by up to two orders of magnitude and also to change sign, when replacing the momentum-space prescription for the running of the coupling (as natural in the calculation of the NLO impact factor) by a coordinate-space prescription (as generally used when solving the Balitsky-Kovchegov equation).
Recently we have found out that the origin of this puzzle lies in the interplay between the Fourier transform from coordinate space to momentum space and the asymptotic freedom of QCD [1]. We present a new coordinate space prescription which avoids this problem and leads to results consistent with the momentum-space prescription ones. The NLO corrections are negative and reduce the LO result by 40% to 50%. We argue that the scheme-dependence could be further reduced by using the momentum-space representation throughout the whole NLO calculation, that is, for both the impact factor and the solution to the BK equation.
[1] "On the use of a running coupling in the NLO calculation of forward hadron production", to appear.
The measurement of D-meson production in jets can provide important insights into the interactions of heavy-flavour quarks with the quark-gluon plasma created in heavy ion collisions. In particular, the role of gluon splitting processes in the production of heavy flavour, which is fundamental for a complete understanding of the quenching mechanisms for both light and heavy quarks, can be explored. Large datasets for proton-proton and PbPb collisions at a nucleon-nucleon center-of-mass energy of 5.02 TeV were collected with the CMS detector during the 2015 LHC run. These data enable measurements of D-meson production as a function of the radial distance between the jet axis and the D meson in different intervals of D-meson transverse momentum. The ratio of the results for PbPb and pp collisions will be compared to similar measurements of jet radial profiles using light particles from the CMS experiment at the same center-of-mass energy.
Heavy quarks (charm and beauty) are produced in hard parton scatterings in the early stages of hadronic collisions. Therefore, they are ideal probes to investigate the properties of the Quark-Gluon Plasma (QGP) produced in ultra-relativistic heavy-ion collisions. The study of angular correlations between heavy-flavour particles and charged particles allows us to characterize the heavy-quark fragmentation process and its possible modification in a hot and dense medium. The measurement of heavy-flavour jets gives more direct access to the initial parton kinematics and can provide further constraints for heavy-quark energy-loss models, in particular adding information on how the radiated energy is dissipated in the medium.
Studies in pp collisions are mandatory to characterize heavy-quark production and fragmentation in vacuum, constituting the necessary reference for interpreting heavy-ion collision results. Differences between results from pp and p-Pb collisions can reflect how the heavy-quark production and hadronization into jets is affected by cold nuclear matter effects.
This contribution will include the latest heavy-flavour correlation and jet measurements with the ALICE detector in pp, p-Pb and Pb-Pb collisions from the LHC Run-2 data. In particular, results on azimuthal correlations of D mesons with charged particles in p-Pb collisions at $\sqrt{{s}_{\rm{NN}}} = 5.02$ TeV will be presented. Measurements of multiplicity and centrality dependent azimuthal correlations of heavy-flavour hadron decay electrons with charged particles in p-Pb and Pb-Pb collisions at $\sqrt{{s}_{\rm{NN}}} = 5.02$ TeV will be shown. In addition, measurements of D-meson tagged and beauty tagged jet production in pp collisions at $\sqrt{s} = 7$ TeV and p-Pb collisions at $\sqrt{{s}_{\rm{NN}}} = 5.02$ TeV will be presented. The status of D-tagged jet measurements in Pb-Pb collisions at $\sqrt{{s}_{\rm{NN}}} = 5.02$ TeV will be discussed.
The nuclear modification of groomed jet splitting in relativistic heavy-ion collisions at RHIC and the LHC energies is studied [1] based on the higher twist formalism. Assuming coherent energy loss for the two splitted subjets, a non-monotonic jet energy dependence is found for the nuclear modification of jet splitting function: strongest modification at intermediate jet energies whereas weaker modification for larger or smaller jet energies. Combined with the smaller size and lower density of the QGP medium at RHIC than at the LHC, this helps to understand the groomed jet measurements from CMS and STAR Collaborations: strong modification of the momentum sharing $z_g$ distribution at the LHC whereas no obvious modification of the $z_g$ distribution at RHIC. In contrast, the observed nuclear modification pattern of the groomed jet $z_g$ distribution cannot be explained solely by independent energy loss of the two subjets. The dependence on the angular separation $\Delta R$ between two subjets is also studied; it is found that the nuclear modification of $z_g$ distribution decreases with decreasing $\Delta R$ but the maximal nuclear modification from CMS energies is always roughly twice of that for STAR energies. Our result may be tested in future groomed jets measurements with lower jet energies at the LHC and larger jet energies at RHIC, for different angular separations between the two subjets.
Reference:
[1] Ning-Bo Chang, Shanshan Cao, Guang-You Qin, arXiv:1707.03767 [hep-ph].
Hard splittings in the evolution of a jet may be modified by the presence of a dense strongly interacting medium. Grooming procedures can be used to isolate such hard components of a jet and allows one to focus on the two subjets resulting from a sufficiently hard partonic splitting. The modification of these splittings in medium could highlight the role of jet induced medium response as well as potential single hard scatterings (higher-twist) and multiple soft medium induced radiation (BDMPS).
Measurements of the symmetry parameter ($z_g$) and angular separation of such subjets are reported as measured with the ALICE Detector in pp and PbPb collisions at $\sqrt{s} = 7$ TeV and $\sqrt{s_{NN}} = 2.76$ TeV respectively. Results are compared to predictions using Monte Carlo generators. The use of recursive splittings and their mappings to identify interesting regions of phase space will also be discussed with comparisons made between Monte Carlo generators and data in pp and PbPb collisions.
Nearly collinear pairs of partons are sensitive to potential novel coherence effects in the parton energy loss process, which can be observed through measurements of jet substructure. This analysis presents a new measurement of jets containing a gluon that splits into a heavy quark pair, i.e., a heavy-quark antenna. Such jets are identified by analyzing the groomed substructure of double b-tagged jets. The grooming procedure allows the identification of the hardest splitting process within the parton shower and is sensitive to the virtuality evolution of the parton. Results of the subjet transverse momentum balance and angular distance between the two subjets are shown in pp collisions at 5.02 TeV, which are the first jet substructure measurements of identified partons.
Correlations of electroweak probes, jets, and charged particles are a powerful tool to study medium modifications of the parton shower. One can impose constraints on jet quenching mechanisms in heavy ion collisions by measuring jet substructure observables, such as fragmentation functions or jet momentum density profiles. Tagging jets with an associated photon helps to constrain the associated parton kinematics and flavor before quenching. Additionaly, parton flavor dependence of these observables can be explored by comparing the results for the inclusive jet sample, which is dominated by gluon-induced jets, to one with a b-tagged jet-selection. Measurements of photon-tagged jet fragmentation functions and the jet shapes for inclusive, b-tagged, and ,for the first time, photon-tagged jet sample in pp and PbPb collisions at sqrt(s_NN) = 5.02 TeV collision energy using data collected by CMS will be reported. Besides, the corresponding measurements for inclusive and b-tagged jets will be shown in context of parton flavor dependence.
Measurements of quarkonium production play an important role in understanding the properties of the Quark-Gluon Plasma (QGP) created in relativistic heavy-ion collisions. Quarkonium suppression in the medium due to the color screening effect has been proposed as a direct signature of the QGP formation. However, other effects, such as cold nuclear matter (CNM) effects and regeneration, add additional complications to the interpretation of the observed suppression. Compared to charmonia, bottomonia not only gain less contribution from regeneration due to the smaller b-quark production cross-section, but are also less affected by the CNM effects. Furthermore, different bottomonium states with different binding energies are expected to dissociate at different temperatures, thus measurement of this "sequential melting" can help constrain the thermodynamic properties of the medium.
In this talk, we will present the latest measurements of $\Upsilon$ production in Au+Au collisions at $\sqrt{s_{\scriptsize\mbox{NN}}}$ = 200 GeV via both di-muon and di-electron channels by the STAR experiment. With combination of the data sets taken in 2011, 2014 and 2016, the precision of $\Upsilon$ measurements will be significantly improved compared to previous preliminary results, especially for the excited $\Upsilon$ states. The nuclear modification factors for the ground and excited $\Upsilon$ states will be shown as a function of transverse momentum and centrality, and compared to those measured at the LHC as well as to theoretical calculations.
The production cross sections of the $\Upsilon(1S)$, $\Upsilon(2S)$, and $\Upsilon(3S)$ states were measured separately using the CMS experimental apparatus, in pp, pPb, and PbPb collisions at 5.02 TeV. New results on the production of the three upsilon states in pPb are reported, including cross sections as a function of transverse momentum (p$_T$) and rapidity ($y$). The data show a stronger suppression of the excited states (2S and 3S) as compared to the ground state (1S). The event activity dependence of the forward-backward ratio of all three upsilon states is also reported. Final results on the differential production cross section and nuclear modification factor of upsilon mesons in PbPb collisions at 5.02 TeV, as a function of centrality, p$_T$ and $y$, show similar suppression pattern, more pronounced than in pPb data. A strong suppression is observed in PbPb collisions, by up to a factor of 2 and 10 for the $\Upsilon(1S)$ and $\Upsilon(2S)$ respectively. The $\Upsilon(3S)$ was not observed in PbPb collisions, being suppressed by more than a factor 14 at the 95% confidence level.
Bottomonium suppression has long been discussed as a probe for the quark-gluon plasma generated in ultra-relativistic heavy ion collisions. The use of a realistic hydrodynamic background which is anisotropic in momentum space has shown to reproduce experimental data for various windows across each experiment. We have recently expanded our model to incorporate a realistic lattice-vetted heavy-quark potential and have implemented a regeneration model. We present bottomonia suppression results for RHIC and CMS collisions with this new potential and regeneration model.
Quarkonium production in high-energy hadronic collisions provides a fundamental test of QCD. Its modification in a nuclear medium is a sensitive probe of the space-time temperature profile and transport properties of the QGP, yielding constraints complementary to the ones obtained form the quenching of light and heavy flavor. We will present new results for the suppression of high transverse momentum charmonium [$J/\psi, \psi(2S)$] and bottomonium [$\Upsilon(1S),\Upsilon(2S),\Upsilon(3S)$] states in Pb+Pb collisions at the Large Hadron Collider. Our theoretical formalism combines the collisional dissociation of quarkonia, as they propagate in the quark-gluon plasma, with the thermal wavefunction effects due to the screening of the $Q\bar{Q}$ attractive potential in the medium. We find that a good description of the relative suppression of the ground and higher excited quarkonium states, transverse momentum and centrality distributions is achieved, when comparison to measurements at a center-of-mass energy of 2.76 TeV is performed. Theoretical predictions for the highest Pb+Pb center-of-mass energy of 5.02 TeV at the LHC, where new experimental results are being finalized, will also presented. Finally, we will show the latest theoretical calculations at forward rapidity and in smaller systems, such as Xe+Xe.
The heavy-quark potential is a highly versatile theoretical tool. It allows one to summarize many aspects of the intricate interactions between a QQbar bound state and its surrounding medium in a single complex valued quantity. It is systematically defined from QCD [1,2] and at the same time provides an intuitive understanding of the physics of in-medium quarkonium modification. I.e. it offers the means to investigate from first principles how e.g. color screening and collisional excitations conspire to lead to quarkonium suppression in heavy-ion collisions [3,4].
Here we present the first direct computation of this potential from realistic lattice QCD simulations with near physical pion masses [5]. Current ensembles with $N_\tau=12$ from the TUMQCD collaboration offer unprecedented high statistics, those with $N_\tau=16$ unprecedented time resolution, making possible a robust extraction of its values from the spectral functions of Wilson line correlators. To this end we deploy a combination of Bayesian reconstruction methods (BR), as well as the Pade approximation, in turn diminishing individual method artifacts.
Re[V] shows a smooth transition from a confining to a Debye screened behavior. At all temperatures its values lie close to the color singlet free energies. Based on Re[V] we estimate the Debye mass. The modification of Im[V] at very high temperatures is compared to predictions of hard-thermal-loop perturbation theory.
Applications of the complex potential in the modeling of charmonium and bottomonium in heavy-ion collisions are briefly touched upon ([3,4,6]).
[1] N. Brambilla, J. Ghiglieri, A. Vario and P. Petreczky PRD78 (2008) 014017
[2] A.R., T. Hatsuda, S. Sasaki PRL 108 (2012) 162001
[3] Y. Burnier, O. Kaczmarek, A.R. JHEP 1512 (2015) 101
[4] N. Brambilla, M. Escobedo, J. Soto, A. Vairo PRD96 (2017) 034021
[5] A.R. & TUMQCD collaboration (in preparation)
[6] B. Krouppa, M. Strickland, A.R. arXiv:1710.02319
New results on quarkonia production in proton-lead collisions at LHCb at 8.16 TeV nucleon-nucleon center-of-mass energy will be presented. Measurements include J/psi and psi', where the prompt and from-b-decay components can be disentangled, and the 1-- bottomonia states. The large data sample allows the determination of nuclear modification factors with high accuracy.
LHCb has the unique capability to study collisions of the LHC beams on fixed targets. Internal gas targets of helium, neon and argon have been used so far. Updated results and prospects on open and hidden charm productions will be presented, which can provide crucial constraints on cold nuclear matter effects and nPDF at large x. These measurements, together with production of antiprotons and other light hadrons, are of great interest to cosmic ray physics.
Recent observations at RHIC and the LHC of two- and multi-particle correlations
in high multiplicity relativistic proton-proton and proton-ion collisions and similarity of the results to those observed in central heavy-ion collisions are often interpreted as an evidence for collective particle production in small collision systems. These results motivate a study in even smaller systems, such as produced in relativistic electron-proton collisions.
A measurement is presented of two-particle correlations in collisions of electron beams at 27.5 GeV with beams of protons at 920 GeV, which corresponds to 318 GeV centre-of-mass energy. A sample of events equivalent to the integrated luminosity of 430 pb$^{-1}$ was recorded with the ZEUS experiment in 2003-2007. The correlations are measured for charged hadrons as a function of event multiplicity for the lab pseudorapidity range $-1.5<\eta_{\rm lab}<2$. To probe the possible contribution due to collective effects, the correlations are studied as a function of the particle's pair separation in pseudorapidity and the pair mean transverse momentum. The observed correlations are compared to available Monte Carlo models of deep inelastic electron-proton scattering. Observations based on the analysis of the ZEUS data put a limit on the possible collective effects in high multiplicity electron-proton collisions.
We investigate the relative importance of initial and final state effects on azimuthal correlations in low and high multiplicity p+Pb collisions at LHC energies. By matching the classical Yang-Mills dynamics of pre-equilibrium gluon fields (IP-GLASMA) to a perturbative QCD based parton cascade for the final state evolution (BAMPS) on an event-by-event basis, we find that signatures of both the initial state correlations and final state interactions are seen in azimuthal correlation observables, such as $v_2\{2P C\}(p_T)$, with their relative strength depending on the event multiplicity and transverse momentum. Initial state correlations dominate elliptic flow in low multiplicity events for transverse momenta $p_T > 2~\mathrm{GeV}$. While final state interactions are dominant in high multiplicity events and at low momenta, we find that initial state correlations strongly affect $v_2\{2P C\}(p_T)$ for $p_T>2$ GeV as well as the pT integrated $v_2\{2P C\}$. By carrying out a systematic multiplicity scan, we can also probe the dynamics on the border of initial state dominated to final state dominated - but not yet fully developed hydrodynamic – regime. We predict at which multiplicity and transverse momentum many-body QCD effects in the initial state can be experimentally unveiled.
Reference: Greif, Greiner, Schenke, Schlichting, Xu: Phys. Rev. D 96, 091504, 2017
We report on recent progress in understanding multi-particle correlations in $pA$ collisions from the initial state. We consider a proof of principle model of eikonal quarks from the projectile proton multiple-scattering off of a dense nuclear target. With this model, we find that many of the features observed in light-heavy ion collisions at RHIC and the LHC which are often ascribed to collectivity can be qualitatively reproduced in an initial state model. These include the ordering of the two-particle azimuthal angle n-th Fourier harmonics, $v_n\{2\}$; a negative four-particle second Fourier cumulant $c_2\{4\}$, giving rise to a real $v_2\{4\}$; the energy and transverse momentum dependence of $v_2\{4\}$; the similarity in multi-particle second Fourier harmonics $v_2\{4\} \approx v_2\{6\} \approx v_2\{8\}$; and the energy dependence of the four-particle symmetric cumulants. Finally, we consider the Glasma graph approximation of our model and find that many of these features cannot be reproduced, leading to the conclusion that multiple-scattering is a key ingredient for the observed multi-particle correlations from the initial state.
[1] K. Dusling, M. Mace, R. Venugopalan. Multiparticle collectivity from initial state correlations in high energy proton-nucleus collisions. arXiv:1705.00745 [hep-ph]
[2] K. Dusling, M. Mace, R. Venugopalan. Parton model description of multiparticle azimuthal correlations in pA collisions. arXiv:1706.06260 [hep-ph]
Recent measurements of correlations between two particles separated in pseudorapidity and azimuthal angles have shown striking similarities between results obtained in $pp$, $p$+A and A+A collision systems. In the $pp$ collision system, unlike in $p$+A and A+A collisions, the strength of the correlations, quantified by the anisotropy parameter $v_2$, shows little dependence on the observed charged-particle multiplicity. Recent theoretical models suggest that this can result from an intrinsically weak correlation between the charged-particle multiplicity and the impact parameter of the $pp$ collision. An independent handle on the impact parameter can be obtained in principle by requiring the presence of a hard-scattering process in the collision. This talk presents the first measurement of two-particle correlations in $pp$ collisions with a presence of Z boson identified via its $\mu\mu$ decay channel. The analysis uses ATLAS data recorded with nominal $pp$ luminosity with high pileup. A new procedure is used to correct for the contribution of tracks arising from pileup vertices. The multiplicity and transverse momentum dependence of the inclusive charged-particle $v_2$ measured in Z-tagged events at $\sqrt{s}=8$ and 13 TeV is compared to the $v_2$ measured in minimum-bias collisions. They are found to be of a similar magnitude.
The observation of collective effects in small systems, such as strangeness enhancement and the appearance of a ridge, have posed a challenge to conventional models for multiparton interactions and hadronization underlying general purpose MC event generators.
In this talk I will present the microscopic model for collective effects recently implemented in the Pythia8 and DIPSY event generators. In this model collectivity is generated from interactions between Lund strings, referred to as “string shoving” and “rope hadronization”. Rope hadronization is shown to give a good description of strangeness enhancement across pp, pA and AA collisions systems, while string shoving qualitatively describes the ridge observed in pp collisions.
