Quark Matter 2017 is the XXVI international conference on ultrarelativistic heavy-ion collisions. The aim of this conference is to bring together theoretical and experimental physicists from across the world to discuss new developments in high energy heavy ion physics. The focus of the discussions is on fundamental understanding of strongly interacting matter at extreme conditions formed in ultrarelativistic heavy-ion collisions which relates to the state of the early universe. Following the previous Quark Matter conference in Kobe, Japan in 2015, the current meeting will be held in Chicago from February 5 to February 11, 2017.
Public Website: QM2017
The quantum fluctuations of the initial state described by the overlap of two highly Lorentz-contracted nuclei traveling on light-cone trajectories are probably imprinted upon the distribution of particles created in the Quark-Gluon Plasma (QGP). Without assessing these quantum fluctuations in nuclei, fundamental properties of the QGP such as its viscosity-to-entropy ratio cannot be determined to a high precision. By studying coherent J/$\Psi$ photoproduction in $\gamma$-Pb interactions, and comparing it to that J/$\Psi$ photoproduction off the proton, the CMS data together with that from ALICE, have showed that the no-nuclear shadowing hypothesis at low Bjorken-x and $Q^{2}$ values is rejected with a significant lager than 5 sigmas. The neutron dependence and energy dependence of J/$\Psi$ photoproduction off the Pb, and its connection to these nuclear gluon effects, will be presented for the first time. Furthermore, the experience gained analyzing vector meson photoproduction has been used to study other $gamma$-Pb processes such as photonuclear jets, and will be discussed in this talk.
Ultra-Peripheral Pb-Pb collisions, in which the two nuclei pass close to each other but at an impact parameter greater than the sum of their radii, provide information about the initial state of nuclei. In particular, heavy vector meson production, where the particle mass sets a hard scale, proceeds in such collisions by photon-gluon interactions, and gives access to nuclear PDFs. The ALICE collaboration has published measurements of UPC J/Psi and psi(2S) production in LHC Run 1 at forward (J/psi) and mid-rapidity, and has obtained a substantially larger data set in 2015 from LHC Run 2, allowing much more detailed studies of the production mechanism to be performed. In particular, the increased energy and more detailed measurements in the forward region in Run 2 give access to significantly lower values of Bjorken-x than in previous studies. In this talk, the latest available results from Run 2 will be given.
Beams of relativistic heavy ions are accompanied by a large flux of equivalent photons, and photon-induced reactions are the dominant interaction mechanism in heavy-ion collisions when the colliding nuclei have transverse separation larger that the nuclear diameter. In these ultra-peripheral collisions (UPC) the photon can provide a clean probe of the partonic structure of the nucleus analogous with deep inelastic scattering. This talk presents measurements of dijet production in ultra-peripheral Pb+Pb collisions performed with the ATLAS detector. Events are selected using requirements on rapidity gaps and forward neutron production to identify the photo-nuclear processes. The relatively clean environment of these events allows for measurements in a region of x and Q^2 where significant nuclear PDF modifications are expected to be present and not strongly constrained by previous measurements.
This talk also presents measurements of four-jet cross sections in pp and p+Pb collisions over a large kinematic range. In addition to higher order QCD effects, the four-jet cross section receives contributions from so-called double parton scatterings (DPS) in which two independent hard scattering processes occur in the same collision. Thus measurements of DPS, which have already been performed in 7 TeV pp collisions, can yield new information on the spacial and momentum correlations between partons in a nucleon beyond single parton distributions. In p+Pb collisions, additional mechanisms for DPS are possible when two partons in the proton scatter off partons in different nucleons in the nuclear target, leading to an enhancement in the four jet rate and a different sensitivity to the underlying correlations. Differential cross sections as well as measurements of momentum and angular correlations, which help disentangle the single and double parton scattering contributions, will be presented.
The large equivalent-photon fluxes accompanying Pb ion beams at the LHC initiate photon-photon and photo-nuclear interactions which dominate when the colliding nuclei have large impact parameter (ultra-peripheral collisions). These electromagnetically-induced processes are sensitive to the nuclear wave-function and in particular the nuclear modifications of the nucleon parton distribution functions (nPDFs). As such, they are complementary to the ongoing p+A program at RHIC and the LHC, as well as the upcoming electron-ion collider (EIC) program in the US. The absolute rates of single and multiple neutron emission into one or both zero-degree calorimeters (ZDCs) will be presented, to test theoretical predictions for the photon fluxes as well as the photonuclear absorption. High-mass dilepton pair continuum rates have been measured and compared with theoretical predictions to test expectations for two-photon interactions, and good agreement with model calculations is obtained. Finally, evidence for the elastic scattering of photons γγ→γγ (“light-by-light” scattering) will be presented, a previously unobserved process made possible by the high photon flux and low event pileup provided by the LHC. While of intrinsic interest as a heretofore-unobserved standard model process, it has also been proposed as a clean channel for searches for beyond the standard model (BSM) physics.
We present an ab-initio approach to compute the longitudinal dependence of the initial state by including small-x evolution of the nuclear gluon distributions.
We extend the IP-Glasma model by consistently including JIMWLK rapidity evolution and compute event-by-event rapidity distributions of produced gluons and the early time energy momentum tensor as a function of space-time rapidity and transverse coordinates. We show how the effects of small-x evolution manifest themselves in longitudinal (rapidity) correlations of event-by-event multiplicities and transverse geometry and compare our results to various phenomenological models and experimental observations.
[1] B. Schenke,S. Schlichting arXiv:1605.07158 (accepted for publication in Phys.Rev. C)
We report measurement of the semi-inclusive distribution of charged-particle jets recoiling from a high transverse momentum ($p_{T}$) hadron trigger, for p--Pb collisions at $\sqrt{s_{NN}} =5.02$ TeV that have been classified by event activity. This coincidence observable is calculable perturbatively in vacuum, and has previously been measured in pp and Pb--Pb collisions at the LHC, providing a new probe to measure quenching. Jets are reconstructed from charged particle tracks using the anti-kT algorithm with low IR cutoff of jet constituents ($p_T^{track}>0.2$ GeV/c). The analysis applies a data-driven statistical approach to correct the complex uncorrelated jet background, including multi-partonic interactions. Recoil jet distributions are reported for $15 < p_{Tjet} <50$ GeV/c, for R=0.2 and 0.4. Events are classified by signal in the ALICE V0A detector, which measures forward multiplicity, and ZNA, which measures the number of neutrons at zero degrees. The semi-inclusive observable corresponds to the ratio of inclusive cross sections, $d\sigma^\mathrm{h_{trig}+jet}/\sigma^\mathrm{h_{trig}}$, and comparison of the recoil jet yield in p--Pb collisions with different event activity therefore does not require knowledge of $T_\mathrm{pPb}$, thereby avoiding the need for geometric modelling. We compare the trigger-normalized recoil jet yield for p--Pb collisions with different event activity to measure the effects of jet quenching in small systems at the LHC.