A defining feature of the microscopic model is that all effects are generated without assuming a deconfined plasma or thermalization. Ongoing efforts aim towards further extending the models to pA and AA.
Simulations of relativistic heavy-ion collisions based on viscous hydrodynamics provide an accurate description of the bulk observables measured at RHIC and LHC beam energies, including identified particle yields, mean $p_T$ and multiparticle correlations. The success of the hydrodynamic framework, however, is naturally expected to break down in the dilute limit where discrete particle degrees of freedom dominate.
It was thus surprising when the multiparticle correlations measured in high-multiplicity proton-lead collisions were found to be similar in magnitude to those observed in lead-lead collisions. The observation suggests that hydrodynamic behavior could be manifest in small droplets of quark-gluon plasma (QGP), and that flow might develop at length scales smaller than a proton.
In this work, we posit the existence of hydrodynamic flow in small collision systems and evaluate the likelihood of our assertion using Bayesian inference. Specifically, we model the dynamics of proton-lead and lead-lead collisions at $\sqrt{s_{NN}}=5.02$ TeV using QGP initial conditions with parametric nucleon substructure, a pre-equilibrium free-streaming stage, event-by-event viscous hydrodynamics, and a microscopic hadronic afterburner.
Bayesian parameter estimation is used to construct the posterior probability distribution for the model's input parameters, calibrated to fit the charged particle yields, meant $p_T$ and flow cumulants of both collision systems. We then sample preferred regions of parameter space and evaluate the performance of the model using optimally chosen parameter values. This semi-exhaustive model validation enables us to to comment on the implied viability of hydrodynamics in small collision systems subject to the approximations of the chosen framework. We conclude by presenting posterior constraints on the shape of the proton and temperature dependence of QGP transport coefficients, and discuss relevant implications for hot and dense nuclear matter.
The multiplicity dependent results of identified particle production allowed the discovery of collective-like behavior in pp collisions at the LHC. Good understanding of the effects attributed to well-understood physics, like multiple hard scatterings, is required to establish the origin of the new phenomena. Experimentally, those effects can be controlled using event shapes, like transverse spherocity or directivity, which allows the classification of the pp collisions either as jetty-like or isotropic events. The transverse momentum ($p_{\rm T}$) spectra of light-flavor hadrons in pp collisions measured over a broad $p_{\rm T}$ range provide important input to study particle production mechanisms in the soft and hard scattering regime of QCD. In this work, they are used to perform a comprehensive study as a function of the event multiplicity, collision energy, and event shapes.
We will present the inclusive charged particle transverse momentum distributions for pp collisions at different center-of-mass energies. The multiplicity and energy dependencies of the particle production at high transverse momentum are studied with the exponent of the power law function which describes the $p_{\rm{T}}$ spectra. For pp collisions at
$\sqrt{s} = 13$ TeV and for a fixed multiplicity interval, the parameters obtained from the blast wave analysis of the $p_{\rm T}$ spectra are used to characterize the evolution of the spectral shapes for different event topologies. The multiplicity and spherocity dependencies of the average transverse momenta and integrated yields as a function of charged-particle multiplicity are discussed. The proton-to-pion and kaon-to-pion particle ratios as a function of $p_{\rm T}$ are also reported. Comparisons between data and QCD-inspired models will be shown.
We present a novel approach to the treatment of thermal fluctuations in the (3+1)-D viscous hydrodynamic simulation MUSIC. We investigate the phenomenological impact of thermal fluctuations on hadronic and electromagnetic observables using the state-of-the-art IP-Glasma + hydrodynamics + hadronic cascade hybrid approach [1]. In particular, we show that these thermal fluctuations influence the result of elliptic and triangular flow measurements for ultra-central collisions, such as those presented by the CMS Collaboration [2]. Consequences on the extraction of QCD transport coefficients from heavy-ion collisions will also be discussed.
The anisotropic flow observed in heavy-ion collision experiments is mostly attributed to the hydrodynamic response to the event-by-event collision geometry and to the sub-nucleon quantum fluctuations. However, hydrodynamic fluctuations are present during the dynamical evolution of the Quark Gluon Plasma (QGP) and are quantified by the fluctuation-dissipation theorem [3]. They can leave their imprint on final-state observables.
By analyzing the thermal noise mode-by-mode, we provide a consistent scheme of treating these fluctuations as the source terms for hydrodynamic fields. These source terms are then evolved together with hydrodynamic equations of motion. Such a treatment captures the non-perturbative nature of the evolution for these thermal fluctuations.
[1] McDonald, S., Shen, C., Fillion-Gourdeau, F., Jeon, S. and Gale, C., Phys. Rev. C 95, 064913 (2017).
[2] The CMS collaboration, Chatrchyan, S., et al., JHEP (2014) 2014:88.
[3] Kapusta, J. I., Müller, B. and Stephanov, M., Phys. Rev. C 85, 054906 (2012).
Multi-particle flow correlations in Pb+Pb collisions provide unique insight into the nature of event-by-event fluctuations of the initial eccentricity as well as final state dynamics in the transverse and longitudinal directions. This talk presents a detailed study of transverse flow fluctuations using 4 and 6-particle cumulants $v_n\{4\}$ and $v_n\{6\}$ for $n=1, 2, 3$, and 4. This includes several new results: the first measurement of a negative dipolar flow $v_{1}\{4\}$; a high-precision measurement of $v_{4}\{4\}$, changing sign around 20-25$\%$ centrality; observation of an intriguing sign-change pattern of $v_2\{4\}$ and $v_2\{6\}$ in ultra-central collisions; a detailed study of the cumulant ratio $v_n\{4\}/v_n\{6\}$ which shows significant deviation of $v_2$ and $v_3$ from both Bessel-Gaussian and elliptic-power distributions. The three-subevent cumulant method is used to show that these results are unlikely to be due to non-flow effects. The talk also presents a detailed study of the longitudinal dynamics of harmonic flow using various correlators involving two, four or six particles. The flow decorrelations for $v_n$ ($n=2, 3$, and 4), as well as their center-of-mass energy dependence are studied over broad range of pseudorapidity ($|\eta|<2.5$) and transverse momentum (0.5<$p_\mathrm{T}$<5 GeV). The decorrelation signals are decomposed into contributions from the forward-backward twist and asymmetry in the flow angle and magnitude, respectively. Furthermore, the decorrelation between $v_n$ and $v_m$ in different $\eta$ is measured to disentangle the longitudinal dependence of the initial-state linear effects and final-state non-linear mode-mixing effects. These results provide a wealth of differential information on event-by-event fluctuations of harmonic flow in both transverse and longitudinal directions, and they can be used to improve event-by-event 3+1D hydrodynamic models.
In relativistic heavy-ion collisions, event-by-event fluctuations in the transverse plane in the initial states of quark-gluon plasma (QGP) could lead to anisotropic flows of the final hardons, which has been successfully described by relativistic hydrodynamics simulation. On the other hand, the initial states fluctuations in the longitudinal direction could lead to the fluctuations and decorrelations (factorization breaking) of anisotropy flows in the pseudorapidity direction [See e.g., 1, 2]. Detailed study of initial state fluctuations and their manifestations in the final-state flows and flow correlations can provide insights into the initial states and dynamical evolution of the hot and dense QGP.
In this work [3], we perform a detailed analysis on initial-state longitudinal fluctuations, the pseudorapidity dependence of anisotropic flows and the longitudinal decorrelations of anisotropic flows in heavy-ion collisions at RHIC and the LHC. The dynamical evolution of the QGP is simulated via a (3+1)-dimensional hydrodynamics model [4]. To study the dependence on initial conditions, we utilize two initial condition models: the AMPT model and a Monte-Carlo Glauber based model with longitudinal fluctuations. For longitudinal fluctuations and decorrelations, the individual contributions from flow magnitudes and flow angles are investigated. The comparison to the ALICE, ATLAS and CMS data is also performed. We also study the correlations among anisotropic flows of different orders, such as flow angle correlations and symmetric cumulants. The pseudorapidity dependence of flow correlations is studied as well.
[1] Long-Gang Pang, Guang-You Qin, Victor Roy, Xin-Nian Wang, Guo-Liang Ma, Phys.Rev. C91 (2015) no.4, 044904.
[2] Long-Gang Pang, Hannah Petersen, Guang-You Qin, Victor Roy, Xin-Nian Wang, Eur.Phys.J. A52 (2016) no.4, 97.
[3] X.-Y. Wu, L.-G. Pang, G.-Y. Qin， X.-N. Wang, in preparation.
[4] Longgang Pang, Qun Wang, Xin-Nian Wang, Phys.Rev. C86 (2012) 024911.
Detailed measurements of collectivity in Au+Au collisions at RHIC provide a key
connection between the initial geometry of the deposited energy and the
hydrodynamic evolution of the medium. Utilizing the PHENIX silicon detectors, we present new measurements of flow coefficients extending over a wide range in pseudorapidity $-3 < \eta < 3 $ and to higher $p_T$. Over a broad range in centrality, we present cumulant results $v_2\{2\}$ - $v_2\{8\}$ and $v_3\{2\}$ - $v_3\{6\}$ with different methods for isolating flow and non-flow contributions. Complementing these results, we present flow coefficient unfolded distributions and compare them directly with theoretical models with event-by-event fluctuations. We also present a first look at symmetric cumulants of different orders.
Longitudinal harmonic flow decorrelation (the "torque" effect [1,2]) is a sensitive probe of the early dynamics of ultra-relativistic nuclear collisions. We propose new decorrelation measures of flow magnitude and event-plane angles and apply them to Pb+Pb collisions at the LHC, modeled via event-by-event hydrodynamic simulations. The basic purpose is to verify a generic feature, namely, that the events with a higher flow magnitude decorrelate significantly less in the event-plane angle, compared to the events with a lower flow magnitude. We find a hierarchy between various flow decorrelation measures and confirm specific factorization relations. The model results are in qualitative agreement with the experimental data from the ATLAS and CMS Collaborations. The proposed generalization of the flow decorrelation measures, with weights involving higher powers of the flow magnitude, can be directly tested in future experimental analyses.
[1] P. Bozek, W. Broniowski, J. Moreira, Phys.Rev. C83 (2011) 034911
[2] P. Bozek, W. Broniowski, arXiv:1711.03325 [nucl-th]
Fluctuations have been playing an important role in understanding observables in high-energy nuclear collisions. Higher harmonics of azimuthal angle distributions, for example, can be attributed to initial fluctuations of transverse profile from event to event. In this presentation, we focus on thermal fluctuations during hydrodynamic evolution (a.k.a. hydrodynamic fluctuations) of the QGP fluids in the intermediate stage and investigate the effects of them on several observables in high-energy nuclear collisions.
We employ an integrated dynamical model [2,3] which combines full three-dimensional relativistic fluctuating hydrodynamics with Monte-Carlo version of the Glauber model for event-by-event initialization and the hadronic cascade model in the late rescattering stage. By using this model, we first adjust initial parameters and transport coefficients to reproduce $dN_{\mathrm {ch}}/d\eta$ and centrality dependence of integrated $v_{2}$ in Pb+Pb collisions at the LHC energy. We next analyze %$r_{n|n;1}(\eta)$ and observables for longitudinal flow correlations of the $n$-th order higher harmonics $R_{n;n|n;n}(\eta)$ [4] for separation of the flow magnitude fluctuations from the event-plane twist along the rapidity. From this analysis, we see how hydrodynamic fluctuations break up longitudinal correlations of the magnitude and the event-plane angle of anisotropic flow parameters.
References
[1] K.~Murase and T.~Hirano, Relativistic fluctuating hydrodynamics with memory functions and colored noises,'' arXiv:1304.3243 [nucl-th].
[2] Koichi Murase,
Causal hydrodynamic fluctuations and their effects on high-energy nuclear collisions'', Ph.~D thesis, the University of Tokyo (2015).
[3] K.~Murase and T.~Hirano, Hydrodynamic fluctuations and dissipation in an integrated dynamical model'', arXiv: 1601.02260 [nucl-th].
[4] Jiangyong Jia \textit{et al.},
Observables for longitudinal flow correlations in heavy-ion collisions'', arXiv:1701.02183 [nucl-th].
In relativistic high energy collisions, hard scattered partons can fragment into two back-to-back jets. These jets can be used as hard probes to study properties of the Quark Gluon Plasma created in nucleus-nucleus collisions. Di-hadron correlations with respect to high $p_T$ trigger particles are a useful tool to study the interactions between jets and the medium in high-energy heavy-ion collisions. The jet-medium interplay depends on the in-medium path length of initial partons and the system evolution.
Centrality dependence of the jet-like recoil peak magnitude opposite the trigger particle in di-hadron correlations has been widely interpreted as a signature of path-length dependence of jet modification. However, the path length differs between in-plane (the same direction as the event plane, the shorter axis of elliptic shape) and out-of-plane (the perpendicular direction to the event plane) trigger particles because of the almond-like shape of two overlapping nuclei. Therefore, one can exert a measure of control over the recoil jet's in-medium path length by selecting the trigger particle's azimuthal direction with respect to the event plane. Event shape engineering (ESE) has been proposed as a powerful tool to control the initial geometrical shape. ESE constrains the event-by-event flow fluctuations by selecting the magnitude of the flow vector $q_n$. A more detailed study for jet-medium interplay will be possible with ESE than previous event plane dependent di-hadron correlations at the STAR experiment. Here, we present di-hadron correlations in Au+Au collisions at $\sqrt{s_{NN}}=$200 GeV as a function of both the trigger azimuthal angle with respect to the event plane and various ESE selections. The result of this analysis will provide new insights on the geometry and path-length dependence of jet-medium interactions as well as the effect of collective medium expansion on jet modification.
A novel equation of state with the surface tension induced by particles’ interactions was generalized to describe the properties of the neutron stars. In this equation the interaction between particles occurs via the hard core repulsion by taking into account the proper volumes of particles. Recently, this model was successfully applied to the description of the properties of nuclear and hadron matter created in collisions of nucleons.
The new approach is free of causality problems and is fully thermodynamically consistent, which enables us to use it for the investigation of the strongly interacting matter phase-diagram properties in a wide range of temperatures and baryon densities, including neutron stars. The considered model with a small number of parameters, fully determined according to the experimental constraints, reproduces very well all the known properties of normal nuclear matter, provides a high quality description of the proton flow constraints, hadron multiplicities created during the nuclear-nuclear collision experiments and equally is consistent with astrophysical data coming from neutron star observations. Accordingly, we found parameter values that are in good agreement with the same ones obtained from the nuclear–nuclear collision data analysis.
The recent spectacular observation of neutron star mergers underlines the importance of developing strong interaction models that can cover the whole range of densities and temperatures, which can be reached in compact stars as well as heavy-ion collisions. As the temperatures in the merger might reach 80 MeV or more, these events connect conditions of compact star physics and of the fireball created in a heavy-ion collision.
To this end we present a newly developed unified flavour SU(3) description of the hadronic and quark matter based on the parity-doublet description of chiral symmetry breaking. We adjust the parameters to describe nuclear ground state isospin symmetric as well as asymmetric matter, and properties of finite nuclei. In addition, lattice results for thermodynamic quantities as well as susceptibilities at vanishing chemical potential are well reproduced. The QCD phase diagram exhibits a first-order liquid-gas as well as a deconfinement transition with a critical endpoint at large chemical potential. We will present the resulting neutron stars that agree with observed heavy neutron star masses and the deformability of the stars are in accordance with the gravitational wave signal from the merger. The compact stars have small radii and consist mainly of a mixed quark-hadron phase.
Furthermore, we will discuss consequences of the model for the matter created in a neutron star merger and how the properties of hot as well as dense matter are interconnected.
The theory of quantum chromodynamics (QCD) is expected to have a rich phase structure at finite chemical potential and temperature. Its study is a central topic of high energy nuclear physics. Theoretical studies employing lattice QCD methods have already established that the transition from hadrons to quarks proceeds as a smooth crossover in the case of vanishing net baryon number density. At very large net baryon densities and low temperatures, astrophysical observations have become more important in constraining the QCD Equation of State. Nuclear matter ground-state properties as well as properties of compact stars and their violent mergers will serve to determine the equation of state at several times nuclear ground-state density.
In this talk I will present the first calculation
of the QCD phase strcuture and thermodynamics which is shown to be consistent with lattice QCD
results at small barychemical potential as well as nuclear matter properties
and known constraints from compact star observations[1,2].
In this context I will discuss the most relevant properties and constraints which should
be satisfied by any model which attempts to predict the QCD phase structure.
Furthermore, I will present results on the baryon number susceptibilities calculated with this
model and discuss how nuclear interactions may strongly influence the measured
baryon number fluctuations in nuclear collisions at low beam energies [1].
Finally, I will also address similarities and differences between the matter created in heavy ion collisions
and in mergers of compact stars and how both can be described in a unified framework [3].
[1] A.Mukherjee, JS and S.Schramm, Phys. Rev. C 96, no. 2, 025205 (2017)
[2] A.Mukherjee, S.Schramm, JS and V. Dexheimer, arXiv:1706.09191 [nucl-th].(accepted for pulication in Atronomy & Astrophysics)
[3] M.Hanauske, JS et al., J. Phys. Conf. Ser. 878, no. 1, 012031 (2017).
The large sample of high quality data taken in pp collisions at $\sqrt{s} = 7$ TeV and 13 TeV, together with smaller data sets at 900 GeV and 2.76 TeV, and in p-Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV at the LHC with the ALICE detector allows for a systematic study of the light (anti-)nuclei production in these collision systems.
The excellent performance of the Inner Tracking System, the Time Projection Chamber and the Time-Of-Flight detector provide a clear identification and separation of primary produced light (anti-)nuclei from secondaries.
Additionally, the high energy deposit of Z=2 particles in the Transition Radiation Detector has been exploited to collect a hardware-triggered data sample in the high-interaction rate p-Pb collisions at $\sqrt{s_{\rm NN}}$ = 8.16 TeV. First findings from this (anti-)nuclei enriched sample will be shown.
Recent results on deuteron production as a function of multiplicity in pp and p-Pb collisions will be presented, as well as the measurement of helium-3 in p-Pb collisions. The goal is to study production mechanisms such as coalescence in small systems, and to compare them to those in heavy-ion collisions. To achieve this, the coalescence parameter $B_{A}$ is studied as function of transverse momentum in the different systems and as a function of the event multiplicity. In addition to this, prospects for measuring (anti-)deuteron production in jets will be presented.