The formation of a QGP in heavy ion collisions and the hydrodynamic expansion of the created medium are well established and reasonably well understood. This state of nuclear matter was not expected to be found in reactions involving smaller systems, such as the pA and pp collisions. Nevertheless, a wealth of experimental evidence in recent years has suggested the presence of collective phenomena and a possible QGP medium being formed also in high-multiplicity pPb collisions. A detailed investigation is needed to establish the cause of the observed collective behavior and to determine if, indeed, a QGP medium is being created or if another mechanism is responsible. Final results of differential $v_{n}$ harmonics in pPb collisions as a function of transverse momentum and pseudorapidity obtained using scalar product, multi-particle cumulant and Lee-Yang Zeros methods are presented in various multiplicity classes. The effect of event plane decorrelations on the observed $\eta$ dependence of $v_{n}$ is studied in detail. Furthermore, new measurements of correlation between $v_{2}$ and $v_{3}$ ($v_{4}$) harmonics are performed using the technique of symmetric cumulants to gain more insight to the initial state and possible medium response of the pPb system. The results shown in this talk relate to our understanding of the origin of azimuthal correlations in high-multiplicity pPb events and shed light on a potential QGP formation in small system collisions.
Two-particle correlations in relative azimuthal angle ($\Delta\phi$) and pseudorapidity ($\Delta\eta$) have been used to study heavy-ion collision dynamics, including medium-induced jet modification. These correlations have been extensively studied in small collision systems by all the four main LHC experiments. Further investigations showed also the importance of Multiple Parton Interactions (MPI). The latter are employed by pQCD-inspired models which provide a consistent way to describe high-multiplicity pp collisions, where the probability of several parton scatterings per nucleon-nucleon collision is high. In this talk we present the latest ALICE measurements using the data from Run I and Run II at the LHC. The MPI results in pp collisons at an energy of $\sqrt{s}$ = 13 TeV will be presented as a function of multiplicity and compared to those in pp collisions at lower energies and in p-Pb collisions at $\sqrt{s_{NN}}$ = 5.02 TeV. Additionally, detailed studies of two-particle azimuthal correlations in pp collisions at $\sqrt{s}$ = 7 TeV will be shown as a function of multiplicity including the studies of the near-side jet peak evolution. These measurements complement the recent ALICE results on the anomalous evolution of the near-side jet peak shape in Pb-Pb collisions and serve as a baseline for studies of long-range correlations in pp collisions.
The availability at the LHC of the largest collision energy in pp collisions
allows a significant advance in the measurement of J/$\psi$ production as function
of event multiplicity. The interesting relative increase observed with data
at the LHC at $\sqrt{s}=7$ TeV and at RHIC at $\sqrt{s}=200$ GeV is studied now
at unprecedented multiplicities at $\sqrt{s}=13$ TeV.
The new measurement, performed at midrapidity in the dielectron channel with
ALICE and facilitated by triggering on high-multiplicity events, allows the
comparison to J/$\psi$ production in p-Pb collisions at similar multiplicities.
The results are also discussed in comparison to predictions from available theoretical models and to data at lower energies.
The study of identified particle production as a function of the proton-proton (pp) collision energy and multiplicity
is a key tool for understanding similarities and differences between small and large interacting systems.
We report on the production of pions, kaons, protons, K$^{0}_{\rm S}$, $\Lambda$, $\Xi$, $\Omega$, K$^{*0}$ and $\phi$ measured in pp collisions for $\sqrt{s}$ ranging from 0.9 to 13 TeV.
The multiplicity dependence of identified particle spectra and yields is presented for $\sqrt{s}$ = 7 and 13 TeV and compared to results obtained in proton-lead (p-Pb) and lead-lead (Pb-Pb) collisions, unveiling remarkable and intriguing similarities among systems and energies.
The production rates of strange hadrons are observed to increase more than those of non-strange particles, showing an enhancement pattern with multiplicity which does not depend on the collision energy.
Even if the multiplicity dependence of spectral shapes can be qualitatively described by commonly-used Monte Carlo event generators, the evolution of integrated yield ratios is poorly described by these models. Finally, these results will also be compared to expectations from hydrodynamics.
Dileptons ($l^{+}l^{-}$) are produced throughout all stages of heavy-ion collisions and escape with minimum interaction with the strongly interacting medium. For this reason, $l^{+}l^{-}$ pair measurements play an essential role in the study of the hot and dense nuclear matter created in heavy-ion collisions. Dileptons in the low invariant mass region (up to M$_{ll}\sim$1~GeV/c$^2$) retain information about the in-medium modification of vector mesons while dileptons in the intermediate mass region (extending out to M$_{ll}\sim$3 GeV/c$^2$) predominantly originate from charm decays and thermal radiation of the medium. At higher invariant masses, recent studies of $J/\psi$ yields in peripheral A$+$A collisions by the ALICE~\cite{alice} and STAR collaborations showed significant excess at very low momentum transfers (p$_T$ $<$ 0.3~GeV/c). These observations may point to evidence of coherent photoproduction of $J/\psi$ in hadronic interactions which conflicts with traditional knowledge of the coherent photoproduction mechanism. It is interesting to investigate the $e^{+}e^{-}$ pair production in a wider invariant mass region (M$_{ee}<$4 GeV/c$^2$) at very low p$_T$ in heavy-ion collisions for different centrality bins in order to study the production mechanism.
This talk will cover $e^{+}e^{-}$ spectra with various invariant mass and p$_{T}$ differentials in Au$+$Au collisions at $\sqrt{s_{NN}}$ = 200 GeV and U$+$U collisions at $\sqrt{s_{NN}}$ = 193 GeV. The structure of the t (t = p$_{T}^{2}$ ) distributions of these mass regions will be shown and compared with the same distributions in ultra-peripheral collisions. Additionally, this talk will cover first measurements of $\mu^{+}\mu^{-}$ invariant mass spectra from STAR's recently installed Muon Telescope Detector (MTD) in p$+$p and Au$+$Au collisions at $\sqrt{s_{NN}}$ = 200 GeV. Physics implications of the $\mu^{+}\mu^{-}$ results will be discussed in the context of STAR's published $e^{+}e^{-}$ results.
We present a first measurement of low-mass electron pairs for a heavy collision-system at SIS18/Bevalac energies. The data is analyzed in terms of excess radiation above a conventional cocktail of contributions from meson decay after thermal freeze-out. We observe a strong excess radiation which is remarkably well described assuming emission from a thermalized system. The high statistics data allows studying multi-differential distributions. The multiplicity of excess radiation in the mass window 300 to 700 MeV/$c^2$ rises with $A_{part}$ stronger than linear. To gain deeper understanding of the microscopic origin of the excess radiation we started to investigate di-electron radiation emitted from baryonic resonances produced off protons in pion-induced reactions. The data is in support of VMD in electromagnetic transition of excited baryons.
Dielectrons produced in ultra-relativistic heavy-ion collisions at the LHC provide a unique probe of the whole system evolution as they are unperturbed by final-state interactions. The dielectron continuum is extremely rich in physics sources: on top of ordinary Dalitz and resonance decays of pseudoscalar and vector mesons, thermal black-body radiation is of particular interest as it carries information about the temperature of the hot and dense system created in such collisions. Dielectron invariant-mass distribution is furthermore sensitive to medium modifications of the spectral function of short-lived vector mesons that are linked to the potential restoration of chiral symmetry at high temperatures. Correlated electron pairs from semi-leptonic charm and beauty decays provide complementary information about the heavy-quark energy loss.
In this talk, we will present an extensive summary of the LHC Run-1 results in all three collisions systems: pp, p-Pb and Pb-Pb, the former two providing crucial vacuum and cold-nuclear matter references for the latter. Furthermore, we will discuss the status of the ongoing Run-2 pp and Pb-Pb analyses with a focus on pp collisions collected with a trigger on high charged-particle multiplicities and conclude with an outlook for future measurements in Run-3 following the ALICE detector upgrades during the second long shutdown phase.
Among the probes used to investigate the properties of the Quark-Gluon Plasma, the measurement of the energy loss of high energy partons can be used to put constraints on energy loss models and to ultimately access medium characteristics (such as energy density or temperature). The study of two particle correlations allows to obtain very different constraints compared to the nuclear modification factor. In particular, the correlation of recoiling charged hadrons with high energy $\pi^{0}$ or direct photons is believed to give a measure of the parton energy loss and insights of the medium induced modification of the fragmentation process.
High energy neutral pions and photons are reconstructed using the ALICE electromagnetic calorimeter EMCal and the charged particles are detected by ALICE main tracking detectors ITS and TPC.
In this talk, we will present the measurements of azimuthal $\pi^{0}$-hadron correlations in pp and Pb-Pb collisions along with the extracted per-trigger yield modification factor ($I_{AA}$) as well as comparisons with models. The status of the isolated $\gamma$-hadron correlations and $\pi^{0}$ elliptic flow measurements will also be presented.
Bulk viscosity has recently been shown to play an important role in describing both photon [1] and hadron [2] observables at the Relativistic Heavy-Ion Cllider (RHIC) and the Large Hadron Collider (LHC). The presence of a temperature-dependent bulk viscosity in the hydrodynamical evolution of the medium modifies the development of the hydronynamic momentum anisotropy differently in the high- and low-temperature regions. Thus, anisotropic flow coefficients of hadronic observables, which are emitted predominantly from low temperatures, are affected differently by bulk viscosity than electromagnetic probes which are radiated during the entire evolution. Starting from the IP-Glasma initial conditions [1,2], we study how thermal dilepton production gets modified owing to the presence of bulk viscosity at RHIC and LHC energies. With calculations at different collision energies we can draw more robust conclusions regarding the role of bulk viscosity in high energy heavy-ion collisions. Dilepton radiation from the dilute phase of the medium will be included for the first time using the Boltzmann-transport model SMASH [3] and compared to previous hydrodynamic approaches to ascertain whether these modifications may be observable in experimental data.
[1] Jean-François Paquet et al., Phys. Rev. C 93 no. 4, 044906 (2016)
[2] S. Ryu et al., Phys. Rev. Lett. 115 no. 13, 132301 (2015)
[3] Janus Weil et al., arXiv:1604.07028 and arXiv:1606.06642
Direct photons, being colorless objects, provide an unmodified control particle that can be used in conjunction with jets to probe the quark-gluon plasma. To leading order the direct photon momentum balances the momentum of opposing jets and can therefore provide a clean handle on the jet energy. Therefore, angular correlations with direct photons provide a mechanism to study the fragmentation of the opposing jet without performing jet reconstruction. Jet fragmentation modification has been measured previously in PHENIX in central Au+Au collisions. Recent RHIC runs offer the potential to study these observables in heavy ion collisions with greater statistics and over different collision systems including asymmetric collision geometries. In this talk we present results of isolated direct photon-triggered correlations in $d$+Au collisions and discuss the constraints of cold nuclear matter effects on the fragmentation functions. We also present the latest results with higher statistics on direct photon-triggered correlations in Au+Au collisions including differential measurements of fragmentation function modification. Finally, we present the status of the centrality and collision species dependence of these observables, including comparisons to related di-hadron correlations. Together these results can give a view of jet modification going from small to large system size.
Jets and their modifications due to partonic energy loss provide a powerful tool to study the properties of the QGP created in ultrarelativistic heavy ion collisions. For correlation studies of jet energy loss, two complementary trigger object choices offer access to the initial hard parton’s energy: On the one hand, direct-photon–tagged jets are a self-generated tomographic medium probe, unaffected by the medium and hence with an unbiased in-medium path length. They are also expected to exhibit a different flavor-dependence of energy loss, since high-$p_T$ direct photons are primarily produced with a quark recoil. On the other hand, reconstructed jets offer the promise of path length control or Jet Geometry Engineering through cut parameters. Previously, $A_J$ measurements at STAR observed significant imbalance for anti-$k_T$ di-jets with a resolution parameter R = 0.4 and a “hard-core” selection using only constituents with $p_T$ above 2.0 GeV/c and a neutral high tower with $E_T$ > 5.5 GeV. When soft constituents are included, this imbalance is found restored to the balance of the p+p reference inside the original jet cone, indicative of milder modification due to more surface-biased production.
We present a study of correlated hadrons and semi-inclusive recoil jets coincident with direct photons and neutral pions with 9 < $E_T^{trig}$ < 20 GeV, as well
as with respect to to the dijet selection from the above $A_J$ measurement. A
comparison between the two measurements, and a comparison to similar measurements at RHIC and the LHC establishes new systematic input into the flavor- and path-length dependence of light flavor energy loss in the QGP.
Jet-hadron correlations are used to extend measurements of the properties of jets beyond classic fixed-R jet reconstruction. New measurements using PbPb and pp collision data at $\sqrt{s_{NN}}$ = 5.02 TeV recorded by CMS use a statistical approach that allows for a reliable subtraction of the underlying event beyond the typical distance parameters of jet reconstruction. Measurements of correlated particle densities are extended out to +/-1.5 units of relative azimuth and pseudorapidity. Double-differential measurements of jet fragmentation functions and jet shapes will be presented up to radial distance of R=1 from the jet axis. New results will be compared to the previous measurements at 2.76 TeV.
We use a Linear Boltzmann Transport model (LBT model) coupled to (3+1)D ideal hydrodynamic evolution in real time with fluctuating initial conditions to simulate both the transport of jet shower partons and jet-induced medium excitation. In this coupled approach, soft partons from medium recoil and induced radiation from propagation of energetic shower partons in the Linear Boltzmann transport (LBT) model provide a source term to the 3+1D hydrodynamic evolution of the medium, which in term provide medium profile in real time for the parton shower propagation. With this coupled approach we investigate the hadrons spectrum in the whole transverse momentum region and focus on gamma-hadron and hadron-hadron correlations to study the effect of both jet-induced medium excitations and jet quenching due to parton energy loss
The final goal of the jet quenching studies is to extract medium parameters that characterize the QGP formed in high-energy nuclear collisions. In our analysis, we combine event-by-event hydrodynamics, within the EKRT formulation, with jet quenching (ASW Quenching Weights) to obtain high-$p_T$ $R_{AA}$, $v_2$ and $v_3$ for charged particles at RHIC and LHC energies for different centralities.