These investigations have direct connections to cosmological and astrophysical studies, in particular for the search of dark matter candidates where one possible signal is the increased flux of light anti-nuclei which has an interplay with the $B_{A}$ measurementes shown here.
The data from RHIC Beam Energy Scan phase I (BES-I) have shown interesting results below $\sqrt{s_{NN}}<$ 19.6 GeV in identified hadron anisotropy ($v_1$, $v_2$, $v_3$), kaon over pion ratios, and net-proton higher moments. These interesting features continue to the lowest energy, $\sqrt{s_{NN}}$ = 7.7 GeV, and motivate the investigation to even lower energy collisions. The STAR fixed-target program extends the energy reach from $\sqrt{s_{NN}}$ =7.7 GeV to $\sqrt{s_{NN}}$ = 3.0 GeV, corresponding to baryon chemical potential 420 MeV to about 700 MeV range. The comparison of the asymmetric system (Al+Au) and symmetric system (Au+Au) at almost equal number of participating nucleons from most central to mid-central collisions provides useful information on nucleon stopping, which is key to understanding the baryon chemical potential.
We present results from Al (beam)+Au (target) collisions at $\sqrt{s_{NN}}$ = 4.9 GeV and Au+Au collisions at $\sqrt{s_{NN}}$ = 4.5 GeV from the STAR fixed-target program. We will report transverse mass spectra, rapidity density distributions, particle ratios, centrality dependence and directed flow of protons, $\pi^{\pm}$, $K_{s}$ and $\Lambda$, elliptic flow of protons, $\pi^{\pm}$ and K, and HBT homogeneity lengths of pions. Pion and proton elliptic flow show mass ordering. Number of constituent quark scaling tests will be presented. For the asymmetric Al+Au system, the peak of the rapidity density distributions is shifted from the nucleon-nucleon center-of-mass rapidity. The magnitude of this shift varies with centrality and is a measure of the nucleon stopping. These newly measured data will be compared with previously published results from the AGS and SPS. The implications of the results on future STAR fixed-target physics runs will be discussed.
In high energy collisions nuclei are practically transparent to each other but produce very hot, nearly baryon-free, matter in the central rapidity region. Where do the baryons go? We calculate the energy loss of the nuclei using the color glass condensate model. Using a space-time picture of the collision we calculate the baryon and energy densities of the receding baryonic fireballs. For central collisions of large nuclei at RHIC and LHC we find baryon densities more than ten times that of atomic nuclei over a large volume which appear at high rapidities. These results can and are being used as initial conditions for subsequent hydrodynamic evolution and could test the equation of state of matter at very high baryon densities.
The main goal of the CBM experiment at FAIR is to study the behavior of nuclear matter at very high baryonic density in which the transition to a deconfined and chirally restored phase is expected to happen. One of the promising signatures of this new states are the enhanced production of multi-strange particles. The CBM detector is designed to measure such rare diagnostic probes with unprecedented precision and statistics.
Important key observables are the production of hypernuclei and dibaryons. Theoretical models predict that single and double hypernuclei, and heavy multi-strange short-lived objects are produced via coalescence in heavy-ion collisions with the maximum yield in the region of SIS100 energies. The discovery and investigation of new hypernuclei and of hyper-matter will shed light on the hyperon-nucleon and hyperon-hyperon interactions.
Results of feasibility studies of these key CBM observables in the CBM experiment are discussed.
the study of charm production in heavy-ion collisions is considered an excellent probe to study the properties of the hot and dense medium created in heavy-ion collisions. Measurements of D- meson nuclear modification, elliptic and triangular flow in PbPb collisions can provide strong constraints into the mechanisms of in-medium energy loss and charm flow in the medium. The measurement of charm flow and, in particular, of the direct flow coefficient is also expected to be sensitive to the extremely strong but short-lived magnetic field induced by the spectator protons in non- central collisions. This strong magnetic field is indeed expected to generate differences in rapidity-odd directed flow for charm and anti-charm mesons, which can be measured with high precision with the CMS apparatus. In this talk, the measurements of the $D^0$ nuclear modification factor, of the elliptic and triangular flow measured by CMS in PbPb collisions at 5.02 TeV will be presented together with the a new measurement of $D^0$ $R_{pPb}$ in pPb collisions at 5.02 TeV, which can provide more constrains into the relevance of cold nuclear matter effects at central rapidity down to very low tranverse momenta. The first measurement of the direct flow of charm and anti-charm in non-central collision will also be shown and compared to those for inclusive charged particles, previously measured by the CMS Collaboration.
Heavy quarks, i.e. charm and beauty, are formed on a shorter time scale with respect to the strongly-interacting Quark-Gluon Plasma (QGP) produced in high-energy heavy-ion collisions. Therefore, they are sensitive probes to study the mechanisms of parton energy loss, hadronisation in the hot and dense medium, the medium evolution and its transport coefficients. The heavy-flavour nuclear modification factor ($R_{\rm AA}$) and the elliptic flow ($v_2$) are two of the main experimental observables that allow us to investigate the interaction strength of heavy quarks with the constituents of the expanding medium. The comparison of the $R_{\rm AA}$ of charm, beauty and light-flavour hadrons provides information about the colour-charge and parton-mass dependence of parton energy loss. At low $p_{\rm T}$ the $v_2$ is expected to give insights into the degree of thermalisation of heavy quarks in the deconfined medium, and at high $p_{\rm T}$ it carries information on the path-length dependence of in-medium parton energy loss.
In this talk, measurements of $R_{\rm AA}$ and $v_2$ of open heavy-flavour hadrons via semi-leptonic decays to electrons at mid-rapidity and muons at forward rapidity in Pb-Pb collisions at LHC energies will be discussed. The progress on the analysis of the production and anisotropy of electrons from beauty-hadron decays will also be discussed. In addition, the $R_{\rm AA}$ of heavy-flavour hadron decay leptons in Xe-Xe collisions will be presented, along with the prospects for measuring the total charm cross section in this collision system. Comparisons with model calculations including the interaction of heavy quarks with the hot, dense, and deconfined medium will be also shown.
We study the propagation of charm and bottom quarks in the quark-gluon plasma (QGP) by means of a relativistic Boltzmann transport approach that in the large M/T limit recovers the standard Langevin dynamics. The non-perturbative interaction between heavy quarks and light quarks is described by means of a quasi-particle approach in which light partons are dressed with thermal masses. The last tuned to lattice QCD thermodynamics naturally induce a non-perturbative interaction that entails only a weak dependence on the temperature especially around the critical temperature $T_c$, which plays a fundamental role to describe simultaneously the experimental data for the nuclear suppression factor $R_{AA}$ and the elliptic flow $v_2(p_T)$ of D mesons from RHIC to LHC energies. In the same scheme we present predictions for B mesons at 5.02 ATeV that shows a quite significant suppression and allow a determination of the space-diffusion coefficient that is practically independent on the transport scheme for HQ (Boltzmann vs Langevin). The last is seem to largely deviate from from pQCD estimate but also to be still somewhat larger than AdS/CFT and quite close to lattice QCD calculations. Finally it will be discussed the relevance of initial state fluctuations that allows to extend the analysis to high order anisotropic flows $v_3(p_T)$ and $v_4(p_T)$ as well as to investigate the role of QCD interaction in developing correlations between the light and the heavy flavor anisotropic flows. These will provide novel and powerful constraints for the transport coefficients.
[1] F. Scardina, S. K. Das, V. Minissale, S. Plumari, V. Greco, Phys.Rev. C96 (2017) no.4, 044905.
[2] S. K. Das, F. Scardina, S. Plumari, V. Greco, Phys.Lett. B747 (2015) 260.
[3] F. Scardina, D. Perricone, S. Plumari, M. Ruggieri, V. Greco, Phys.Rev. C90 (2014) no.5, 054904.
[4] S. Plumari, G. L. Guardo, F. Scardina, V. Greco, Phys.Rev. C92 (2015) no.5, 054902.
Heavy flavor quarks are unique tools for studying the properties of the Quark Gluon Plasma (QGP) produced in high-energy nuclear collisions. In this talk we will present measurements of various charm hadrons ($\Lambda^{\pm}_{c}$, $D^{\pm}_{s}$, $D^{*\pm}$, $D^{\pm}$ and $D^0$ ($\bar{D}^0$), as well as open bottom production through displaced decay daughters ($B\rightarrow J/\psi$, $D^0$, $e$), at mid-rapidity in Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV, using the STAR Heavy Flavor Tracker. With the high statistics data collected in 2016 and the use of supervised machine learning methods for topological reconstruction of charm hadrons, the $\Lambda^{\pm}_{c}$ and $D^{\pm}_{s}$ signal significances are improved significantly. This allows us to study the $p_T$ and centrality dependences of their production. We will also report on $D^{*\pm}$, $D^{\pm}$ and $D^0$ spectra measured in various centralities and the total charm quark cross section extracted from these extensive measurements as well. In addition, we will present the nuclear modification factors for daughters from decays of bottom hadrons and compare them to those for charm hadrons as well as to theoretical calculations. Physics implications of these measurements for the mass dependences of parton interactions with the QGP, as well as the charm quark hadronization in the medium will be discussed.
Experimental results at RHIC and at the LHC show the same strong suppression for
light and heavy quark probes at high $p_T$, and a possible quark mass
dependence at low $p_T$. More high precision measurements of separated charm and bottom are needed to quantify the dependence of medium energy loss on the quark mass.
The PHENIX Experiment measures electrons from heavy flavor decays using displaced
vertex distributions at mid-rapidity $|y|<$0.35. The nuclear modification of
charm and bottom decays in the semi-electronic channel is obtained for
$1 < p_T < 8$ GeV/c from the new 2015 $p$+$p$ reference data and a much-improved analysis of the 2014 high statistics Au+Au data. PHENIX is also able to measure $B\rightarrow $J$/\psi$ decays in the rapidity range $1.2<|y|<2.2$ for $p_T>0$.
This presentation will report on the nuclear modification factor of separated
charm and bottom yields at mid-rapidity along with the status of the forward rapidity $B\rightarrow$J/$\psi$ analysis using the large statistics obtained during the 2014 Au+Au run.
Beauty quark production in heavy-ion collisions is considered to be one of the key measurements to address the flavour-dependence of in-medium energy loss in heavy-ion collisions. On the other hand, the measurement of the production of strange beauty mesons can provide fundamental insights into the relevance of mechanisms of beauty recombination in the quark-gluon plasma. In this talk, we will present the state of the art of beauty measurements in PbPb collisions in CMS that includes the $R_{AA}$ measurement of fully reconstructed $B^{+}$ mesons and the latest measurements of non-prompt $D^0$ and $J/\psi$ from B decay over a wide transverse momentum range in the same colliding system. The first measurement of the $B_{s}$ $R_{AA}$ in PbPb collisions will also be presented as well as the ratio between the production yield of $B_{s}$ and $B^{+}$.
Heavy quarks serve as valuable probes of the QGP properties as well as the mass hierarchy of parton energy loss. Experimental data at the LHC indicate significant nuclear modification of heavy flavor ($D$ & $B$) meson production that is comparable to light flavor hadrons, which seem contradictory to one’s earlier expectation of $\Delta E_g > \Delta E_q > \Delta E_c > \Delta E_b$. We extended the Linear Boltzmann Transport (LBT) model coupled to hydrodynamic medium to heavy quark jet evolution in the QGP including both elastic and inelastic scattering processes [1,2]. Within the LBT model, we obtain good descriptions of heavy flavor meson suppression, elliptic and triangular flow coefficients observed at the LHC and RHIC.
The time-ordered transport model is further combined with a virtuality-ordered parton shower scheme into a multi-stage evolution approach [3,4] that includes a rare-scattering multiple emission formalism at momentum large compared to heavy quark mass (sensitive only to the transverse diffusion coefficient), and a single scattering induced emission formalism at momentum comparable to mass (sensitive to the transverse diffusion as well as longitudinal drag and diffusion coefficients). This multi-stage approach reduces the difference of energy loss between $c$ and $b$ quarks and simultaneously describes $D$ and $B$ meson $R_\mathrm{AA}$. In addition, the mass (or velocity) dependence of jet-induced medium excitation is explored for the first time. Its effects on the mass hierarchy of parton energy loss and heavy-light hadron correlation functions are investigated.
[1] S. Cao, T. Luo, G.-Y. Qin, and X.-N. Wang, Phys. Rev. C94 (2016) 1, 014909.
[2] S. Cao, T. Luo, G.-Y. Qin, and X.-N. Wang, arXiv:1703.00822.
[3] S. Cao, A. Majumder, G.-Y. Qin, and C. Shen, arXiv:1711.09053.
[4] S. Cao, et al., Phys. Rev. C96 (2017) 2, 024909.
We analyze the recent STAR collaboration results on net-kaon fluctuations in the framework of the Hadron Resonance Gas (HRG) model and lattice QCD. In the latter, the kaon contribution is isolated using the Boltzmann approximation [1]. Our purpose is to extract the freeze-out temperature and chemical potential as functions of the collision energy. In our HRG model, we use the complete hadron spectrum from the latest PDG list. These results are compared to the freeze-out parameters obtained from a combined analysis of electric charge and net-proton fluctuations. Predictions for moment ratios of the net-Lambda multiplicity distribution are obtained along the kaon freeze-out line. They can be compared to forthcoming experimental results from RHIC Beam Energy Scan.
[1] J. Noronha-Hostler et al., arxiv: 1607.02527.
At energies below $\sqrt{s_{NN}}\approx2.55$ GeV, strange quarks can not be produced in binary nucleon-nucleon collisions because of the higher production threshold of the lightest hadrons carrying strangeness. Hence, the investigation of sub-threshold strangeness production in heavy-ion collision is one of the most promising probes, to access the properties of the created system, as the missing energy must be provided by the latter one.
For the first time, a nearly complete set of strange particles has been reconstructed in the 40% most central Au+Au collisions at 1.23A GeV. The data sample includes multi-differential representations of charged and neutral Kaons, Lambdas and Phi-mesons.
We observe a stronger than linear and universal scaling of all strange hadrons yields with increasing centrality, which does not reflect the different nucleon-nucleon thresholds of the various hadrons carrying strangeness. The data are confronted with various phenomenological approaches.
Previous calculations of the shear viscosity to entropy density ratio in the hadron gas have failed to reach a consensus, with $\eta/s$ predictions differing by almost an order of magnitude. This work addresses and solves this discrepancy by providing an independent extraction of this coefficient using the newly-developed SMASH (Simulating Many Accelerated Strongly interacting Hadrons) transport code and the Green-Kubo formalism. We compare the results from SMASH with numerical solutions of the Boltzmann equation for various systems using the Chapman-Enskog expansion as well as previous results in the literature. Substantial deviations of the coefficient are found between transport approaches mainly based on resonance propagation with finite lifetime (such as SMASH) and other (semi-analytical) approaches with energy-dependent cross-sections, where interactions do not introduce a timescale other than the inverse scattering rate. Our conclusion is that long-lived resonances strongly affect the transport properties of the system, resulting in significant differences in η/s with respect to other approaches where binary collisions dominate. We argue that the relaxation time of the system —which characterizes the shear viscosity— is determined by the interplay between the mean-free time and the lifetime of resonances. We finally show how an artificial shortening of the resonance lifetimes or the addition of a background elastic cross section nicely interpolate between the two discrepant results. To turn this around, we finally note that the temperature dependence of $\eta/s$ can be used to constrain the properties of the hadron gas.
The Hyperon-Nucleon (Y-N) interactions play an important role for understanding the strong interaction. It is suggested that alternative Y-N couplings can be a possible solution to the recent observations of neutron star exceeding two solar masses, the so-called "hyperon puzzle". A precise measurement of masses and lifetimes of hypertriton and anti-hypertriton can enrich our knowledge on Y-N interactions. In addition, the light nuclei distributions provide an excellent tool for understanding the freeze-out conditions of system created in high-energy nuclear collisions. For example, the yield ratio of triton, $N(t)$, deuteron, $N(d)$, and proton, $N(p)$, which is defined as $N(t)N(p)/N^2(d)$ may be utilized as an alternative variable in the search of the QCD critical point.
In this talk, we will present the first precise measurement of hypertriton and anti-hypertriton masses in heavy-ion collisions at STAR with the Heavy Flavor Tracker (HFT). Hypertritons and anti-hypertritons are reconstructed through both two-body decay channel ($^3He+\pi^-$) and three-body decay channel ($p+d+\pi^-$) using the high-statistics data collected in 2014 and 2016 Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV. The binding energies and lifetimes of the (anti-)hypertriton will be extracted from this precise measurement. We will also present the centrality dependence of the mid-rapidity $p_T$ spectra of triton ($t$) from Au+Au collisions at $\sqrt{s_{NN}}$ = 7.7, 11.5, 14.5, 19.6, 27, 39, 62.4, 200 GeV. The collision energy and centrality dependence of the yield ratio, $N(t)N(p)/N^2(d)$, and the coalescence parameters of $d$ ($A$ = 2) and $t$ ($A$ = 3) will be also presented. Physics implications of these measurements will be discussed.
We present a measurement of the hyper-triton lifetime with the ALICE detector at the LHC, aiming at shedding light on the hyper-triton lifetime puzzle. During the LHC Run 2, the ALICE experiment recorded Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV that complement the Pb-Pb datasets acquired at $\sqrt{s_{\rm NN}}$ = 2.76 TeV from Run 1. These datasets allow for a systematic study of light (anti-)hypernuclei production in Pb-Pb collisions, in particular, for the hyper-triton lifetime determination.
The identification of the hyper-triton is based on the excellent performance in terms of energy loss measurement in the Time Projection Chamber. In addition, the Inner Tracking System is used to discriminate secondary vertices, originating from weak decays, from the primary vertex. This is of particular importance for the measurement of (anti-)(hyper-)triton that decays weakly with a decay length of several centimetres.
The study of (anti-)(hyper-)nuclei production at both energies will be discussed and compared to model predictions. Special emphasis will be put on new results of the hyper-triton lifetime determination in its 2- and 3-body decay modes. Indeed most calculations on the lifetime of hyper-triton give similar lifetimes, which are close to the lifetime of free $\Lambda$ decays. On the experimental side, all results in Au+Au collision at RHIC and in Li+C collisions at GSI show a significantly shorter lifetime in comparison with that of the free $\Lambda$ decay.
A detailed discussion of the experimental results and the effort needed on both the experimental and the theoretical side in this sector will be presented.