By defining a $K$-factor that quantifies the departure of the transport coefficient, $\hat{q}$, from an ideal estimate, $K = \hat{q}/(2 \epsilon^{3/4})$, we fit the single-inclusive experimental data for charged particles. Then, using the fitted $K$-value for each energy and centrality we also compute high-$p_T$ $v_2 $ and $v_3$, getting a good agreement with data. As obtained already in previous analyses, this $K$-factor is larger at RHIC than at the LHC but, surprisingly, it is almost independent of the centrality of the collision. We provide some possible explanations to this finding.
We study the various transverse-momentum-dependent (TMD) gluon distributions entering the cross section for forward di-jet production in dilute-dense collisions. For each TMD distribution we identify their operator definitions at small $x$ and finite $N_c$ as correlators of Wilson lines. With the result, we show the equivalence between the nearly back-to-back limit of the Color Glass Condensate cross section and the small-$x$ limit of the TMD factorization, at finite $N_c$. We obtain an analytical result for the gluon distributions in the Golec-Biernat-Wusthoff model, their perturbative behavior at large transverse momentum in the McLerran-Venugopalan model, and, numerically, their JIMWLK evolution towards small $x$. We observe geometric scaling regime for all the TMDs after some evolution.
We use effective kinetic theory to simulate equilibration in heavy-ion collisions. We construct a map for out-of-equilibrium initial state to the energy-momentum tensor at a time when hydrodynamics becomes applicable. We apply this map to IPGlasma initial conditions and demonstrate a smooth transition to hydrodynamics. In a phenomenologically favorable range of $\eta/s$ values, equilibration can be well approximated by a fixed function of a scaled time variable $(\tau T)/(\eta/s)$. This scalable kinetic equilibration can be readily applied to other initial state models to provide perturbatively controlled description of pre-equilibrium energy and transverse momentum flow evolution.
References:
JHEP 1608 (2016) 171 [hep-ph/1605.04287]
Phys.Rev.Lett. 115 (2015) 18 [hep-ph/1506.06647]
A primary goal of heavy-ion physics is the measurement of the fundamental properties of the quark-gluon plasma (QGP) and the characterization of the initial state that leads to its formation.
While these properties—such as temperature-dependent transport coefficients—are not directly measurable, they may be quantitatively estimated through computational models of heavy-ion collisions.
The properties of interest are input as model parameters and tuned so that a variety of simulated observables optimally fit corresponding experimental data.
Previous studies [1, 2] have applied Bayesian parameter estimation methods to simultaneously constrain a variety of QGP properties, including the temperature dependence of the specific shear viscosity $(\eta/s)(T)$ and the scaling of initial entropy deposition, and confirmed a finite QGP bulk viscosity $\zeta/s > 0$.
However, this work also demonstrated that more precise estimates of the shear and bulk viscosities could be achieved by including experimental data from multiple beam energies and improving several aspects of the computational model.
In this work, we utilize Bayesian methodology to estimate the parameters of an updated heavy-ion collision model [3] including the parametric initial conditions TRENTO [4], a pre-equilibrium free streaming phase [5], viscous 2+1D hydrodynamics, improved Cooper-Frye particlization, and UrQMD.
We calibrate the model to multiplicity, transverse momentum, and flow data from multiple RHIC and LHC beam energies, report the latest quantitative estimates of the temperature dependence of QGP shear and bulk viscosities as well as initial state properties, and validate model predictions of higher-order observables such as flow correlations.
[1] J. E. Bernhard, J. S. Moreland, S. A. Bass, J. Liu, U. Heinz,
Phys. Rev. C94, 024907 (2016), arXiv:1605.03954 [nucl-th].
[2] J. E. Bernhard, P. W. Marcy, C. E. Coleman-Smith, S. Huzurbazar, R. L. Wolpert, S. A. Bass,
Phys. Rev. C91, 054910 (2015), arXiv:1502.00339 [nucl-th].
[3] C. Shen, Z. Qiu, H. Song, J. E. Bernhard, S. A. Bass, U. Heinz,
Comput. Phys. Commun. 199, 61 (2016), arXiv:1409.8164 [nucl-th].
[4] J. S. Moreland, J. E. Bernhard, S. A. Bass,
Phys. Rev. C92, 011901 (2015), arXiv:1412.4708 [nucl-th].
[5] J. Liu, C. Shen, U. Heinz,
Phys. Rev. C91, 064906 (2015), arXiv:1504.02160 [nucl-th].
The study of particle production in proton-nucleus ($pA$) collisions provides essential
information about high-density effects (like gluon saturation) in the nuclear wavefunction and offers a benchmark for the corresponding studies in nucleus-nucleus collisions. The cross-sections for particle production in $pA$ can in principle be computed within perturbative QCD, using the framework of the Color Glass Condensate (CGC). However, recent efforts trying to extend such calculations beyond the leading-order (LO)
approximation met with an unexpected difficulty: the next-to-leading order (NLO) prediction for the hadron multiplicity suddenly turns negative at transverse momenta of the order of a few GeV, in a range where perturbation theory was expected to be reliable.
This problem triggered much interest and several studies over the last 5 years, but not satisfactory solution has emerged.
In a recent publication [1], we have revisited the previous proposals for the CGC
factorization at NLO and identified the source of the negativity problem: this is related
to the subtraction method used to separate LO from NLO contributions. To overcome this difficulty, we proposed a new factorization scheme which involves no such a subtraction: the relevant, LO or NLO, perturbative contributions are included once and only once. We have thus obtained a manifestly positive expression for the cross-section for hadron multiplicities in $pA$. On this occasion, we have also extended the resummation program that we recently proposed [2] for the BK and JIMWLK evolution equations to the calculation of cross-sections. Besides its phenomenological implications, this new factorization scheme should provide a better framework for computing particle production in QCD at high energy.
[1] ``CGC factorization for forward particle production in proton-nucleus collisions at next-to-leading order,'' E. Iancu, A. Mueller, and D. Triantafyllopoulos, e-Print: arXiv:1608.05293 [hep-ph].
[2] E. Iancu et al, Phys.Lett. B744 (2015) 293; Phys.Lett. B750 (2015) 643; JHEP 1608 (2016) 083.
The dependence of particle production on the size of the colliding system (pp, p-Pb, and Pb-Pb) is studied using the most recent measurements at $\sqrt{s_{\rm{NN}}}=5.02~\rm{TeV}$ from ALICE for -3.4 < η < +5.0 employing the same methodology for the three colliding systems. Comparing particle production between Pb-Pb and p-Pb collisions to pp collisions as a reference over a wide pseudorapidity range provides insight into the longitudinal (and low-x) nature of the hot and dense medium created in heavy-ion collisions.
Studies from pp collisions at various energies show that the width of the charged-particle pseudorapidity density distribution increases with increasing collision energy. This underlines the importance of investigating the system size dependence of the particle production at the same collision energy. The approximate linearity versus η that is found in the ratio of the pseudorapidity density distributions of the different systems indicates a coherent particle production throughout the longitudinal extent of the collision region, while the dependence on pseudorapidity may reflect underlying mechanisms, for example colour fluctuations or Colour-Glass Condensate initial conditions.
Understanding the longitudinal dependence of flow harmonics and possible event plane decorrelations is an important part of properly extracting information on the matter created in heavy ion collisions. Asymmetric systems, by their nature, provide unique insight on the relation between geometry, transverse expansion, and longitudinal dynamics. In 2016, RHIC operations included $d$+Au beam energy scan at 200, 62.4, 39, and 19.6 GeV. In this talk we present results on the pseudorapidity dependence of elliptic and triangular flow in the 2016 $d$+Au beam energy scan. Investigations into longitudinal event plane decorrelations over wide pseudorapidity ranges, including between the projectile and target directions, will be presented and compared with model calculations.
In nucleus-nucleus collisions, the linear dependence found for the elliptic flow harmonic of both positive or negative charged particles as a function of event charge asymmetry ($A_{ch} = (N^{+} – N^{-})/(N^{+}+N^{-}) $, where $N^{+}$ and $N^{-}$ are the number of positive and negative charged particles, respectively) is predicted by the phenomenon known as the Chiral Magnetic Wave (CMW) due to its induced electric quadrupole moment. However, other scenarios are also possible and may provide alternative explanations for the experimental results. New measurements of elliptic ($v_{2}$) and triangular ($v_{3}$) flow for positive and negative charged particles as a function of $A_{ch}$ in pPb and PbPb collisions at $\sqrt{s_{NN}}$ = 5.02 TeV are presented, using data collected by the CMS experiment during the LHC runs 1 and 2. The slopes and intercepts of the charged-dependent $v_{n}$ harmonics vs. $A_{ch}$ are directly compared for pPb and PbPb collisions with similar charged-particle multiplicities, where a strong CMW effect is not expected in very high multiplicity pPb events. Moreover, a comparison is made of the slope parameters between $v_{2}$ and $v_{3}$ harmonics normalized by the inclusive charge particle $v_{n}$ in PbPb collisions as a function of centrality. These results provide a means to discriminate between the CMW and other scenarios such as local charge conservation as possible explanations for the observed charge dependent behavior.
ATLAS measurements of two-particle correlations in $\Delta\phi$ and $\Delta\eta$ and multi-particle azimuthal correlations using four, six and eight-particle cumulants are presented for $pp$, $p$+Pb and low multiplicity Pb+Pb collisions.
For the two-particle correlations, a template fitting procedure is used to subtract the dijet contribution and to extract the genuine
long-range ridge correlations. In all collision systems, the ridge correlations are shown to be present even in events with a low
multiplicity of produced particles, implying that the long-range correlations are not unique to rare high-multiplicity events. The
properties of the correlation are shown to exhibit only a weak energy dependence and are remarkably similar to that observed in $p$+Pb collisions. Another new aspect of this talk is a detailed study of ridge properties in collisions containing hard processes,
characterized by large four-momentum transfer. This may help answering the question whether the ridge arises from hard or semi-hard processes, or if it is the result of mechanisms unrelated to the initial hardness scale.
In order to assess the collective nature of multi-particle production, the correlation measurements are extended to include azimuthal
correlations measured using multiparticle cumulants. The presented measurements of multi-particle cumulants $c_2 \{2–8\}$ confirm the evidence for collective phenomena in $p$+Pb and low-multiplicity Pb+Pb collisions. However, for $pp$ collisions, the measurements of cumulants do not yet provide clear evidence for collectivity as they are susceptible to event-by-event multiplicity fluctuations. In order to address this, results from a new modified cumulant method which suppress both the contribution of multiplicity fluctuation and non-flow effects are presented.
Results on elliptic flow in p+p and p/d/$^3$He+A have raised the question of how small a system can be while still exhibiting collective behavior. In 2016, RHIC operations included $d$+Au collisions at 200, 62.4, 39, and 19.6 GeV. In this talk we present results on elliptic and triangular flow at midrapidity as a function of transverse momentum and event multiplicity in $d$+Au collisions at various energies. We compare these results with several theoretical predictions in scenarios including pre-equilibrium flow, hydrodynamic flow, partonic scattering, and purely hadronic scattering in order to assess the role of each stage in the system evolution for producing collective effects in small systems.
Recent observation of collective-flow-like behaviors in small colliding systems
attracts significant theoretical and experimental interests.
In large colliding systems,
large collective flow has been interpreted as manifestation of almost-perfect
fluidity of the quark gluon plasma (QGP).
So it is quite intriguing to explore how small
the QGP can behave as a fluid.
In this presentation,
we newly develop an initialization model
for hydrodynamic simulations
by combining a
Monte-Carlo version of the Glauber model (MC-Glauber)
with an event generator PYTHIA.
We further implement this model into
an integrated dynamical framework [1]
which is based on
fully three-dimensional ideal hydrodynamic description
of the QGP fluid and kinetic description of the hadron gas.
Using this new version of integrated dynamical model,
we analyze multiplicity fluctuations and
collective flow in small colliding systems
at RHIC and LHC energies.
Multiplicity fluctuations play a crucial role in centrality definition of the events
in small colliding systems
since the fluctuations
are, in general, more important as the system size is getting smaller.
To consider the correct multiplicity fluctuations,
we employ
PYTHIA which
naturally describes multiplicity distribution in $p$+$p$ collisions.
We superpose $p$+$p$ collisions
by taking into account the number of participants and that of binary collisions
from MC-Glauber
and evaluate initial entropy density distributions which
contain not only multiplicity fluctuations but also fluctuations of longitudinal profiles.
Solving hydrodynamic equations followed by the hadronic afterburner,
we calculate $p_{T}$ spectra, elliptic ($v_{2}$) and triangular ($v_{3}$) flow
in $p$+Au, $d$+Au and $^3$He+Au collisions
at the RHIC energy and
$p$+Pb collisions at the LHC energy.
Although a large fraction of final $p_{T}$-integrated $v_{2}$ and $v_{3}$ comes from
the fluid-dynamical stage, the effects of hadronic rescatterings turn out to be also important as well
in understanding of the flow data in small colliding systems.
Reference
[1] T.Hirano, P.Huovinen, K.Murase and Y.Nara,
``Integrated Dynamical Approach to Relativistic Heavy Ion Collisions,''
Prog. Part. Nucl. Phys. 70, 108 (2013)
Direct photon measurement in heavy-ion collisions provides a valuable set of observables to study the hot QCD medium since these photons are
produced at different stages of the collision and escape the medium unaffected. In pp collisions,
the direct photon yield at high transverse momentum ($p_T$) are produced in hard scattering (prompt photons),
but also in the fragmentation of high $p_T$ partons. Their measurement provides a direct test of pQCD and can constrain the parton distribution functions.
The access to the prompt photon production can be achieved experimentally with isolation techniques.
In heavy-ion collisions, the high $p_T$ component provides information on the initial parton dynamics and nuclear parton densities in nuclei, whereas
the low momentum component (below $p_T <$5 GeV/c) of the direct photon production is dominated by thermal radiation from the hot and dense matter created,
carrying information on its space-time evolution, collective flow and temperature.