In a quantum field theory, apparent thermalization can be a consequence of entanglement as opposed to scatterings. I will discuss how this can help to explain open puzzles such as the success of thermal models in electron-positron collisions. It turns out that an expanding relativistic string described by the Schwinger model (which also underlies the Lund model) has at early times an entanglement entropy that is extensive in rapidity. At these early times, the reduced density operator is of thermal form, with an entanglement temperature $T_\tau=\hbar/(2\pi k_B \tau)$, even in the absence of any scatterings.
The measurement of strange and resonance particle production in relativistic heavy ions collisions is of great interest to investigate the properties of the hadronic matter under extreme conditions. The enhanced production of strange and multi-strange hadrons with respect to non-strange ones was
historically considered as one of the signatures of the formation of a partonic phase during the evolution of the system created in such collisions. Moreover, hadronic resonances are used to study the energy dependence of the hadronic interactions and jet quenching, giving us the possibility to constrain the lifetime of the hadronic phase.
In this talk, we present a comprehensive set of measurements on hadronic resonance, strange and multi-strange particle production in collisions of Xe-Xe and Pb-Pb at the center-of-mass energies of $\sqrt{s_{\rm NN}} = 5.44$ and 5.02 TeV, respectively, measured by the ALICE experiment at the LHC.
Transverse momentum spectra, integrated yields, mean transverse momenta and particle ratios are presented as a function of centrality for $K^{0}_{S}$, $\Lambda$, $\Xi^{-}$, $\bar{\Xi}^{+}$, $Omega^{-}$, $\bar{\Omega}^{+}$, $\rho(770)^{0}$, $K*(892)^{0}$, $\phi(1020)$, $\Sigma(1385)^{\pm}$,
$\Lambda(1520)$ and $\Xi(1530)^{0}$. Measurements of the nuclear modification factors are also shown for resonances. Our results are discussed and compared to statistical hadronisation models calculations and with predictions of QCD inspired event generators. Additionally, comparisons with lower energy measurements, including an improved re-analysis of the 2.76 TeV sample for the strangeness sector, are also presented.
The ALICE Collaboration started to investigate the baryon-baryon interaction through the search for exotic bound states via invariant mass analyses of different possible decay channels.
In this poster the study of the $d^{*}(2380)$ production in p-Pb collisions at $\sqrt{s_{\rm NN}}$=5.02 TeV with the ALICE detector at the LHC will be presented.
This dibaryon was recently observed at WASA-at-COSY. A narrow resonance with I($J^{P}$) = 0($3^{+}$) - named now $d^{*}(2380)$ - has been observed in all relevant two-pion decay channels as well as in np-scattering. Quark model calculations predict this state to be a compact hexaquark structure of size 0.8 fm, whereas Faddeev calculations with hadronic interactions favour a molecular structure.
Thanks to the excellent tracking and particle identification capabilities of the ALICE apparatus, the reconstruction and identification of the products of the $d^{*}(2380)$\rightarrow$d+pi^{+}+pi^{-}$ decay over a large momentum range is possible with high efficiency. A detailed Monte Carlo study of the expected correlated and uncorrelated background sources in the $d^{*}(2380)$ relevant invariant mass region will be presented in order to have an estimation of the statistical significance of the measurement.
The confirmation of the existence of the $d^{*}(2380)$ state would provide a new candidate for an exotic degree of freedom in the nuclear equation of state at high densities (e.g. neutron stars) and will open the hunt for other possible non-trivial exotic states produced in heavy ion collisions.
In late 2015 the ALICE collaboration recorded Pb--Pb and pp collisions at $\sqrt{s_{\rm NN}}$ ($\sqrt{s}$) = 5.02 TeV.
The availability of data at the highest energy ever achieved in laboratory for heavy-ion collisions together with a pp reference at the same energy opens up the possibility for a detailed study of the nuclear modification factors ($R_{\rm AA}$) of identified particles.
The excellent particle identification capabilities of the ALICE experiment allow to measure the production of pions, kaons and protons over a wide range of transverse-momenta ($p_{\rm T}$).
Particle ratios as a function of the collision centrality are compared to previous results at lower energy to investigate the dynamics of particle production.
In light of the new set of results, a discussion of the latest model predictions of light flavor particle production is presented.
Finally, to quantify the effect of the energy loss in the QCD medium created in the collision, the nuclear modification factors ($R_{\rm AA}$) are computed and compared with results obtained at lower energy.
Recently there has been rapid progress in understanding in-medium dynamics of a quarkonium based on the framework of open quantum system [1-5]. The stochastic potential model [5] introduces thermal fluctuations on Debye screened potential and hence incorporates wave function decoherence. This model however lacks quantum dissipation, which has so far limited its application to early times and has prevented comparison with experimental measurements. In this contribution, we present two strategies that overcome this limitation:
quantum-classical matching
by investigating the localization properties of wave function and in turn of the reduced density matrix, we show, how, at intermediate time scales, one can switch from the quantum dynamics of the stochastic potential model to the classical dynamics of a Langevin equation. It involves matching the Wigner quasi-probability function of the former to the phase space distribution function of the latter. The validity of this matching procedure is discussed.
quantum state diffusion
by directly implementing the quantum state diffusion formalism [6] for the Lindblad master equation [1] of a quarkonium, i.e. mapping the Lindblad master equation to a non-linear stochastic Schrödinger equation (NLSSE) for the wave function. This for the first time provides a direct link between QCD and phenomenological models based on non-linear Schrödinger equations. By numerically solving the corresponding NLSSE, we can capture quantum dissipation and thus the thermalization of quarkonia in a quantum mechanical manner. For simplicity, we discuss this formalism for a single heavy quark in the QGP.
[1] Akamatsu, Phys. Rev. D91 (2015) 056002.
[2] Blaizot et al., Nucl. Phys. A946 (2016) 49.
[3] De Boni, JHEP 08 (2017) 064.
[4] Brambilla et al., Phys. Rev. D96 (2017) 034021.
[5] Akamatsu and Rothkopf, Phys. Rev. D85 (2012) 105011; Kajimoto et al., arXiv: 1705.03365, PRD in press.
[6] Gisin et al., J. Phys. A: Math. Gen. 25 (1992) 5677.
Heavy flavoured jets are important in many of today's studies both as tests of QCD and as probes of hot and dense medium created shortly after the hard scattering. We notice that recently $b\bar{b}$ dijet correlations in proton-proton collisions have been measured by the CMS and ATLAS collaborations at the LHC, NLO+PS p+p baseline could give a rather perfect description of the experimental data than PYTHIA. Moreover detailed mechanisms of heavy flavoured jets propagation and energy loss in dense QCD matter are not yet fully investigated.
In this talk, we present the first predictions of the $b\bar{b}$ dijet angular correlations in Pb+Pb collision. In this work, a NLO+PS event generator SHERPA has been employed to give the p+p baseline and events, A Langevin evolution is used to describe the heavy quark evolution ,and the higher-twist approach is implemented to simulate the radiative energy loss of the gluon,heavy and light quarks simultaneously,and the hard-thermal-loop calculation is used to describe the collisional energy loss of light quarks and gluon. We predict the azimuthal angle $\Delta \phi$ , angular distance $\Delta R$ , and rapidity variables $y_B $ distributions of the normalized $b\bar{b}$ dijet production at the LHC 8.8 TeV. We find the energy loss of the $b\bar{b}$ dijet will suppress and broden the near side(small $\Delta \phi$)peak and also enhance and sharp the away side (near $\Delta \phi=\pi$ ) peak. We have also calculated the distribution of transverse momentum $p_T$ of $b\bar{b}$ dijet, the transverse momentum imbalance $X_j$ of back-to-back $b\bar{b}$ dijet pairs and the flavour asymmetry $A_b$ of mixed-flavour dijet pairs to gain new insight into heavy flavour dynamics in the quark-gluon plasma.
Presented is a feasibility study of hypernuclei mesurments for the upcoming NICA/MPD Project. The DCM-LAQGSM model was used as well as the full realistic MPD reconstruction chain. Presented here are invaraint mass spectra for three decay modes. A good resolution with 3 MeV/c$^{2}$ was achieved.
The relativistic heavy ion collisions undergo extremely hot and dense phases, which are postulated to resemble parts of the cosmological early stages. This suggests that the collisions could provide a QCD laboratory, in which phenomena of strong interactions are studied. The investigations of colour interactions in the collisions are made in a Monte-Carlo computational model which implements dynamical interactions. The dynamics are in present work modelled as a superposition of parameterized hydrodynamics and a media-modulated hard state. In the simulations, observations are differentiated in terms of density, position, and production modes; in combination with a higher order analysis. The heavy ion yield appears to have been reproduced to great detail in present model. The reproduction of both elliptic-, and triangular flow speaks of a fluctuating-, and density-characterized first order geometric mode, in addition to higher order features. In the simulations, it seems that the particle fragmentation is density characterized, thus providing a channel for pressure differentiated observations. Therefore it is concluded that the present computational model is reproducing the heavy ion yield to higher orders, thus supporting observables which differentiates strong phenomena within the simulated evolving matter.
A charge-sensitive in-event correlator ($R(\Delta S)$) is proposed and tested for its efficacy to detect and characterize charge separation associated with the Chiral Magnetic Effect (CME) in heavy ion collisions~[1]. For CME-driven charge separation, the correlator gives a concave response relative to the second-order event plane ($\Psi_2$), and a null response relative to the third-order plane ($\Psi_3$), consistent with the correlation (de-correlation) of the $\vec{B}$-field with the $\Psi_2$ ($\Psi_3$) plane. For non-CME background, the correlator gives responses, relative to $\Psi_2$ and $\Psi_3$, which allows a distinction between CME-driven charge separation and non-CME backgrounds. We discuss the $R(\Delta S)$ correlator and present results for its detailed response and sensitivity, to both signal and background, in several reaction models. They include (but are not limited to) a 3+1-dimensional hydrodynamic model [2], the Anomalous Viscous Fluid Dynamics (AVFD) model [3] and the Multi-Phase Transport model (AMPT) [4]. The implications for the use of the $R(\Delta S)$ correlator in the upcoming Isobar Run at RHIC will also be discussed.
[1] N. Magdy, S. Shi, J. Liao, N. Ajitanand, and R. A. Lacey, arXiv:1710.01717
[2] P. Bozek, arXiv:1711.02563
[3] S. Shi, Y. Jiang, E. Lilleskov, J. Liao, arXiv:1711.02496
[4] Lin Z W et al. Phys.Rev. C72 064901 (2005)
The NA61/SHINE experiment at the CERN SPS experiment is planning to upgrade the detector and extend the heavy-ion programme after 2020 to allow precise measurements of particles with short lifetime (charmed particles in particular).
The study of heavy flavour production is a sensitive tool for new detailed investigations of the properties of hot and dense matter formed in nucleus-nucleus collisions. In particular, it offers new possibilities for studies of such phenomena as in-medium parton energy loss and quarkonium dissociation and possible regeneration, thus providing new information to probe deconfinement. Recently a silicon Small-Acceptance Vertex Detector was installed to measure production of open charm mesons. It is planned to significantly expand the vertex detector with more sensors both in longitudinal and transverse directions, as well as tho increase by an order of magnitude the read-out rate. In addition to improve measurements of open charm particles, the larger size will also significantly increase the capabilities to reconstruct the secondary vertices of relatively long-lived (multi strange) particles that decay inside the detector.
The physics motivation as well as the proposed detector layout simulations and hardware implementation will be discussed.
We show that certain ideas developed in the last few years of heavy ion physics research could be used to produce key features of the standard cosmological model, in the context of a beyond the standard model pure gauge theory with a high (~TeV) equivalent of the QCD scale.
In particular, the peak in bulk viscosity argued to exist within QCD [1,2] can be used to generate inflation, while glueballs within the same theory are viable dark matter candidates.
We present solutions of the FRW equations for matter with these characteristics, in the hope of establishing weather the right number of e-foldings and dark matter abundance can be generated in such a model.
[1] https://inspirehep.net/search?p=find+eprint+0805.0442
[2] https://inspirehep.net/search?p=find+eprint+0707.4405
DIRC-like Time-of-Flight detector (DTOF) is an innovative TOF utilizing internally reflected Cherenkov light for high energy charged particle identification. It achieves a high level of performance at the extreme data taking conditions under high luminosity and high backgrounds. The basic structure of DTOF is composed of a Fused Silica radiator connected to fast photomultiplier (MCP-PMT or SiPM) array, readout by a dedicated front-end electronics. The challenge comes from the fact that the limited flight length for charged particles in detectors requires its timing measurement achieves the level of tens of pico seconds, summing up all uncertainties, to meet the requirement for current and future high luminosity particle colliders. For example, the high luminosity upgrade of the Large Hadron Collider (HL-LHC) at CERN is expected to provide instantaneous luminosities of 1034cm-2s-1 and above, which requires TOF's time resolution ~30ps to suppress pileup in collisions.
To meet these requirements, we developed a pico-second timing TOF prototype based on DTOF technology, using 1.5cm x 1.5cm x 2cm square Cherenkov radiator connected by high-resolution fast MCP-PMT(Hamamastu R3809U). And we also developed high speed readout electronics (Bandwidth 2.4GHz, Gain 6dB ~ 26.6dB): Programmable Differential Amplifier(PDA) LMH6881/2 and Dual-threshold Differential Discriminator(DDD). It applied dual thresholds to process signals: low threshold to measure signals’ arrival time, while high threshold to identify real signals and exclude noise. It is a much simpler and faster timing method than wave sampling. The beam test in H4 at CERN shows that: the intrinsic time resolution readout by DDD and PDA is <20ps. Since there is no tracking selection and the readout electronics (PDA and DDD) are preliminary designation, there is still potential to improve its performance. We also plan to test larger radiator and various MCP-PMTs and SiPMs in the future.
THERMINATOR model [1] is a Monte Carlo event generator invented to study the statistical production of particles created in relativistic heavy-ion collisions. Its current description allows one to study the highest collision energies achieved by LHC and RHIC colliders. However it is possible to adapt THERMINATOR model to the lower energy spectrum as is used in Beam Energy Scan (BES) program at RHIC.
Femtoscopy of two particles investigates the properties of matter produced in heavy-ion collisions. Two-particle correlations use Quantum Statistics and the Final State Interactions which allow one to examinate the space-time characteristics of the medium.
For the first time we present single- and two-particle momentum distributions of identified particles generated for the energy spectrum from BES program. To verify how model predictions agree with experimental results, we compare transverse momenum distributions of pions, kaons and protons and the correlation functions obtained for identical pions in Au+Au collisions to the results from the STAR experiment from BES program.
[1] Comput.Phys.Commun. 183 (2012) 746-773
We compute the suppression, angular, and rapidity distribution of single open heavy flavour and the momentum, angular, and rapidity correlations for pairs of open heavy flavour in pA and AA collisions at RHIC and LHC from an AdS/CFT-based energy loss model. We quantitatively compare the strongly-coupled QGP predictions for AA collisions to the weakly-coupled QGP predictions of Nahrgang et al.
When restricted to leading order production processes, we predict similar angular correlations for open heavy flavour pairs in a strongly coupled plasma and a weakly coupled plasma, but with an order of magnitude difference in the low momentum momentum correlations. We find that the difference in momentum correlations from the AdS model compared to the pQCD model is, surprisingly, due to tighter momentum correlations in the AdS model than the pQCD model.
When initialised at next-to-leading order (aMC@NLO matched to Herwig++), we observe significant additional broadening of azimuthal correlations. However, the momentum correlations remain even when NLO production mechanisms are included. Thus, our conclusion for differences in momentum correlations with leading order production processes should carry over to next-to-leading order production processes once comparable predictions for a weakly-coupled QGP emerge.
We then show quantitative agreement between all measured high momentum open heavy flavour observables in AA collisions at RHIC & LHC and predictions from this new NLO production plus AdS energy loss model using a recent AdS derivation for the spectrum of momentum fluctuations. Finally, we present first results from this model for open heavy flavour observables in p+A collisions.
The initial conditions and particle emission in proton-proton collisions is much better constrained than in heavy-ion collisions. This allows for a precise investigation of the interaction between pairs of produced baryons such as proton-$\Lambda$ and $\Lambda$-$\Lambda$ in this system.
In this analysis femtoscopic correlations of proton-proton, proton-$\Lambda$ and $\Lambda$-$\Lambda$ pairs have been studied for the first time in pp collisions at $\sqrt{s}$=7 TeV and 13 TeV recorded with the ALICE detector.
A new formalism to separate the background contributions from the genuine correlation arising from the baryon-baryon interaction was developed. The measured correlations were fit with the parametrization obtained by the "Correlation Analysis Tool using the Schrödinger Equation (CATS)" .
The sensitivity to different interaction potentials of the proton-$\Lambda$ and $\Lambda$-$\Lambda$ correlation function is investigated and a comparison to previous measurements by the STAR collaboration is presented.
Supervised learning with a deep convolutional neural network (CNN) is used to identify the QCD equation of state (EoS) employed in event-by-event (2+1)-D relativistic viscous hydrodynamics coupled to a hadronic cascade afterburner" simulations of heavy-ion collisions from the simulated final-state pion spectra $\rho(p_T, \phi)$. High-level correlations of $\rho(p_T,\phi)$ are learned by the neural network, which acts as an effective
EoS-meter" in distinguishing the nature of the QCD transition. The EoS-meter is robust against many simulation inputs, such as shear viscosity, freeze-out temperature, equilibration time and collision energy. Thus the EoS-meter provides a powerful tool as the direct connection of heavy-ion collision observables with the bulk properties of QCD.
Recent measurements of charge-dependent azimuthal correlations in high-energy heavy-ion collisions at RHIC and the LHC have indicated charge-separation signals perpendicular to the reaction plane, and have been related to the chiral magnetic effect (CME) (see a review in Ref [1]). The discovery of this phenomenon in heavy-ion collisions will signify simultaneously three important physics ingredients: the strongest magnetic field ever made by mankind, the chirality imbalance caused by vacuum transition, and the chiral symmetry restoration in the deconfined nuclear matter. However, the correlation signal is contaminated with the background driven by the elliptic flow ($v_2$) of the collision system [2], and an effective approach is needed to remove the flow background from the correlation.
In this talk, we will disclose a few shortcomings of a previous attempt of the event shape engineering (ESE) based on the "event-by-event $v_2$" [3]. We will further present a novel ESE technique [4] utilizing the magnitude of the flow vector to select spherical events in heavy-ion collisions, which leaves the charge separation measurements free of flow contributions. The simplified Monte Carlo simulations and a multi-phase transport model (AMPT) are employed to develop the ESE scheme to reveal the true CME signals from the experimental observation. Caveats regarding artificial effects and extreme conditions in this method will also be discussed.