In this talk, we will present ALICE results of direct photon production in pp reactions at $\sqrt{s}$=7 TeV using isolation techniques.
The measurement of the direct photon flow in Pb-Pb collisions at $\sqrt{s_{NN}}$=2.76 TeV will also be presented.
The results will be discussed and compared to theoretical predictions and earlier measurements.
PHENIX has discovered a large yield of low momentum direct photons emitted with large azimuthal anisotropy in 200 GeV Au+Au collisions. The large yield suggests early emission at high temperature, while the large anisotropy points towards late emission when the radial flow of the matter is fully developed, but the temperature is already reduced. This apparent contradiction poses a significant challenge to models that aim to calculate thermal photon production. To further constrain the sources of the low pt photons, PHENIX is analyzing data from Au+Au collisions at lower beam energies of 39 and 62.4 GeV, as well as data from smaller collisions systems Cu+Cu and Cu+Au at 200 GeV. First results from these analyses and from a larger statistics sample of Au+Au at 200 GeV will be presented.
We discuss photon emission at the stage of hadronization as a possible resolution to the direct-photon puzzle. In an ordinary plasma, it is well known that photon emission occurs when a plasma goes back to a normal state through recombination processes such as $e^- + p^+ \to H + \gamma$ for an electron-proton plasma. This is called the “radiative recombination”. A similar process should take place when a QGP hadronizes. For example, meson formation from a quark and an antiquark will be accompanied by photon emission $q + \bar q \to meson + \gamma$ to compensate the energy difference between the initial and final states.
In order to compute the number of photons emitted at hadronization, we employ the “recombination model” developed by the Duke group. There, the number of produced hadrons is computed under the assumption that coalescence of valence (anti)quarks just occurs without emission of additional particles, which surely violates energy and entropy conservation. With the photon emission added in this coalescence process, however, energy and entropy can be made conserved. We reinterpret the production formula of hadrons in the original recombination model as that of artificial “resonant states” whose invariant masses are not necessarily equal to the masses of any physical hadrons. We further assume that the “resonant state” decays into a physical hadron and a photon.
This “improved” recombination model has a potential to resolve the direct-photon puzzle: (1) a larger yield of photons since we add photon production at hadronization, which has been overlooked so far, and (2) radiated photons flow similarly as hadrons because photons are emitted in a collimated way with the resonant state's motion. Moreover, the pt distribution of emitted photons mimics thermal distribution whose effective temperature is essentially given by blue-shifted quark’s temperature and thus becomes much higher than critical temperature.
Large scale classical statistical simulations confirm [1] that the correct kinetic theory describing the off-equilibrium Glasma is the "bottom-up” thermalization scenario [2]. Detailed simulations demonstrate that the bottom-up results match on to relativistic viscous hydrodynamics on time scales of order 1 Fermi for realistic values of the coupling [3]. We explore the detailed implications of this scenario for photon production in the Glasma relative to the QGP [4]. In particular, we argue that the "reheating” phase of the bottom-up scenario will lead to enhanced production rates for photons and may possibly generate the significant flow anisotropies essential for resolving the v2 puzzle. We report on first kinetic simulations of photon production in the expanding Glasma that will quantify our estimates and determine how brightly the Glasma shines relative to the QGP [5].
References:
[1] J. Berges, K. Boguslavski, S. Schlichting, R. Venugopalan, Phys. Rev. D89 (2014) no. 7, 074011; ibid., no. 11, 114007.
[2] R. Baier, A. H. Mueller, D. Schiff, D. T. Son, Phys. Lett. B502 (2001) 51.
[3] A. Kurkela and Y. Zhu, Phys. Rev. Lett. 115 (2015) no. 18, 182301.
[4] N. Tanji, J. Berges, K. Reygers, R. Venugopalan, in preparation.
[5] N. Tanji and R. Venugopalan, in preparation.
The polarization of direct photons
produced in an ultrarelativistic heavy-ion collision reflects the
anisotropy of the quark-gluon plasma created in the collision. We describe a general framework, based on the photon spectral functions in the plasma, for analyzing the angular distribution and thus the polarization of dileptons in terms of the plasma anisotropies. The rates of dilepton production depend in general on four independent spectral functions, corresponding to two transverse polarizations, one longitudinal polarization, and -- in plasmas in which the anisotropy is not invariant under parity in the local rest frame of the matter -- a new spectral function, $\rho_n$, related to the anisotropy direction in the collision. The anisotropy appears in the difference of the two transverse spectral functions, as well as in $\rho_n$. As an illustration we delineate the spectral functions for dilepton pairs produced in the lowest order Drell-Yan process of quark-antiquark annihilation to a virtual photon.
Within the context of a hybrid strong/weak coupling model of jet quenching, we study the
modification of the angular distribution of the energy within jets in
heavy ion collisions, as partons within jet showers lose energy
and get kicked as they traverse the strongly coupled plasma
produced in the collision. To describe
the dynamics transverse to the jet axis, we add
the effects of transverse momentum broadening
into our hybrid construction, introducing a parameter $K\equiv \hat q/T^3$
that governs its magnitude. We show that, because of the quenching
of the energy of partons within a jet, even when $K\neq 0$ the jets
that survive with some specified energy in the final state are narrower
than jets with that energy in proton-proton collisions. For this reason,
many standard observables are rather
insensitive to $K$.
We
propose a new differential jet shape ratio observable in which
the effects of transverse momentum broadening are apparent.
We also analyze the response of the medium to the passage
of the jet through it, noting that the momentum lost by the jet
appears as the momentum of a wake in the medium. After
freezeout this wake becomes soft particles with a broad angular
distribution but with net momentum in the jet direction, meaning that
the wake contributes to what is reconstructed as a jet. This effect must
therefore be included in any description of the angular structure
of the soft component of a jet.
We show that the particles coming from the response of the medium to the momentum
and energy deposited in it leads to a correlation between the momentum of soft particles
well separated from the jet in angle with the direction of the jet momentum, and find
qualitative but not quantitative agreement with experimental data on observables designed
to extract such a correlation.
More generally, by confronting the results that we obtain upon introducing transverse momentum
broadening and the response of the medium to the jet
with available jet data, we highlight the importance of these processes for
understanding the internal, soft, angular structure of high energy jets.
Dijet, dihadron, hadron-jet angular correlations have been reckoned as important probes of the transverse momentum broadening effects in relativistic nuclear collisions [1]. When a pair of high-energy jets created in hard collisions traverse the quark-gluon plasma produced in heavy-ion collisions, they become de-correlated due to the vacuum soft gluon radiation associated with the Sudakov logarithms and the medium-induced transverse momentum broadening [2, 3]. For the first time, we employ the systematical resummation formalism and establish a baseline calculation to describe the dihadron and hadron-jet angular correlation data in $pp$ and peripheral $AA$ collisions where the medium effect is negligible. We demonstrate that the medium effects, especially the so-called jet quenching parameter $\hat q$, can be extracted from the angular de-correlations observed in $AA$ collisions. A global $\chi^2$ analysis of dihadron and hadron-jet angular correlation data renders the best fit $\langle \hat q L\rangle_{\textrm{tot}} \sim 14 \textrm{GeV}^2$ for a quark jet at RHIC top energy, with $L$ the typical traversed medium length. Our approach [1] stands as a new and complimentary method for the extraction of the jet quenching parameter $\hat{q}$ as compared to the JET Collaboration effort [4]. 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 unprecedented knowledge of jet quenching parameter in relativistic heavy-ion collisions.