References
1. D.E. Kharzeev et al., Prog. Part. Nucl. Phys. 88 (2016) 1.
2. A. Bzdak et al., Phys. Rev. C 83 (2011) 014905 .
3. L. Adamczyk et al., [STAR Collaboration], Phys. Rev. C 89 (2014) 044908.
4. F. Wen et al., Chinese Phys. C 42(1) (2018) 014001 [arXiv:1608.03205].
The chiral magnetic effect (CME) and the chiral magnetic wave (CMW) have been predicted to arise from the coupling of domains with quark chirality imbalances in the quark-gluon plasma (QGP) and the strong magnetic field produced by energetic spectator protons. Searches for these quark chirality effects in nucleus-nucleus collisions have been performed at RHIC and the LHC as major scientific goals. For example, the RHIC 2018 run will be devoted to the isobaric collisions of $^{96}$Ru+$^{96}$Ru and $^{96}$Zr+$^{96}$Zr at √s$_{NN}$ = 200 GeV, where one may expect an up-to-20% difference in the experimental observables related to the magnetic-field-induced effects.
Current data indicate that the experimental sensitivity to the chirality effects also depends on the beam energy and the colliding system size, presumably owing to variations in the magnetic field and/or the size of the QGP droplets. Therefore, another venue to enhance the experimental sensitivity could be Au+Au collisions at lower beam energies. We will demonstrate with the AMPT simulations that the number of net protons (N$_{\mathrm{net-p}}$) at mid-rapidity is anti-correlated with the number of spectator protons, and hence provides an excellent handle on the magnetic field from spectator protons in Au+Au collisions at lower RHIC beam energies. Equipped with the event-shape engineering technique [1], the search for chirality effects by varying N$_{\mathrm{net-p}}$ in Au+Au collisions at lower energies (with √s$_{NN}$ still higher than 10 GeV) will complement the isobaric collision data. The future RHIC Beam Energy Scan II program will facilitate the application of our method and discern the true contribution due to the quark chirality effects.
[1] F. Wen et al., Chinese Phys C 42(1) (2018) 014001 [arXiv:1608.03205].
More than 15 years ago a longitudinal effective string rope model was proposed [1] to construct nucleus-nucleus collision Initial State (IS) for realistic 3+1D relativistic fluid dynamical models. This model reflected correctly not only the energy-momentum, but also angular momentum conservation, initial shear flow, and local vorticity [2]. Recent experimental and theoretical developments indicate that angular momentum, local vorticity and the subsequent particle polarization is observable and provides valuable information: a significant Λ polarization was detected and analyzed in detail in the RHIC BES program [3].
On the other hand, recent developments in parton kinetic and field dominance models provide a rather different initial state configuration, more compact for non-central collisions, see for example [4], what makes us revisit the early IS model [1] in that direction.
We will present a new initial state model for hydrodynamical simulation of relativistic heavy ion collisions, which is based on Bjorken-like solution applied streak by streak in the transverse plane [5]. The proposed model satisfies all the conservation laws including conservation of a strong initial angular momentum which is present in non-central collisions. As a consequence of this large initial angular momentum we observe the fluid shear in the IS, which leads to large flow vorticity. Another advantage of the proposed model is that the initial state can be given in both [t,x,y,z] and [τ, x, y,η] coordinates.
[1] V.K. Magas, L.P. Csernai, D.D. Strottman, Phys. Rev. C64 (2001) 014901; Nucl. Phys. A712 (2002) 167.
[2] L.P. Csernai, V.K. Magas, D.J. Wang, Phys. Rev. C87 (2013) 034906.
[3] L. Adamczyk et al. (The STAR Collaboration), Nature 548 (2017) 62.
[4] L.G. Pang, H. Petersen, G.Y. Qin, V. Roy and X.N. Wang, Nucl. Phys. A956 (2016) 272.
[5] V.K. Magas, J. Gordillo, D.D. Strottman, Y.L. Xie and L.P. Csernai, arXiv:1712.00283 [nucl-th].
Energetic heavy quarks passing through the hot and dense medium of a quark-gluon plasma (QGP), represented by the resulting mesons, are viewed as a suitable probe for the interactions inside of the QGP, in particular the mechanisms of energy loss, as they are less likely to thermalize within the medium and are mostly created at early stages of the medium evolution.
However, models of both, purely collisional energy loss as well as combinations of collisional and radiative energy loss are equally successful for reproducing the nuclear modification factor $R_{AA}$ and the elliptic flow $v_2$ [1]. In an attempt to discriminate between the two different energy-loss mechanisms,an alternative observable, the angular correlations between two mesons were investigated. Azimuthal correlations between pairs of heavy mesons, like $D$-$\bar{D}$ pairs, allow for distinguishing the energy-loss scenarios [2].
We continue these studies by investigating the angular correlations between pairs of heavy and light mesons ($D$ and $\pi$), originating from a heavy quark jet. This is motivated by the fact that the emitted gluon in radiative collisions hadronizes and these hadrons are correlated to the emitting heavy quark.
We created a Monte-Carlo code for the parton splitting in the vacuum together with an effective medium model. This program represents a consistent framework to study the influences of either collisional or radiative processes on parton propagation, and the resulting two-particle correlations. In order to learn at which stages of their space-time evolution jets are affected the most by interactions with the medium we studied contributions to angular correlations from different jet topologies.
[1] P. B. Gossiaux, J. Aichelin, T. Gousset and V. Guiho,
J. Phys. G 37 (2010) 094019
[arXiv:1001.4166 [hep-ph]].
[2] M. Nahrgang, J. Aichelin, P. B. Gossiaux and K. Werner,
J. Phys. Conf. Ser. 509 (2014) 012047
[arXiv:1310.2218 [hep-ph]].
Due to the different energy scales involved in the production of charmonium states in proton-proton collisions, they provide important testing grounds for the theory of Quantum Chromo-Dynamics (QCD). The initial charm-quark pairs are produced in large-$Q^2$ processes that allow for a perturbative treatment while the hadronization into a bound system is non-perturbative.
Different effective theories for the description of charmonium production exist, like the Color Singlet Model, the Color Evaporation Model or the non-relativistic QCD approach. However, none of them describes the production cross-sections and the polarization simultaneously. The correlations of J$/\psi$ mesons and hadrons can provide constraints on the color-singlet or color-octet nature of the pre-resonant charmonium state by measuring the amount of hadronic activity in the vicinity of the J$/\psi$.
In this poster, preliminary ALICE results on the angular correlations between inclusive J$/\psi$ mesons and charged hadrons at mid-rapidity in pp collisions at $\sqrt{s} = 13~\text{TeV}$ will be shown. The high multiplicity triggered data taken by ALICE in Run-2 allows for measurements in high multiplicity events, in addition to the analysis of minimum-bias data. Projections for the LHC Run-3 and Run-4 will also be reported. Our measurements will be compared to model calculations.
The angular correlation function (CF) refers to the correlation of particles in the relative pseudorapidity ($\Delta\eta$) and relative azimuthal angle ($\Delta\phi$). CF is influenced by various physical phenomena such as conservation laws, collective particle flow, resonance decays, final state interactions, or particle production mechanism - e.g., correlation of particles within the single jet. By analysis of long-range correlations (of pairs with $\Delta\eta \geq$1.0) it is possible to access the early stage of the system created during heavy-ion collision and its longitudinal and azimuthal dynamics.
The STAR Beam Energy Scan data allows one to perform a detailed CF analysis to investigate the phase diagram of strongly interacting matter. Recently [1] STAR reported an angular correlation measurements of $\pi$-$\pi$, K-K, and p-p pairs with $\Delta \eta \leq$1 in 0-5% central Au+Au collision at $\sqrt{s_{NN}}$ = 7.7-200 GeV. These results show a significant difference between CF of given particle species combinations.
In this poster, we extend this results to $\Delta \eta \leq\;$2, which allows for analysis of long-range correlations of $\pi$-$\pi$, K-K, and p-p pairs. We describe the data by fitting a multi-component function. Such an approach allows for disentanglement of various correlation sources. The study is conducted in nine centrality classes (70-80%, 60-70%, 50-60%, 40-50%, 30-40%, 20-30%, 10-20%, 5-10% and 0-5%) of Au+Au collisions at $\sqrt{s_{NN}}$ between 7.7 and 200 GeV. The collision energy and centrality dependence of the best fit-to-data function parameters, describing short- and long-range correlations, will be presented.
Reference
[1] S. Jowzaee (for the STAR Collaboration), Nucl. Phys. A967, 792 (2017).
Anisotropic flow plays a crucial role to characterize the momentum anisotropy of the final state particles. In order to probe the properties of the system created in high multiplicity pp collisions at LHC energies, we study within the percolation color sources, the effects of initial state geometry, profile distribution, size and eccentricity fluctuations in pp collisions at the LHC energies. The results on the higher harmonic flow modes shown how the initial state geometry and the density of color string sources vary significantly the contribution of the higher flow modes (vn) due to the size effect. The correlation of the higher flow harmonics with the corresponding η/s is compared with hydrodynamic calculations.
Anisotropic flow at SPS energies was measured by the NA49 Collaboration more than 10 years ago. Recently new data for Pb-Pb collisions were collected by the NA61/SHINE experiment during the Pb-ion beam energy scan program at the SPS. This motivated a new analysis of the available NA49 data, based on modern flow measurement techniques that will also utilize the spectator fragments for reaction plane determination.
The new results on directed and elliptic flow in Pb-Pb collisions at beam energy of 40 A GeV recorded with the fixed target experiment NA49 at CERN SPS are presented. Event classification is based on the multiplicity of produced particles as well as on the energy of the projectile spectators using the procedure implemented within the Centrality Framework developed for the future CBM experiment at FAIR. To account for the azimuthal asymmetry of the fixed target setup of the NA49 experiment, a three-subevent technique is used for the determination of the reaction plane resolution. The reaction plane is estimated both from the azimuthal asymmetry of the produced particles measured with the NA49 TPCs as well as by using the transverse granularity of the NA49 forward VETO calorimeter. Corrections for the detector acceptance anisotropy in the transverse plane are applied using an extension of the Qn-Corrections Framework developed originally for the ALICE experiment at the LHC.
The results are compared with those previously obtained by the experiments STAR at RHIC and NA49 at the SPS. The new study is complementary to the ongoing analysis of the recently collected Pb-Pb data of the NA61/SHINE experiment at the CERN SPS and provide an important reference for the performance investigations for the future CBM experiment at FAIR.
Anisotropic flow plays a critical role in understanding the properties of the quark- gluon plasma. In this poster we present the elliptic and triangular flow of multi-strange particles in Pb--Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV. The measurements are presented at mid-rapidity for a wide range of particle transverse momenta. The results are compared to those for elliptic and triangular flow for other identified hadrons and measurements for Pb-Pb collisions at lower energy.
We consider accelerated and rotating media of weakly interacting fermions in local thermodynamic equilibrium on the basis of kinetic approach. Kinetic properties of such media can be described by covariant Wigner function calculated on the basis of relativistic distribution function of particles with spin. We obtain the formulae for axial current by summation of the terms of all the orders over thermal vorticity tensor, chemical potential and temperature arising from the distribution functions under consideration, both for massive and massless particles and calculate axial current divergence. We show, that in the massless limit all the terms, since the fourth order over vorticity and third order over chemical potential and temperature equal zero. It is shown, that axial current gets a topological component along the 4-acceleration vector. The similarity between different approaches to baryon polarisation is established.
J/$\psi$ mesons and other hadrons containing a charm or a beauty quark are
excellent probes to study the Quark-Gluon Plasma (QGP) produced under
extreme temperature and energy density conditions in heavy-ion collisions. Because of their large mass,
heavy quarks are produced in hard parton-scattering processes at the
beginning of the collisions and they are therefore present in the QGP
during all stages of its evolution.
At mid-rapidity ($|y|<0.8$), ALICE can reconstruct J/$\psi$ mesons via their decay
into the dielectron channel, down to zero transverse momentum $p_{\rm T}$. However,
particularly at very low $p_{\rm T}$ and in central collisions, the measurement is
limited by the low signal to background ratio. Increasing the significance of the
measurement in the low $p_{\rm T}$ region is extremely important for several reasons:
First, in the study of prompt J/$\psi$ production, higher precision will
shed light on the interplay between J/$\psi$ dissociation and regeneration.
Second,
the non-prompt J/$\psi$ analysis can give access to low $p_{\rm T}$ beauty
measurements.
Using multivariate methods helps to reduce the background and increase the
significance while keeping as much signal as possible.
A study of the multivariate methods with data from Pb--Pb collisions at
$\sqrt{s_{\rm NN}}=5.02$ TeV will be presented in this poster. Different
choices of training variables were tested, with respect to background
rejection and good stability of the efficiency corrections.
Averaged jet charge characterizes the electric charge distribution inside jets, and provides a powerfull tool to distinguish quark jets from gluon jets. In this talk, we give the first prediction for the medium nodification of averaged jet charge in heavy-ion collision at the LHC energy, where the jet productions in $pp$ collisions are simulated by pythia6+FastJet, and parton energy loss effects in the QGP are calculated with two Monte Carlo codes of jet quenching: PYQUEN and JEWEL. In $pp$ collisions, the value of jet charge for quark jets goes up with increasing jet transverse momentum, while for gluon jets it approximates to zero in the whole range of $p_{T}$ because gluon carries no electric charge. It is shown that the distribution of averaged jet charge is significantly suppressed by initial state nuclear effects due to the participants of neutrons with zero electric charge during nuclear collisions. Considerable enhancement of averaged jet charge in central $PbPb$ collisions relative to peripheral collisions is observed, since jet quenching effect is more pronounced in central collisions. In the jet quenching calculations, a fast gluon will lose more energy in the QGP than a fast quark due to its large color-charge($\Delta E_{g}/\Delta E_{q}=C_{A}/C_{F}=9/4$), more quarks with electric charge may survive in central collisions as compared with that in peripheral $PbPb$ collisions. The fraction of quark jet will be increased, which results in the larger value of averaged jet charge in central collisions than that in peripheral reactions. Distinct feature of averaged jet charge between quark and gluon jets, together with the sensitivity of jet charge alternations to flavour dependence of parton energy loss, could be very useful to discriminate the energy loss pattern between quark and gluon jets in heavy-ion collisions.
Heavy quark yields is a powerful tool to study the quark gluon plasma (QGP)
created in high energy heavy ion collisions.
PHENIX separated electrons from the charm and bottom decays by measuring the distance of the closest approach with the silicon vertex detector, and found the suppression of bottom quarks is smaller than that of charm quarks at low $p_T$. Heavy quark measurements also show a strong coupling with the QGP medium. It is important to measure the the flow of bottoms and charms separately.
In this poster, we present the analysis method and current status of b$\rightarrow$e and c$\rightarrow$e flow measurement at mid-rapidity in Au+Au 200GeV collisions with the PHENIX detector
PHENIX measured two-particle angular correlations between high $p_T$ ($2 < p_T
< 11 $ GeV/c) $\pi^{0}$ at midrapidity $|\eta| < 0.3 $ and hadrons emitted at
forward $(3.1<\eta<3.9)$ or backward $(-3.7<\eta<-3.1)$ rapidity in 200 GeV
p+p and d+Au collisions at $\sqrt{s_{NN}}$=200 GeV. In the Au-going direction the azimuthal correlations of these particle pairs with this large rapidity gap exhibit a ridge-like structure. This structure persists up to $p_{T} \approx 6$ GeV/c and strongly depends on collision centrality. It is reminiscent of collective behavior in A+A collisions. The ridge-like structure is absent in the d-going direction as well as in p+p collisions.
We present the final results of two particle correlations with a large rapidity
gap in 200 GeV d+Au and p+p collisions and discuss the physics implication of
the results.
The ALICE experiment at the Large Hadron Collider (LHC) is dedicated to study the properties of the Quark-Gluon Plasma (QGP), a de-confined state of strongly-interacting partons formed in relativistic heavy-ion collisions. Heavy quarks, produced by parton-parton hard scatterings in the early stages of such collisions, stand out as unique probe to study the QGP, as they are expected to experience the whole evolution of the system formed in the collision.\
The azimuthal correlations between heavy-flavour particles and charged particles give insight on the modification of charm-jet properties in nucleus-nucleus collisions and the mechanisms through which heavy quarks lose energy inside the medium. Studies in pp collisions are necessary as a reference for nucleus-nucleus collisions and also important for testing expectations from pQCD-inspired Monte Carlo generators. In this contribution, we will present the first study of azimuthal correlations of $D^{0}$ mesons with charged particles in pp collisions at $\sqrt{s}=13$ TeV, so far the highest available energy at the LHC, performed with the ALICE apparatus. A comparison with pp collisions results at $\sqrt{s}=7$ TeV will help to evaluate the energy dependence of the correlation function. Data will also be compared with expectations from POWHEG and PYTHIA event generators.
In the first part of the talk we shall investigate how the averaging
over a large number of events influences the shape of the observed
correlation function. We demonstrate that a shape characterised by Levy
distribution may result from an average over Gaussian sources with
varying sizes and orientations. We then propose to sort the events
according to their similarity and investigate azimuthal dependence of
the correlation radii on events classes which differ in shape. The
method is explained and demonstrated on events simulated with different
event generators.
To understand the dynamics of cluster formation, starting from homogeneous distribution, we set up classical molecular dynamics simulation of the baryon motion, supplemented by a Langevin equation to model the effect of a meson heat bath. Quantum mechanical kinetic energy is included via an effective potential, tuned to reproduce known properties of nuclear matter. We then modify the inter-nucleon potential, in a way expected in the vicinity of the hypothetical QCD critical point, and calculate the rate of baryon cluster formation. Then we compare it with the time available in real heavy-ion collisions.
Interaction cross-sections for baryon pairs are of fundamental interest
and they are actively investigated theoretically. They are known well for
pairs of common (anti-)baryons, however there is a lack of precise
data for heavier baryons, including the ones carrying strangeness. The
so-called kaonic atoms are also investigated theoretically and their
properties crucially depend on the kaon-nucleon interaction. The
two-particle correlation formalism (femtoscopy) is sensitive to the
interaction kernel for a pair of particles, which is related to the
pair interaction cross-section [1]. The formalism is extensively used
to measure two-particle correlations in heavy-ion collisions. In
particular the collisions at RHIC and LHC produce simultaneously large
number of baryons and anti-baryons and even larger number of kaons. We
show how this formalism can be used to extract the cross-sections from
the femtoscopic baryon-(anti-)baryon correlation functions [2], as
well as from proton-charged kaon functions. The analysis is
complicated by the presence of the so-called "residual correlations"
arising from weak decay products in the measured sample. We show how
this effect can be exploited to gain further insight into the
cross-sections of even heavier baryons. We discuss the limitations of
the measurement technique and estimate the discovery potential of
currently available and soon-to-be-collected heavy-ion collision
datasets at RHIC and at the LHC.
[1] A. Kisiel, H. Zbroszczyk, M. Szymanski; "Extracting
baryon-antibaryon strong interaction potentials from pÎÂ¯ femtoscopic
correlation functions"; Phys.Rev. C89 (2014) 5, 054916
[2] R. Lednicky, V.L. Lyuboshits; "Final State Interaction Effect on
Pairing Correlations Between Particles with Small Relative Momenta";
Sov.J.Nucl.Phys. 35 (1982) 770, Yad.Fiz. 35 (1981) 1316-1330
Recent years have seen significant theoretical progress in the transport description of open heavy flavor in QCD matter -- a number of models are now able to simultaneously describe a subset of the most important heavy flavor observables -- a simultaneous description of a comprehensive set of observables at all available collision energies still poses a challenge. A global analysis encompassing all available collision systems and energies as well as an improved treatment of known uncertainties for different observables would significantly improve our ability to distinguish between different theoretical models and constrain the heavy flavor diffusion coefficient in an unbiased way.
In this study, we show that describing the heavy quark energy loss with Langevin diffusion and a radiative energy loss component in a state-of-the-art hydrodynamic medium, including heavy meson hadronic interactions modeled with UrQMD allows for a simultaneous description of the $D$-meson $R_{\mathrm{AA}}$ and $v_n$ at both RHIC (200 GeV) and the LHC (2.76 \& 5.02 TeV) energies. The heavy flavor interaction with the quark-gluon plasma is encoded in the diffusion coefficient, which is calibrated on experimental measurements using a systematic model-to-data Bayesian analysis. The estimated diffusion coefficient $D_sT$ has a positive temperature dependence and a non-trivial momentum dependence. The constrained diffusion coefficient is validated by comparing $D$-meson $R_{\mathrm{AA}}$ in pPb collisions along with $B$-meson measurements. New observables are proposed to further constrain the diffusion coefficient.
The study of the Quark-Gluon Plasma created in ultrarelativistic heavy-ion collisions at the CERN-LHC is complemented by reference measurements in proton-lead (p--Pb) and proton-proton (pp) collisions, where the effects of multiple-parton interactions and hadronization beyond independent string fragmentation can be investigated.
In this poster, we present a Bayesian unfolding procedure
to reconstruct the correlation between transverse momentum ($p_{\mathrm{T}} $) spectra of charged particles and the corresponding charged particle multiplicities $N_{\mathrm{ch}}$.
The unfolded spectra are presented in single multiplicity ($\Delta N_{\mathrm{ch}}$ = 1) bins and are used to derive moments of the $p_{\mathrm{T}} $ distributions.
We illustrate the unfolding procedure of the $p_{\mathrm{T}} $ spectra
with MC simulations for pp collisions and compare the resulting $\langle p_{\mathrm{T}}\rangle $ of different systems (pp, p--Pb, Pb--Pb) and collision energies.
We present new differential measurements of charge separation relative to the second- ($\Psi_2$), third- ($\Psi_3$) and fourth-order ($\Psi_4$) event planes for Au+Au collisions at $\sqrt{s_{NN}}$= 200, 39, 27 and 19.6~GeV, U+U at $\sqrt{s_{NN}}$= 193 GeV and Cu+Au, Cu+Cu, d+Au and p+Au at $\sqrt{s_{NN}}$=200 GeV. The measurements are performed with a charge-sensitive correlator $R(\Delta S)$ [1] and the three-particle mixed harmonic correlator $\mathrm{C_{m,n,m+n} = \left< cos(m\phi_1 + n\phi_2 -(m+n)\phi_3)\right>}$ [2]. These are expected to give different responses to the CME-driven charge separation and non-CME background correlations. The $R(\Delta S)$ measurements are found to be flat relative to $\Psi_3$ in all systems and $\Psi_2$ in p(d)+Au systems, consistent with the expectation of random $\vec{B}$-field orientations relative to these event planes. In contrast, the heavy-ion measurements relative to $\Psi_2$ show concave-shaped $R(\Delta S)$ distributions, which is consistent with the presence of CME-driven charge separation characterized by an out-of-plane Fourier dipole coefficient $a_1$. We will present and discuss $R(\Delta S)$ and $\mathrm{C_{m,n,m+n}}$ measurements for a broad range of transverse momenta, pseudorapidity, and centrality intervals and compare it with model predictions [1]. The implication of these measurements for the upcoming isobar collisions at RHIC will also be discussed.
References
[1] N. Magdy, S. Shi, J. Liao, N. Ajitanand, and R. A. Lacey, arXiv:1710.01717.
[2] L. Adamczyk et al. (STAR Collaboration), arXiv:1701.06496.
Studies of collisions of highly accelerated ions are the key to understand the creation of quark matter. Experimental physicists put considerable effort in collecting information characterizing the various processes occurring during such collisions. In order to describe such scenarios, complex models have been constructed, one of them being the EPOS approach. It applies Parton-based Gribov-Regge theory as an initial condition, introduces the core-corona approach, hydrodynamical evolution and hadronic cascades as well. The model is used by the experimental physicist at the LHC or in cosmic ray physics.
At the Brookhaven National Laboratory, the STAR collaboration is currently investigating an interesting project called Beam Energy Scan. The QCD phase diagram is studied in order to understand the phase transitions close to the critical point, which should be in the energy domain studied in this program. Models have difficulties to describe this energy range properly. The aim of our investigation is to adapt the EPOS model to describe correctly collisions of ions with energies studied in the framework of the BES program.
The detailed description of the theory included in EPOS model will be presented. The energy dependence of the separation into the core and corona will be discussed, and the way it affects transverse momentum spectra of identified particles and the observables of the azimuthal anisotropy of expanding matter. The particles from the corona are strongly affected by the radial flow and the flow asymmetries. The results of different types of analysis of elliptic flow will be discussed. The simulation results for collisions of Au+Au at selected BES energies will be presented in comparison with the published STAR data.
Heavy quarks are unique probes to study the medium created in heavy-ion collisions. Detailed measurements of the production of bottom hadrons can supply information crucial to understanding the properties of the strongly interacting QCD matter and the flavor dependence of parton energy loss. In this poster, the measurement of transverse momentum spectra of $D^0$ from beauty-hadron decays in pp and PbPb Collisions at 5.02 TeV performed by the CMS collaboration is presented. The $D^0$ from B decay are distinguished from prompt $D^0$ by their different geometrical distributions relative to the collision point, due to the large decay length of B meson. The measured spectrum in pp collisions is compared to perturbative QCD calculations. The Nuclear Modification Factor ($R_{AA}$) of $D^0$ from B decay will also be reported, comparing with prompt $D^0$, light flavor hadrons, as well as model predictions.
Cross sections for direct photon production in hadronic scattering processes have been calculated according to an effective chiral field theory following Turbide et al. For $\ \pi + \rho \rightarrow \pi + \gamma$ and $\ \pi + \pi \rightarrow \rho + \gamma$ processes, these cross sections have been implemented into a novel hadronic transport approach (SMASH), which is suitable for collisions at low and intermediate energies. Comparisons of the obtained thermal rates in infinite matter calculations to theoretical predictions and to the ones used in hydrodynamic calculations are shown. This constitutes a benchmark for future non-equilibrium calculations. Employing SMASH for the final state rescattering in a hybrid approach will allow to assess the importance of the hadronic stage in the generation of direct photon flow.
References:
J. Weil et al, “Particle production and equilibrium properties within a new hadron transport approach for heavy-ion collisions”. In: Phys. Rev C 94 (2016) no. 5, 054905
Simon Turbide, Ralf Rapp, and Charles Gale. “Hadronic production of thermal photons”. In: Phys. Rev. C69 (2004), p. 014903
For Bjorken models with gradual freeze out, the resulting post freeze out momentum distribution practically does not depend on the layer thickness. Using such a model we calculate the pion correlation function produced in Pb+ Pb central collisions The correlation function is in qualitative agreement with other publications but our model allows us to perform a more detailed study of how this affects the final observables. We also find a good agreement with experimental data from ALICE and we deduce that long freeze out averaged correlation functions are more similar to experimental data than sharp freeze out.
We present a model of the dynamical evolution of relativistic heavy ion collisions, which combines second-order viscous hydrodynamics and microscopic transport. In particular, we present a hybrid approach with MUSIC hydrodynamics, particlization with improved treatment of resonance masses based on spectral functions, and SMASH (Simulating Many Accelerated Strongly-interacting Hadrons) afterburner. In this work, we focus on low-$p_T$ hadronic observables --- identified hadron $p_T$ spectra and anisotropic flow coefficients. Given that the low-$p_T$ bulk dynamics in hadronic re-scattering is dominated by resonance excitations and decays, it is expected that implementation of mass sampling at the particlization and better treatment of resonances in microscopic transport play important roles in the late stage of heavy ion collisions. This motivates us to compare other hybrid approaches, such as MUSIC+UrQMD, to MUSIC+SMASH hybrid with an emphasis put on the importance of mass sampling at the particlization and effect of hadronic re-scatterings.
Baryon stopping, experimentally established by the changing shape of net-proton rapidity distributions as a function of beam energy, is still lacking a proper theoretical understanding. In this work, baryon stopping in heavy ion collisions is investigated. In a hadronic transport approach the colliding nucleons form a string, which fragments, producing new hadrons. From the comparison with data, it is possible to fix parameters of the string model (for example the formation time of the produced hadrons) and to find out whether baryon stopping can be described within the string model or other mechanisms are needed.
We study the interplay of the fugacity expansion for the Grand Canonical Partition Function, and the Taylor and virial expansion for the number density. We compare results from the Vladivostok group lattice QCD study [1], and from a toy model of QCD with the predictions of a Cluster Model Expansion. We outline different strategies for the search of singularities in the complex chemical potential plane, including a possible QCD critical point for real chemical potential.
[1] V.G.Bornyakov, D.L.Boyda, V.A.Goy, A.V.Molochkov, A.Nakamura, A.A.Nikolaev and V.I.Zakharov,
Phys.Rev.D95(2017)9,094506
We present a relativistic causal description of conserved-charge diffusion for heavy-ion collisions and show that it produces measurable effects in observables such as the charge balance functions. Other descriptions, based on ordinary diffusion, are known to produce charge fluctuations which propagate with infinite velocity, thus violating a fundamental postulate of special relativity. We present an alternative approach based on Cattaneo diffusion which restores relativistic causality, and show how to generalize this approach to dynamical, rapidly evolving systems such as heavy-ion collisions. We demonstrate that this approach leads to measurable consequences for the balance functions constructed from electrically charged hadrons in a simple 1+1 dimensional Bjorken hydrodynamic model. We find that limiting the speed of propagation of charge fluctuations increases the height and reduces the width of these balance functions when plotted versus separation in rapidity. We conclude by estimating the numerical value of the associated diffusion time constant from AdS/CFT.
Dissipative relativistic fluid dynamics is not always causal. We discuss the causality structure of high energy nuclear collisions. When the fluid evolution equations are hyperbolic, one can bring them to a characteristic form describing the radial expansion of the fireball. This dynamics is causal if the characteristic velocities are smaller than the speed of light such that the domain of dependence of a space-time point is within its past light cone. Within the second order theory of Denicol, Niemi, Molnar and Rischke, we obtain a concrete inequality from this constraint and discuss how it can be violated for certain initial conditions sometimes used in phenomenological studies. We argue that causality poses an important bound to the applicability of relativistic fluid dynamics in particular at very early times.
Due to their large masses, heavy quarks are considered to be an excellent probe to study the properties of the quark gluon plasma through their interactions with the medium. In this presentation, we report on improved measurements, achieved by using supervised machine learning technique, of $D^0$-meson and $D^{\pm}$-meson transverse momentum ($p_{\rm T}$) spectra at mid-rapidity ($|y|<$1) in Au+Au collisions at $\sqrt{s_{_{\rm NN}}}$ = 200$\ $GeV. The data were taken in 2014 by the STAR experiment with the Heavy Flavor Tracker, a high resolution silicon vertex detector. $D^0$ and $D^{\pm}$ mesons are measured through their hadronic decay channels, $D^0\rightarrow K^-+\pi^+$ and $D^{\pm}\rightarrow K^\mp+\pi^\pm+\pi^\pm$, via topological reconstruction of the secondary decay vertices. After being corrected for the detector acceptance, tracking and topological cut efficiencies, invariant yields of $D^0$ and $D^{\pm}$ mesons are presented in various centrality intervals covering a wide transverse momentum region (0 $<$ $p_T$ $<$ 10 GeV/$c$). The charmed hadron freeze-out properties and radial collectivity are discussed within the Blast-Wave model. Nuclear modification factors ($R_{\rm CP}$, $R_{\rm AA}$) in various centrality bins are calculated and compared to phenomenological model calculations.
The goal of relativistic heavy ion collider experiments is to explore the properties of the strongly interacting matter produced with very high temperature and energy density, conditions under which the formation of a Quark-Gluon Plasma (QGP) is expected. Heavy quarks, i.e. charm and beauty are sensitive probes of the QGP as they are produced in the initial stages of the collision and witness the entire evolution of the system. Measurements in p-Pb collisions help understanding Cold Nuclear Matter (CNM) effects such as the modification of the nuclear Parton Distribution Function (nPDF) with respect to the expectation from proton PDF, parton momentum ($k_{\rm T}$) broadening from soft scattering processes and parton energy loss in nuclear matter. Studies of heavy-flavour production in different centrality intervals can provide information on the dependence of CNM effects on the collision geometry and on the density of final-state particles. In addition, these allow us to study the interplay between the hard and soft processes in heavy-flavour production. In this contribution, we will present the $p_{T}$-differential cross-section of heavy-flavour decay electrons for different centrality intervals in p-Pb collisions at $\sqrt{s_{NN}}$ = 5.02 TeV at mid-rapidity ($|\eta| <$ 0.8). The Time Projection Chamber (TPC) was used to identify the electrons in 2 $< p_{T} <$ 8 GeV/c and the Electromagnetic Calorimeter (EMCal) was used to extend the $p_{T}$ range up to 16 GeV/c. The nuclear modification factor, $Q_{pPb}$, ratio of transverse momentum spectra in p-Pb collisions and the corresponding cross section in pp collisions scaled by the nuclear overlap function, and the central-to-peripheral ratio $Q_{cp}$, ratio of transverse momentum spectra measured at central collisions to that from peripheral collisions, scaled by the corresponding nuclear overlap functions, will be presented for different centrality intervals of p-Pb events.
The ALICE experiment at the LHC is designed to investigate the properties of the Quark-Gluon Plasma by studying high-energy pp, p-Pb, Pb-Pb and also in the recently for the first time recorded Xe-Xe collisions. Medium effects like parton energy loss can be examined by measuring the production of charged particles and their nuclear modification factor at high transverse momentum ($\textit{p}_{\text{T}}$).
In this poster, we present the analysis of $\textit{p}_{\text{T}}$-spectra for primary charged particles as well as the nuclear modification factor ($R_{\text{AA}}$) in Xe-Xe collisions at $\sqrt{s_{\text{NN}}}=$ 5.44 TeV measured with ALICE. In particular, we focus on a comparision of the nuclear modification factor in Pb-Pb and Xe-Xe collisions to investigate a possible system size dependence of $R_{\text{AA}}$.
The Event Plane Detector (EPD) is an upgrade to the STAR experiment that will significantly improve event plane resolution and provide a measure of collision centrality at forward rapidity ($2.1<|\eta|<5.1$). The complete detector, composed of two scintillator wheels at $\pm\eta$ and 2.1$<$$|\eta|$$<$5.1, will be operational in the 2018 run, but in 2017, a quarter of one wheel was commissioned. Results from this run including the partial EPD from Au+Au collisions at $\sqrt{\rm s_{\rm NN}}$$=$54 GeV will be presented. The track densities at this energy are considerably higher than those expected for RHIC Beam Energy Scan energies ($\sqrt{\rm s_{\rm NN}}$$\leq$20 GeV) for which the detector was originally designed. Nevertheless, the detector performed very well in this higher density environment. Preliminary pseudorapidity distributions and anisotropic flow ($v_1$ and $v_2$) results will be shown for forward rapidities measured by the EPD during the 2017 run.
We have investigated the properties of charmonium states through the in- medium modifications to both perturbative and nonperturbative term of the Cornell potential. We have then extended our exploration of quarkonium to a medium which exhibits a local anisotropy in the momentum space. For that, we have first visited the anisotropic corrections to the retarded, advanced and symmetric propagators through their self-energies in the hard-loop resummation technique and apply these results to calculate the medium corrections to the perturbative and nonperturbative term of the Cornell potential. The flavor dependence of the binding energies of the heavy quarkonia states and the dissociation temperature for isotropic as well as anisotropic case have been obtained by employing the Debye mass for pure gluonic and full QCD case, which is computed by employing the quasi-particle picture. Finally we observe that overall the anisotropy makes the dissociation temperatures higher, compared to isotropic medium. By using these dissociation temperatures as an input we further explore the sensitivity of prompt and sequential suppression of these states to the shear viscosity-to-entropy density ratio, η/s from perturbative QCD and AdS/CFT predictions. Our results show excellent agreement with the recent experimental results at LHC energy.
We present for cluster produced with the new combined PHQMD+FRIGA model for Nuclotron and NICA energies. PHQMD is a new n-body approach to simulate heavy ion collisions starting from FAIR/NICA energies. The FRIGA clusterisation algorithm, which can be applied to n-body transport approaches, is based on the simulated annealing technique to obtain the most bound configuration of fragments and nucleons. This configurations is close to the finally observed configuration.
The PHQMD+FRIGA model is able to predict isotope yields as well as hyper-nucleus production.
In high-energy nuclear collisions, light nuclei provide a unique tool to explore the QCD phase structure. The production of light nuclei is sensitive to the temperature and phase-space density of the system at freeze-out. In addition, phase transition will lead to large baryon density fluctuations, which will be reflected in the light nuclei production. For example, the ratio of proton ($N(p)$) and triton ($N(t)$) to deuteron ($N(d)$) yields, which is defined as $N(t)\cdot$N(p)/$N^{2}(d)$ , may be used as a sensitive observable to search for the QCD critical point [1].