References:
[1]. Probing Transverse Momentum Broadening via Dihadron and Hadron-jet Angular Correlations in Relativistic Heavy-ion Collisions, by Lin Chen, Guang-You Qin, Shu-Yi Wei, Bo-Wen Xiao, Han-Zhong Zhang, arXiv:1607.01932 [hep-ph].
[2]. Soft Gluon Resummations in Dijet Azimuthal Angular Correlations in Hadronic Collisions, By Peng Sun, C. -P. Yuan, Feng Yuan, arXiv:1405.1105 [hep-ph], Phys.Rev.Lett. 113 (2014) no.23, 232001.
[3]. Medium Induced Transverse Momentum Broadening in Hard Processes, By A.H. Mueller, Bin Wu, Bo-Wen Xiao, Feng Yuan, arXiv:1608.07339 [hep-ph].
[4]. Extracting the jet transport coefficient from jet quenching in high-energy heavy-ion collisions, By JET Collaboration (Karen M. Burke et al.), arXiv:1312.5003 [nucl-th], Phys.Rev. C90 (2014) no.1, 014909.
In relativistic heavy-ion collisions at the Large Hadron Collider (LHC), conditions are met to produce a hot, dense and strongly interacting medium known as the Quark Gluon Plasma (QGP). Quarks and gluons from incoming nuclei collide to produce partons at high momenta early in the collision. By fragmenting into collimated sprays of hadrons, these partons form "jets". Within the framework of perturbative QCD, jet production is well understood in pp collisions and can be used as a baseline reference for comparing to heavy ion collision systems when studying jet quenching. One approach is to measure the azimuthal correlations of a trigger and the associated hadrons in the event. For a jet trigger, these are known as jet-hadron correlations, while a hadron trigger leads to di-hadron correlations. Such correlations are examined in transverse momentum bins of the trigger, transverse momentum bins of the associated hadrons, and studied as a function of collision centrality. The correlations are expected to be sensitive to broadening and softening of the associated recoil jet due to jet quenching. We present azimuthal jet-hadron correlations constructed from a trigger R=0.2 full (charged + neutral) jet, which is correlated with charged hadrons. In an effort to control the path length of the recoil jet and reduce the impact of the background, jets are required to pass a leading constituent cut and are reconstructed using only high energy and momentum constituents. We present the current status of this analysis in Pb--Pb collisions at $\sqrt{s_{NN}}=2.76$ TeV. To further probe the path length dependence, we will also present Pb-Pb jets relative to the event plane, which include a highly robust and precise background subtraction method to remove the complex, flow dominated, heavy ion background. The jet yields and widths will be presented for the Pb--Pb analyses and compared to our baseline measurements in pp collisions. In addition, yields and widths for di-hadron correlations in Pb-Pb will be presented.
Jets are an important tool to study the hot, dense matter produced in Pb+Pb collisions at the LHC.
Due to the loss of some of the jet’s energy outside the jet cone, jet rates have been found to be reduced by approximately a factor of two, in the most central events and over a wide kinematic range. In order to understand precisely how the jets are modified, it is important to measure how the jet momentum is carried by its fragmentation products. The longitudinal momentum fraction of charged particles in jets from Pb+Pb, p+Pb, and p+p collisions have been measured using the ATLAS detector. Proton-proton and p+Pb collisions provide necessary baseline measurements for quantifying the modifications in Pb+Pb collisions. In Run 1, ATLAS collected samples of p+p and Pb+Pb collisions at a center of mass energy of 2.76 TeV and a sample of p+Pb collisions at 5.02 TeV. In Run 2, large samples of p+p and Pb+Pb collisions at 5.02 TeV have been collected providing a complete set of collision systems at 5.02 TeV. In this talk, we present the status of fragmentation function measurements at 5.02 TeV in the context of detailed studies of the fragmentation in p+p and Pb+Pb collisions at 2.76 TeV.
Recently our group analyzed how the probability distribution for the jet
opening angle is modified in an ensemble of jets that has propagated
through an expanding cooling droplet of plasma [1]. Each jet in the
ensemble is represented holographically by a string in the dual 4+1-
dimensional gravitational theory with the distribution of initial energies and
opening angles in the ensemble given by perturbative QCD. In [1], the full
string dynamics were approximated by assuming that the string moves at
the speed of light. We are now able to analyze the full string dynamics for
a range of possible initial conditions, giving us access to the dynamics of
holographic jets just after their creation. We show that, after a period of
time that we compute, the string nullifies: the force of gravity accelerates
each section of string until it approaches the speed of light. The
nullification timescale and the features of the string when it has nullified
are all results of the string evolution. This emboldens us to analyze the full jet shape, rather than just the opening angle of each jet in the ensemble as in [1]. We find the
striking result that the jet shape scales with the opening angle at any
particular energy. We construct an ensemble of dijets with energies and
energy asymmetry distributions taken from events in proton-proton
collisions, opening angle distribution as in [1], and jet shape taken from
proton-proton collisions and scaled according to our result. We study how
all of these observables are modified after we send the ensemble of dijets
through the strongly-coupled plasma.
[1] Krishna Rajagopal, Andrey V. Sadofyev, Wilke van der Schee, "Evolution of the jet opening angle distribution in holographic plasma", PRL 116, 211603 (2016)
Transport coefficients of QCD like the shear viscosity $\eta$ and the diffusion of baryon number $D$ have been determined at leading order in perturbation theory by Arnold, Moore and Yaffe (AMY). I will show how these transport coefficients are sensitive to $O(g)$ corrections arising from interactions with soft gluons. These NLO effects enter as corrections to the transverse momentum broadening coefficient $\hat {q}$, to longitudinal momentum broadening, to quark-gluon conversions, to collinear 1<->2 processes and to wider-angle bremsstrahlung (semi-collinear processes). These corrections have been computed using a Euclidean formalism pioneered by S. Caron-Huot, which exploits the analytical properties of amplitudes supported on light fronts. There remain only two coefficients whose $O(g)$ corrections are unknown, as I will show.
I will show the effect of all known corrections to the value of the transport coefficients. In particular, the large $O(g)$ contribution to $\hat{q}$ is the leading NLO effect and it reduces the value of the transport coefficients very significantly. I will also estimate the effect of the unknown coefficients.
We develop an effective field theory for QCD at finite temperature which takes into account the global symmetries of the problem, including the fact that Lorentz invariance is broken. We discuss regularization and fixing of parameters in the theory. Some of the predictions of the theory are presented, for example the curvature of the critical line. The degree of agreement between the predictions and lattice results shows both that such an approach captures much of the essential physics, and could be useful to discover the nature of the major remaining corrections.