In this poster, we will report the first results of the collision energy and centrality dependence of triton production in Au+Au collisions at $\sqrt{s_{\mathrm{NN}}}$ = 7.7, 11.5, 14.5, 19.6, 27, 39, 62.4 and 200 GeV, measured by the STAR experiment at RHIC. We will present the beam energy dependence of the coalescence parameter $B_3(t)$, directed flow ($v_1$) of light nuclei (d, t, ${}^3$He), and the yield ratio $N(t)\cdot$N(p)/$N^2(d)$. We will also show the energy dependence. Their physics implications will be discussed.
Reference
[1] K.J. Sun, L.W. Chen, C.M. Ko and Z.B. Xu, Phys. Lett. B 774, 103 (2017), arXiv:1702.07620.
The mass dependence of anisotropic flow as a function of $p_T$ in small systems
observed at both RHIC and the LHC provided strong evidence of collective
behavior and suggests the formation of the smallest QGP droplets in these systems. If the cause of this mass dependence is indeed radial flow, this should be reflected in the spectral shapes at low $p_T$. Further, one would expect hard scattered partons to lose energy in these QGP droplets.
PHENIX has measured particle production from a broad set of projectile-target
combinations including p+Au, d+Au, $^{3}$He$+$Au, Cu+Cu, Cu+Au, Au+Au, and U+U. At low $p_T$ the spectra can reveal how radial flow emerges with system size. At high $p_T$ they carry information about energy loss.
We will present a comprehensive study of identified pion, kaon, proton, and
$\eta$ spectral shapes and nuclear modification factors as a function of system size and discuss the implications about radial flow and in-medium energy loss.
The Berry curvature is a fundamental quantity to describe the chiral magnetic effect and chiral kinetic theory. While it can be analytically tractable in non-interacting systems, numerical simulations are necessary in interacting systems. We formulated the lattice simulation to calculate the Berry curvature in interacting systems. We present the first result in quenched lattice QCD.
Confinement/deconfinement phase transition in dense medium
In this talk we report the lattice observation of deconfinement in dense matter. The study of the deconfinement transition was conducted within lattice simulation of dense two-color QCD at zero temperature. We reach very large baryon density (up to quark chemical potential $\mu_q > 2000 \mathrm{~MeV}$). In the region $\mu_q\sim 1000 \mathrm{~MeV}$ we observe for the first time the confinement/deconfinement transition which manifests itself in a rising of the Polyakov loop and vanishing of the string tension. After the deconfinement at $\mu_q > 1000 \mathrm{~MeV}$ we observe a monotonous decrease of the spatial string tension which ends up with its vanishing at $\mu_q > 2000 \mathrm{~MeV}$. From this observation we draw the conclusion that the confinement/deconfinement transition at finite density and zero temperature is quite different from that at finite temperature and zero density. Our results indicate that in very dense matter the quark-gluon plasma is similar to a weakly interacting gas of quarks and gluons without magnetic screening mass in the system, sharply different from a quark-gluon plasma at large temperature. Implications of our results to properties of real QCD are briefly discussed.
Correlated electron-positron pairs produced in heavy-ion collisions provide an excellent probe of the hot and dense strongly-interacting medium, i.e. the Quark-Gluon Plasma (QGP), created in such systems. They are produced at all stages of the collision without significant final-state interactions. Moreover, thermal radiation from the medium, both during the partonic and the hadronic phase, can be investigated through its internal conversion to e+e- pairs. In the intermediate mass region (1.2 – 2.8 GeV/c2), the measurement of thermal dileptons from the QGP is nevertheless challenging due to the dominant contribution from simultaneous semileptonic decays of correlated open heavy-flavour hadrons. The continuum yield in this mass region is sensitive to the energy loss of charm and beauty quarks in the QGP and further medium effects on the heavy-flavour hadron production. In order to understand the resulting modifications of the dielectron spectrum in heavy-ion collisions, a good understanding of the relevance of the various heavy-quark production mechanisms in proton-proton collisions is mandatory.
In this poster, the latest dielectron invariant-mass spectrum measured with ALICE in pp collisions at √s = 13 TeV is used to study the charm production mechanisms implemented in PYTHIA event generator. It will be shown how the relative importance of these processes can lead to model dependencies of the extracted charm cross section from a fit of the data.
The equation of state is one of the fundamental properties of the QCD matter created in relativistic nuclear collisions. Lattice QCD simulations now provide a realistic equation of state at vanishing density, but it may differ from what we should see in the experiments because heavy-ion systems may be out of chemical equilibrium, in strong magnetic fields and affected by finite size effects. It is thus important to understand the imprints of the equation of state on the data.
We find that the mean transverse mass as a function of the multiplicity density reproduces, up to proportionality factors, the energy over entropy ratio as a function of the entropy density [1]. We perform viscous hydrodynamic simulations using a variety of equations of state and compare the results to the experimental data. The equations of state with the effective number of degrees of freedom equal to or larger than that of lattice QCD are favored. We also discuss how we can constrain the equation of state at finite baryon density.
[1] A. Monnai and J.-Y. Ollitrault, Phys. Rev. C 96, 044902 (2017), arXiv:1707.08466 [nucl-th]
One of the main goals of Beam Energy Scan program of Relativistic Heavy-ion collision experiment is to map the QCD phase diagram. Measurement of higher order cumulants of net-proton and net-charge distributions are regarded as one of the potential tools to locate the QCD critical point in the phase diagram. Knowing the probability distributions of net-proton are useful for quark-meson (QM) model within the functional renormalization group approach to study the O(4) criticality. However, there are a lot of challenges to eliminate the detector effect from the experimentally measured distributions. In this poster, we will discuss the Pearson curve method to construct the efficiency corrected probability distribution of net-proton from the experimental results of STAR experiment. Furthermore, the beam energy dependence of sixth and eighth order cumulants estimated from the constructed distributions will be discussed. The predicted cumulants ratio results will be compared with various statistical models.
Construction and beam test results of the new prototype of electromagnetic calorimeter (EMCal) module for the sPHENIX detector are presented. sPHENIX will collect high statistics proton-proton, proton-nucleus and nucleus-nucleus data at the Relativistic Heavy Ion Collider (RHIC) from the early 2020's. The sPHENIX capabilities will enable investigations of jet modification, upsilon suppression and open heavy flavor production to probe the nature of Quark Gluon Plasma, and will allow a broad range of cold QCD studies. The EMCal will be the principal sub detector for identification of photons and electrons. Prototype EMCAL modules have been constructed by embedding scintillating fibers into a tower of Tungsten powder and epoxy. Performance results obtained from a run at the Fermilab Test Beam Facility in Feb 2018 will be discussed.
The Event Plane Detector (EPD) is an upgrade to the STAR experiment. It is similar to the Beam Beam Counter (BBC) which has been a part of the STAR experiment since the beginning, but will provide more pseudorapidity coverage ($2.2<|\eta|<5.1$ compared to $3.3<|\eta|<5.0$) and higher granularity (744 distinct channels compared to 32), leading to an increase in first-order event plane resolution by a factor of at least 1.5. Additional benefits from the EPD include TPC-independent centrality determination and event planes at BES energies where the VPD and ZDC suffer from low occupancies. The EPD is a set of disks consisting of 1.2cm-thick scintillator tiles optically isolated with reflective epoxy and embedded with wavelength-shifting fibers held in place with optical epoxy which are coupled to Silicon photomultipliers via clear fiber optics. The detector was constructed in $2\pi/12$ azimuthal sections called supersectors, each of which were tested for tile quality with cosmic rays and tile crosstalk with a radioactive source. In this poster, I will discuss the process of constructing the supersectors that make up the disks as well as the multiple tests performed on the finished supersectors to characterize their quality.
Freezeout in relativistic collisions occurs as a result of competition between interaction of the fireball constituents and fireball expansion. The magnitude of interaction of the fireball constituents is expected to go down as we go from nucleus-nucleus (A-A) to proton-nucleus (p-A) to proton-proton (pp) collisions which should show up in the thermal model fits of the hadron yields. However, on the contrary, it has been found that within the unified freezeout scheme (1CFO), the fits to hadron yields are insensitive to system size. In this talk we extend the 1CFO scheme to multiple freezeout with early freezeout of strangeness (2CFO) and analyse the system size dependence of the freezeout scheme. We find unlike 1CFO that is blind to system size, 2CFO fits clearly distinguish between large and small system sizes.
We discuss the freezeout conditions in pp, p-Pb and Pb-Pb collisions at the LHC energies by analysing the data on hadron yields and transverse momentum spectra. We have studied three different schemes of freezeout, i) 1CFO, ii) strange hadrons freeze out along with non-strange with an additional strangeness undersaturation factor $\gamma_{S}$ which accounts for the non-equilibrium production of strangeness (1CFO+$\gamma_{S}$), and iii) 2CFO. A comparison of the fit to data suggests that different freezeout schemes are preferred for different collision systems. For small systems (pp and p--Pb) and peripheral Pb--Pb, data prefer 1CFO+$\gamma_{S}$ where $\gamma_{S}$ starts from ~0.8 for pp and reaches close to unity for central p-Pb and peripheral Pb-Pb. For Pb-Pb, 2CFO describes data better than 1CFO and 1CFO+$\gamma_{S}$.
Multi-particle azimuthal correlations have recently been measured in proton/deuteron-nucleus collisions at RHIC and at the LHC, and call for theoretical explanations. In particular, whether they originate from the initial or final state interaction is a matter of intense debate. We propose a new, initial-state mechanism to generate multiple correlations like $c_2\{4 \}$ from the combined effect of multi-parton scattering and the elliptic gluon Wigner distribution of the nucleus. This can arise even if the individual parton-nucleus scatterings are independent of each other. We present a numerical estimate of this effect by using the Wigner distribution computed from the impact parameter dependent Balistky-Kovchegov equation.
Following our earlier finding based on RHIC data about the dominant jet production from nucleus corona region, we reconsider this effect in nucleus-nucleus collisions at LHC energy. Our hypothesis was based on the experimental data, which raised the idea of a finite formation time for the produced medium. At RHIC energy and in low density corona region this time reaches about 2~fm/c. In the center of interaction region it's about 0.7 fm/c. All observed high $p_t$ particles are produced in the corona region and have a chance to escape during this 2~fm/c. After that, the formed high density matter absorbs all jets. Following this hypothesis, the nuclear modification factor $R_{AA}$ should be independent on particle momentum and be flat versus $p_t$. At the same time, we can describe at RHIC the finite azimuthal anisotropy of high $p_t$ particles, $v_2$. A separate prediction held that, at LHC energy, the formation time in the corona region should be two times smaller, about 1~fm/c. New data at LHC show that $R_{AA}$ is not flat and is rising with $p_t$. We add to our original hypothesis an assumption that a fast parton traversing the produced medium loses the fixed portion of its energy. A shift of about 7~GeV from the original power law $p^{-6}$ production cross section in $pp$ explains well all the observed $R_{AA}$ dependencies at all centrality. The shift of about 7~GeV is also valid at RHIC energy, where the cross section follows a power law with about $p^{-8}$ and this shift explains a very slow rise of $R_{AA}$ seen for neutral pions with $p_t$ above 15~GeV/c. We also show that the observed at LHC dependence of $v_2$ at high $p_t$ and our previous predictions agree. It is very attractive to call this value of 7 GeV as a parton binding energy.
Coulomb effects on charged pion transverse momentum spectra produced in Au-Au collisions at RHIC-BES energies are investigated. From these spectra the negative-to-positive pion ratios as a function of transverse momentum are obtained and used to analyze the Coulomb final state interaction between the charged pions and the positive net-charge of the particle source. The „Coulomb kick” (a momentum change due to Coulomb interaction) and initial pion ratio for three different collision energies and various centrality classes were obtained. The Coulomb kick shows a decrease with the increase of beam energy and clear centrality dependence, with largest values for the most central collisions. These results are connected with the kinetic freeze-out dynamics and results are discussed.
Fluctuations of conserved charges (B, Q, S) are sensitive observables to explore the QCD phase structures in high-energy nuclear collisions. The STAR experiment has reported the energy dependence of the cumulants of net-proton, net-charge and net-kaon distributions in Au+Au collisions at RHIC. Non-monotonic energy dependence has been observed in the net-proton fluctuations in the most central (0-5%) Au+Au collisions for the energies in the RHIC beam energy scan.
In this poster, we will report the collision energy and centrality dependence of net-proton higher moments for Cu+Cu collisions at $\sqrt{s_{NN}}$ = 22.4, 62.4 and 200 GeV in STAR. In a smaller colliding system, the final freeze-out of the hot QCD matter is closer to the phase boundary so more genuine information on the phase structure, including QCD critical point, is retained compare to much larger systems. We will compare the results from Cu+Cu collision to that of Au+Au collisions as a function of both initial system size ( $N_{part}$) and the final size ($N_{mult}$) at these collision energies.
One of the goals of heavy-ion collisions is to search for the Quark-Gluon Plasma (QGP) and study its properties. Due to their large masses, heavy quarks are mainly produced in the initial hard scatterings during the early stage of heavy-ion collisions and experience the entire space-time evolution of the system. At the STAR experiment, utilizing high-precision secondary vertex reconstruction provided by the Heavy Flavor Tracker (HFT), $D^{0}$ mesons have been comprehensively studied to investigate the charm quark transport in the QGP. Measurement of $D^{*\pm}$ production is complementary to the $D^{0}$ measurement in studying the medium modification to the open charm meson production. It also provides useful information on feed-down contributions to the $D^{0}$ yields.
In this poster, measurement of $D^{*\pm}$ production at mid-rapidity ($|y|<1$) in Au+Au collisions at $\sqrt{s_{_{\rm NN}}}$ = 200 GeV is reported. $D^{*\pm}$ are reconstructed via the hadronic decay channel ($D^{*+}\rightarrow D^{0}\pi^{+}$, $D^{0}\rightarrow K^{-}\pi^{+}$, and its charge conjugate channel) utilizing the STAR HFT detector. The invariant yields of $D^{*\pm}$ and the ratios of $D^{*\pm}/D^{0}$ yields will be shown as a function of transverse momentum in different centralities. The nuclear modification factor ($R_{AA}$) for $D^{*\pm}$ will be presented as well, and physics implications will be discussed.
The ALICE experiment studies Pb-Pb collisions at the LHC in order to investigate the properties of the hot and dense QCD matter at extreme energy densities. Heavy quarks are sensitive probes to test the medium properties, since they are formed at a shorter time scale with respect to the deconfined state. In particular, the elliptic flow parameter $v_2$ of D mesons is sensitive to the degree of thermalization of charm quarks within the QGP medium and, at high-$p_{\rm T}$, to the path-length dependence of parton energy loss. In addition, the role of recombination mechanisms can be studied by comparing the D mesons with and without strange-quark content.
The elliptic flow of the ${\rm D}^0$, ${\rm D}^+$, ${\rm D}^{*+}$ mesons reconstructed in their hadronic decay channels in the central rapidity region in Pb-Pb collisions at $\sqrt{s_{NN}}=5.02$ TeV will be presented in the centrality classes 10-30% and 30-50%. The first ALICE measurement of the ${\rm D_s}$ $v_2$ in semi-central collisions will be also presented. The D meson $v_2$ in the 30-50% centrality class will be compared to model calculations to constrain the values of the heavy quarks diffusion coefficients at the critical temperature $T_{\rm c}$.
Multiplicity and event-shape variables like spherocity can be used to select events according to their topology. They provide a powerful tool to study soft-QCD processes (low Q$^{2}$), such as multiple parton interactions (MPI) and colour reconnection (CR) mechanisms which are expected to produce more isotropic events with respect to events dominated by jet production.
At the Large Hadron Collider (LHC) energies, heavy quarks are produced in hard scattering processes and their production can be described using perturbative quantum chromodynamics (pQCD). The measurements of open heavy-flavour hadrons as a function of spherocity and charged-particle multiplicity could improve the theoretical understanding of the production mechanisms, and the interplay between hard and soft processes.
In this contribution, recent results of the production of prompt D$^{0}$-meson as a function of event transverse spherocity (S$_{\rm O}$) in minimum bias pp collisions at $\sqrt{s}$ = 7 TeV will be presented. The results will be compared to predictions obtained with PYTHIA event generator.
The heavy-flavour production in proton--nucleus collisions is sensitive to Cold Nuclear Matter effects (CNM), related to the presence of nuclei in the colliding system such as the modification of the parton distribution functions of nuclei (e.g. shadowing or saturation effects), and parton energy loss in cold nuclear matter. These effects can induce a modification of the heavy-flavour production at low momentum and their measurement is required to understand final-state effects in Pb--Pb collisions related to the presence of the Quark-Gluon Plasma.
The study of heavy-flavour production as a function of multiplicity of charged particles produced in the collision provides information on the dependence of CNM effects on the collision geometry and on the density of final-state particles. In addition, the study of heavy-flavour transverse-momentum modification in high-multiplicity p-Pb collisions could give insight into the possible presence of collective effects.
In this contribution the ALICE measurements of open heavy-flavour production via the reconstruction of $\rm{D^{0}}$-meson hadronic decay are presented, focusing on recent results in p--Pb collisions at $\sqrt{s_{\rm NN}}$= 5.02 TeV collected during LHC Run2. In particular, the production cross sections and the nuclear modification factors down to $p_{\rm T}$ = 0 will be shown. The $\rm{D^{0}}$ nuclear modification factor $Q_{\rm pPb}$, measured as a function of the centrality of the collision and the central-to-peripheral ratio, $Q_{\rm{cp}}$, will be presented as well.
The results will be compared with theoretical model predictions.
A recent result from the STAR experiment shows that in 10-40% central Au+Au collisions at the top RHIC energy the elliptic flow ($v_2$) of the $D^0$-meson follows the Number-of-Constituent-Quark scaling in the same way as it does for light flavor hadrons. This suggests that charm quarks have gained sufficiently large collectivity through their interactions with the Quark-Gluon Plasma (QGP). It is of great interest to see whether this scaling holds in more peripheral collisions, which will shed more lights on how charm quarks interact with the QGP and gain collectivity.
In this poster, we present the centrality and transverse momentum dependences of the $D^0$-meson $v_{2}$ measured in Au+Au collisions at $\sqrt{s_{\mathrm{NN}}}$ = 200 GeV by the STAR experiment. The measurement is based on the combined datasets recorded in 2014 and 2016, which yield about a factor of 3 times of the statistics compared to the previously published results. In order to better understand the non-flow contribution in peripheral collisions, the azimuthal anisotropy parameter $v_{2}$ is measured as a function of the pseudo-rapidity gap between the $D^0$-meson and tracks used for reconstructing the event plane. The results will be compared to those of light hadrons in various centrality intervals and physics implications will be discussed.
In the early stage of relativistic heavy-ion collisions, coherent and anisotropic classical Yang-Mills field emerges.
This field, referred to as glasma, has several instabilities from the anisotropy, so small fluctuations in glasma grow exponentially.
Glasma is also known to have chaoticity, which make the field configuration complex and produces the entropy.
Instability and chaoticity of glasma rapidly drive the pressure isotropization and are expected to cause thermalization.
In this work, we compare the time scale of the entropy production and the pressure isotropization in the classical Yang-Mills field.
We regard the classical field as a coherent state, and evaluate the quantum entropy (von Neuman entropy) obtained by ignoring the off-diagonal density matrix elements, namely decoherence.
We find that the growth of fluctuations cause entropy production and pressure isotropization in the same time scale.
We also discuss how the classical Yang-Mills field loses coherence as a result of time average of the density matrix.
Electron-positron pairs are an excellent probe to investigate the properties of the Quark-Gluon Plasma (QGP) created in ultra-relativistic heavy-ion collisions. Because they are produced at all stages of the collision and do not interact strongly with the medium, their spectra reflect the entire space-time evolution of the system. At low invariant mass ($m_{\textrm{ee}} < 1.2 \textrm{ GeV/c}^2$), the dielectron production is sensitive to the properties of vector mesons in the dense medium which is related to the predicted restoration of the chiral symmetry. In the intermediate-mass region ($1.2 < m_{\textrm{ee}} < 2.9 \textrm{ GeV/c}^{2}$), the dielectron continuum gives further insight into the heavy-quark energy loss in the QGP via the measurement of correlated electron-positron pairs from charm- and beauty- hadron decays. Finally, contributions of thermal radiation from the medium, both during the partonic and the hadronic phases, are predicted in a broad mass range and provide information on the temperature of the system.
In this poster, the status of the dielectron measurements in Pb-Pb collisions at $\sqrt{s_{\textrm{NN}}} = 5.02 \textrm{ TeV}$ with ALICE will be presented. In order to interpret the data, the findings will be compared to the expected yield of known hadronic sources, i.e. the hadronic cocktail, as a function of the invariant mass and pair transverse momentum. Finally, the status of the measurement of virtual direct photons and modifications of the dielectron yield in Pb-Pb collisions will be discussed.
Dileptons are a prime probe of the deconfined state of strongly-interacting matter, the Quark-Gluon Plasma (QGP), produced in high-energy heavy-ion collisions, as they are not affected by final-state interactions and produced at all stages of the collision. A measurement of the thermal radiation from the QGP in the dielectron intermediate-mass region gives information on the medium temperature. In this region the main component of the dielectron continuum is coming from correlated semi-leptonic decays of charm and beauty hadrons, which may be affected by the energy loss and collectivity of charm and beauty quarks in the QGP. Therefore, it is crucial to understand the primordial heavy-flavour production in vacuum and find a way to separate this contribution from the thermal signal of the QGP. This can be studied in proton-proton collisions.
In this poster, the measurement of correlated e$^+$e$^-$. pairs in pp collisions at $\sqrt{s}$ = 7 TeV with ALICE will be presented. In particular, we will show how the measured distance of closest approach (DCA) of the electrons to the primary vertex of the collision gives the possibility to separate prompt and non-prompt dielectron pairs. The results will be compared with the expectations from known hadronic sources as a function of $m_{\rm ee}$, $p_{\rm T,ee}$ and $DCA_{\rm ee}$. The extraction of the charm and beauty cross sections from a fit of the data with different Monte-Carlo generators will be discussed, as well as the measured fraction of direct photons to inclusive photons.
Low-mass dielectrons are an important probe for the hot and dense medium which is created in ultra-relativistic heavy-ion collisions. Since leptons do not interact strongly and are produced throughout the whole collision process, they carry information from all collision stages with negligible final-state interaction.
The ALICE detector is well-suited to perform this measurement due to its excellent tracking and particle identification capabilities at low momenta. However, Dalitz decays and photon conversions lead to a high combinatorial background with a signal-to-background ratio of 1:10 to 1:1000 in Pb-Pb collisions, depending on the invariant mass. Therefore, the minimization of the background is a key aspect of this analysis.
The reconstruction efficiency of low-$p_{\rm{T}}$ electrons can be increased by reducing the magnetic field of the ALICE central barrel solenoid from 0.5 T to 0.2 T. This allows a better rejection of the electron background and simultaneously gives the opportunity to increase the accessible phase space of the dielectron measurement. Such a configuration is planned in ALICE for part of the Pb-Pb campaigns in LHC Run 3 and 4 from 2021 on.
This poster will present the status of the dielectron measurement in pp collisions at $\sqrt{s}$ = 13 TeV from pilot runs taken with B=0.2 T in the ALICE central barrel. It will be shown how the analysis was adapted to the reduced-field configuration. The results will be compared to reference data recorded with the nominal field, to illustrate the benefits of the low magnetic field setting. Finally, the invariant-mass and pair-transverse-momentum distributions will be compared to the expected yield from known hadronic sources.
Dijet, dihadron and hadron-jet angular correlations as well as dijet transverse momentum asymmetry have been reckoned as important probes of the transverse momentum broadening effects in relativistic nuclear collisions [1,2]. Dijets become de-correlated due to the vacuum soft gluon radiation associated with the Sudakov logarithms and the medium-induced transverse momentum broadening.
We first employ the systematic Sudakov resummation formalism to describe the dihadron and hadron-jet angular correlation data in $pp$ and central $AA$ collisions [1]. For a quark jet at RHIC top energy, a global $\chi^2$ analysis of dihadron and hadron-jet angular correlation data renders the best fit for the medium-induced broadening $\langle p_\perp^2\rangle$ and the so-called jet transport coefficient $\hat q$ in central $AA$ collisions.
Then we develop a systematic theoretical approach to dijet asymmetries in $pp$ collisions based on pQCD expansion and Sudakov resummation formalism [2]. We find that the NLO pQCD calculation is indispensable to describe experimental data, while the resummation formalism is vital near the end points where the pQCD expansion fails to converge due to appearance of large Sudakov logarithms. Utilizing our resummation-improved pQCD approach, we extract jet transport coefficient for quark-gluon plasma in $PbPb$ collisions at 2.76A~TeV.
We can also use this method to study the properites of cold nuclear matter and other related topics [3].
Further experimental and theoretical efforts along the direction of this work shall significantly advance the quantitative understanding of transverse momentum broadening and help us acquire precise knowledge of jet quenching parameter in heavy-ion collisions.
[1] L. Chen, G. Y. Qin, S. Y. Wei, B. W. Xiao and H. Z. Zhang, Phys. Lett. B 773, 672 (2017).
[2] L. Chen, G. Y. Qin, S. Y. Wei, B. W. Xiao and H. Z. Zhang, arXiv:1612.04202 [hep-ph].
[3] L. Chen, G. Y. Qin, S. Y. Wei, B. W. Xiao and H. Z. Zhang, in preparing.
As electromagnetic probes dileptons open a window to the in-medium properties of vector mesons. In this talk, medium effects to vector mesons are discussed for heavy ion collisions in the low kinetic energy regime of $1 - 3A$ GeV, where the dielectron emission is accessed by the HADES experiment at GSI. A new hadronic transport approach named SMASH (Simulating Many Accelerated Strongly-interacting Hadrons) is employed to study dilepton production, which is based on vacuum resonance properties and consistently includes dilepton emissions below the hadronic threshold. The approach is validated by an excellent agreement with experimental data up to system sizes of carbon-carbon collisions. After establishing this well-understood baseline in elementary and small systems, the significance of medium effects is investigated with a coarse-graining approach based on the same hadronic evolution. Interestingly, the effect of explicit in-medium modifications to the vector meson spectral functions is already important for dilepton invariant mass spectra in ArKCl and larger systems, even though the transport approach with vacuum properties reveals similar features due the coupling to baryonic resonance and the intrinsically included collisional broadening. In addition, the validated dilepton production allows to assess the importance of a microscopic evolution of the hadronic stage by studying the non-equilibrium dilepton radiation in late, dilute stages of high-energy heavy ion collisions, i.e. in hybrid approaches.
Reference:
J. Staudenmaier, J. Weil, V. Steinberg, S. Endres, H. Petersen, "Dilepton production and resonance properties within a new hadronic transport approach in the context of the GSI-HADES experimental data", arXiv:1711.10297 [nucl-th]
We investigate the quark number density and the quark number holonomy at finite imaginary chemical potential in the lattice QCD using the Dirac-mode expansion. The quark number holonomy is defined by the quark number density and it can be an order parameter which detects the quark-deconfinement [1,2]. We find some analytical formulae of the quark number density. In the large quark mass regime, the quark number density is expressed in terms of the Polyakov loop. On the other hand, in the small quark mass region, the quark number density is expressed by the eigenmodes of the Wilson-Dirac operator on a lattice. It is found that the quark number density strongly depends on the low-lying Dirac modes. However, the quark number holonomy is not sensitive to the low-lying Dirac modes. Based on these results, we discuss the confinement-deconfinement transition [3].
References:
[1] K. Kashiwa, A. Ohnishi, Phys. Lett. B750, 282 (2015), arXiv:1505.06799.
[2] K. Kashiwa, A. Ohnishi, Phys. Rev. D93, 116002 (2016), arXiv:1602.06037.
[3] T. M. Doi, K. Kashiwa, arXiv:1706.00614.
Dielectrons produced in ultra-relativistic heavy-ion collisions provide a unique probe of the system evolution as they are unperturbed by final-state interactions. Among the different physics sources of dielectrons, thermal radiation in the form of real and virtual photons is of particular interest as it carries information about the temperature of the hot and dense system created in such collisions.
In heavy-ion collisions, the very low dielectron mass region ($m_{\rm ee} < 0.3 GeV/c^2$) provides information on the temperature of the system via the measurement of thermal radiation in the form of quasi-real virtual photons.Recently, one has observed collective phenomena in high-multiplicity pp collisions that are similar to the ones observed in heavy-ion collisions. If such collisions produce a thermalised system, it should emit thermal radiation as well.
We present the status for the search of such radiation in high-multiplicity pp collisions together with a vacuum reference measurement in minimum bias pp collisions at $\sqrt{s}$ = 13 TeV.
The medium modification of jets continues to be studied in greater and greater detail, ranging from their absolute yields to substructure measurements. A key problem has always been to accurately determine the jet energy calibration in order to establish the influence of the hot QCD medium on the observed jet properties. A way to circumvent this ambiguity is the measurement of direct $\gamma$-hadron correlations. The direct photon, produced in hard scatterings back-to-back with a parton, serves as a calibration of the away-side jet and thus provides less-biased insight into how the medium affects the away-side jet fragmentation. We ultimately aim to measure the modified fragmentation function f($z_{\mathrm{T}}$) and its dependencies on several quantities like the $\gamma$-trigger $p_{T}$ or associated hadron $p_{T}$ to understand the $z_{T}$ dependence.
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This poster will demonstrate our capability to measure inclusive $\gamma$-hadron and $\pi^{0}$-hadron correlation functions. Results will be presented for Pb--Pb data at \ensuremath{\sqrt{s_\mathrm{NN}}}=5.02\,TeV, measured with the EMCal and DCal detectors of the ALICE experiment.
Despite of a generally very successful description of the elliptic and higher harmonic flow in heavy-ion collisions by theoretical models, there is no single model that explains the dependence of the directed flow on pseudorapidity, collision energy, system size, and the particle type. This indicates that an important piece in our picture of ultrarelativistic heavy-ion collisions is still missing. Directed flow is thought to arise from two main mechanisms: the so-called "tilted" source and the dipole-like initial density asymmetry. The asymmetric Cu+Au collisions provide a unique possibility to identify the roles of theses two mechanisms.
We present the results of directed flow measurements for charged and identified particles in Cu+Au and Au+Au collisions at $\sqrt{s_{\rm NN}}$ = 200 GeV as a function of centrality, pseudorapidity, and transverse momentum. We show how the comparison of pseudorapidiy dependence of $v_1$ and $\langle p_x\rangle$ can be used to quantify the contribution of different mechanisms of directed flow formation. We compare the results with Pb+Pb collisions at the LHC energies, and discuss the centrality and energy dependence of the relative contributions to the directed flow from the initial source tilt and from the density asymmetry.
The equations of relativistic hydrodynamics can be obtained from
the Boltzmann equation via the Chapman-Enskog (CE) procedure and
Grad’s 14 moments approximation. These approaches give different
results for the transport coefficients, which reduce to the same
expressions in the non-relativistic limit.
In this contribution, the propagation of a harmonic longitudinal
wave is considered in the frame of the first- and second-order
relativistic hydrodynamics theories. The ensuing hydrodynamic
equations are solved in the linearized regime (valid for small
wave amplitudes). The analytic solutions corresponding to the
CE and Grad transport coefficients are compared to the numerical
solution of the relativistic Boltzmann equation for massless
particles in the Anderson-Witting (AW) approximation, obtained
using the lattice Boltzmann (LB) method. The comparison clearly
confirms the validity of the CE prediction.
For particular initial conditions, the first-order formulation
gives incorrect predictions for the wave evolution even when
the AW relaxation time is small. This is remedied in the
second-order formulation, the solution of which is confirmed
by numerical simulations.
Relativistic hydrodynamics has played a key role in our understanding of the novel properties of quark-gluon plasma. However, the validity of hydrodynamical models in describing the extreme conditions produced in heavy ion collisions has still not been properly justified theoretically. Even more, the gradient expansion, commonly used to derive hydrodynamics from microscopic theory, has been recently shown to diverge for conformal fluids or relativistic gases undergoing Bjorken flow [1,2], putting under question the definition of hydrodynamics itself. Alternative derivation of the hydrodynamic series have been proposed recently, such as the slow-roll expansion [1], and can be promising candidates to define hydrodynamics.
In this contribution, we present general analytical and semi-analytical solutions of the hydrodynamic attractor of Israel-Stewart theory [2] and kinetic theory for Bjorken and Gubser expanding fluids. We show that the gradient expansion diverges in both cases. For Israel-Stewart theory, we show for the first time that even the slow-roll expansion, a commonly used approach to characterize the attractor, diverges. Finally, we construct the slow-roll expansion for a general flow scenario and find the effective shear viscosity [3] and relaxation time that are able to describe a more general hydrodynamic regime.
[1] M. P. Heller and M. Spalinski, "Hydrodynamics Beyond the Gradient Expansion: Resurgence and Resummation," Phys. Rev. Lett. 115, no. 7, 072501 (2015).
[2] G. S. Denicol and J. Noronha, "Analytical attractor and the divergence of the slow-roll expansion in relativistic hydrodynamics,'' arXiv:1711.01657 [nucl-th].
[3] P. Romatschke, "Relativistic Hydrodynamic Attractors with Broken Symmetries: Non-Conformal and Non-Homogeneous,'' arXiv:1710.03234 [hep-th].
We study effects of dynamical initialization with a core-corona picture in hydrodynamic description of small colliding systems at RHIC and the LHC energies. We previously proposed an idea of dynamically initializing hydrodynamic fields by utilizing source terms in hydrodynamic equations [1]: Instead of setting initial conditions at a fixed hydrodynamic initial time, we make initially produced partons propagate and deposit energy and momentum into the bulk matter via the source terms to form the quark gluon plasma (QGP) fluids gradually. Under this idea, not only initial fluctuations of geometry but also those of velocity fields are naturally generated. In the present study, we further introduce the core-corona picture to the idea by considering spatial density of the initially produced partons. Here, partons produced in the dilute region fragment into hadrons separately instead of taking part in the initial QGP fluid formation. This picture strongly affect the fluid profile in small colliding systems and must be taken into account.
To demonstrate the above idea, we employ a QGP fluid + jet model [2] in which the QGP medium evolution is described by (3+1)-D hydrodynamic equations with source terms. As an input, we generate partons by using the latest version of PYTHIA 8.230 which involves heavy-ion collisions. As propagating after the production, these partons deposit energy and momentum into the bulk QGP fluid according to the parton density around them in the core-corona picture. From simulations with this newly developed framework, we calculate hadron spectra including contributions from both the surviving partons in the dilute regions with PYTHIA fragmentation and the fluid part in the dense region and show that the core-corona picture plays a crucial role in small colliding systems.
[1] M.Okai, K.Kawaguchi, Y.Tachibana, T.Hirano, Phys. Rev. C 95, no. 5, 054914 (2017).
[2] Y.Tachibana, T.Hirano, Phys. Rev. C 90, no. 2, 021902 (2014).
In this work, we extend the resummation of multiple medium-induced emissions to apply to dynamically expanding media. This is done by recasting the quenching weight as the solution of a rate equation with medium-induced partonic splitting functions that are sensitive to the expansion. We perform the calculations in the framework of Baier-Dokshitzer-Mueller-Peigne-Schiff-Zakharov (BDMPSZ) formalism for multiple soft scatterings with a time-dependent transport coefficient. Furthermore, we discuss the validity of a dynamical scaling law that relates the spectrum in an expanding medium to the equivalent static case with rescaled medium parameters [1] and test the size of energy loss fluctuations in a realistic medium.
References:
[1] Carlos A. Salgado and Urs Achim Wiedemann "A Dynamical Scaling Law for Jet Tomography", Phys. Rev. Lett. 89, 092303.
Understanding the early out-of-equilibrium dynamics of heavy-ion collisions (HIC) remains one
of the biggest theory challenges. So far, there are no first principle calculations for the equilibration
process of the quark gluon plasma and the dynamics close to the phase transition. In particular
describing the behavior close to the conjectured critical point, where critical slowing down leads
to off-equilibrium dynamics, poses difficulties.
Here we study the initial stages of a HIC using a low-energy effective theory of QCD, the
quark-meson model, in order to gain insight into the thermalization process. This model manifests
a central and physically relevant feature of QCD: chiral symmetry breaking in vacuum and its
restoration at finite temperature and density. At the critical endpoint this model is expected to
be in the same universality class as QCD and hence a viable model to explore dynamical critical
phenomena.
We solve the non-perturbative real-time quantum equations of motions for the quark and meson
fields in the two-particle irreducible effective action framework. Similar to a HIC, our system is
prepared in a high-energy initial state and suddenly quenched out of equilibrium, evolving towards
a thermal f