We want to study thermodynamical observables at finite density. Since direct lattice simulations at finite muB are hindered by the sign problem an efficient way to study the QCD phase diagram at small finite density is to extrapolate observables from imaginary chemical potential. In this talk we present results on several observables for the equation of state. The observables are calculated along the isentropic trajectories in the (T, muB ) plane corresponding to the RHIC Beam Energy Scan collision energies. The simulations are performed at the physical mass for the light and strange quarks. muS was tuned in a way to enforce strangeness neutrality to match the experimental conditions; the results are continuum extrapolated and systematic effects are taken into account for the error estimate.
Many precision measurements of quarkonium suppression at the LHC, e.g. the nuclear modification factor R_AA of J/Psi, are well described by a multitude of different models [1]. Thus pinpointing the underlying physics is difficult and first principles guidance is needed. In-medium spectral properties, e.g. mass shifts or the broadening of states can help us to understand quarkonium production in a kinetically equilibrated setting. While potential based approaches with lattice input [2] have been used to estimate such modifications, a direct and quantitative determination from first principles lattice QCD is still outstanding.
Advancing towards this goal we present here a high statistics study of bottomonium and charmonium S-wave and P-wave spectral properties at finite temperature using the effective field theory NRQCD on the lattice. This EFT allows us to capture the physics of quarkonium without modelling assumptions in a realistic thermal QCD medium, described by state-of-the-art lattices of the HotQCD collaboration at almost physical pion mass [3]. The availability of two Bayesian methods for spectral functions (MEM and BR [4]) makes it possible to thoroughly test the systematic uncertainties of their reconstruction.
Our new lattice QCD correlation functions and reconstructed spectra corroborate a picture of sequential modification of states with respect to their vacuum binding energy. We find that remnant features of the bottomonium S-wave may survive up to T~400MeV, while the P-wave ground state disappears around T~300MeV. The charmonium analysis hints at melting of the P-wave below T~190MeV while some S-wave remnant feature might survive up to T~245MeV.
With the inclusion of charmonium spectra, an extended temperature range and increased statistics by more than an order of magnitude our study provides a coherent picture of in-medium quarkonium modification extending significantly beyond our previous results of Ref. [5].
[1] A. Andronic et.al., Eur.Phys.J. C76 (2016) no.3, 107
[2] Y. Burnier, A.Rothkopf, O. Kaczmarek JHEP 1512 (2015) 101 and arXiv:1606.06211
[3] A. Bazavov et. al., Phys.Rev. D85 (2012) 054503 and Phys.Rev. D90 (2014) 094503
[4] Y. Burnier, A. Rothkopf, Phys.Rev.Lett. 111 (2013) 182003
[5] S. Kim, P. Petreczky, A. Rothkopf, Phys.Rev. D91 (2015) 054511 and arXiv:1512.05289 (QM2015)
The QCD Equation of State (EoS) is fundamental for our understanding of the properties
of strong-interaction matter at non-zero temperature and density. In view of the
upcoming Beam Energy Scan II program at RHIC, it is important to gain control
over the EoS in the entire range of chemical potentials ($\mu_B$) accessible at
RHIC, $0\le \mu_B/T \le 3$. This will provide crucial input for the hydrodynamic
modeling of hot and dense matter and will allow to clarify whether or not a
critical end-point exists in this parameter range.
We present results for the QCD Equation of State at non-zero chemical potentials
corresponding to the conserved charges in QCD using Taylor expansion upto $6^{th}$ order
in the baryon number, electric charge and strangeness chemical potentials. The latter
two are constrained by strangeness neutrality and a fixed electric charge to
baryon number ratio. In our calculations, we use the Highly Improved Staggered Quarks
(HISQ) discretization scheme at different values of the lattice spacings to
control lattice cut-off effects. The light and strange quark masses are
adjusted to reproduce physical values of pion and kaon masses. Furthermore we calculate the
pressure along lines of constant energy density, which serve as proxies for the
freeze-out conditions and discuss their dependence on $\mu_B$, which is necessary for
hydrodynamic modeling near freezeout.
We also provide an estimate of the radius of convergence of the Taylor series from the $6$th order
coefficients which gives a new constraint on the location of the critical end-point
in the $T$-$\mu_B$ plane of the QCD phase diagram.
We study the thermodynamics of the quark gluon plasma with
lattice simulations in the continuum limit up to 1 GeV
temperature where we show that a perturbative description
already applies. We calculate the effect of the presence
of charm quark in the equation of state and also describe
the topological features of quantum chromodynamics.
The talk is based on the paper 1606.07494 (Nature, in press).
Strong fluctuations of the elliptic flow in heavy ion collisions allow an efficient selection of the events corresponding to a specific initial geometry. This technique, Event Shape Engineering, was applied to select events corresponding to the
same centrality, but having very different values of elliptic flow. For those events, we present results on the centrality dependence of the charge-dependent two- and three-particle correlators in Pb-Pb collisions at
$\sqrt{s_{\rm NN}} = 2.76$ TeV recorded by the ALICE detector. The charge dependence of the three-particle
correlator is often employed as evidence for the Chiral Magnetic Effect (CME). The interpretation of the
experimental results is complicated by possible background contributions, including the modulation from elliptic flow.
We have used these measurements and a Monte-Carlo Glauber simulation of the magnetic field to derive an upper limit on
the CME contribution.
The quark-gluon matter produced in relativistic heavy-ion collisions may contain local domains in which P and CP symmetries are not preserved. When coupled with an external magnetic field, such P- and CP-odd domains will generate electric currents along the magnetic field --- a phenomenon called the chiral magnetic effect (CME). Recently, the STAR Collaboration at RHIC and the ALICE Collaboration at the LHC released data of charge-dependent azimuthal-angle correlators with features consistent with the CME expectation. However, the experimental observable is contaminated with significant background contributions from elliptic-flow-driven effects, which makes the interpretation of the data ambiguous. In this Letter, we show that the collisions of isobaric nuclei, $^{96}_{44}$Ru + $^{96}_{44}$Ru and $^{96}_{40}$Zr + $^{96}_{40}$Zr, provide an ideal tool to disentangle the CME signal from the background effects. Our simulation demonstrates that the two collision types at 200 GeV have more than 10% difference in the CME signal
and less than 2% difference in the elliptic-flow-driven backgrounds for the centrality range of 20-60\%.
We present measurements of the charge-dependent, three-particle correlator \gamma=<cos(\phi1+\phi2-2\phi3)> and elliptic flow v2 in central and ultra-central U+U and Au+Au collisions at sqrt{sNN}=200 GeV from STAR. The difference \Delta \gamma = \gamma(like- sign)-\gamma(un-like sign) measures charge separation across the reaction plane, a predicted signal of the Chiral Magnetic Effect (CME) [1]. Although charge separation has been observed, it has been argued that the measured separation can also be explained by elliptic flow related backgrounds [2]. To disentangle the two effects, we use triggered samples of U+U and Au+Au collisions for which predictions from flow related backgrounds and magnetic field dependent effects diverge. Our analysis includes events selected based on asymmetries in spectators observed in the Zero-Degree-Calorimeters which preferentially select body-tip events [3]. We find that for cases where known, flow driven, background expectations differ from CME related predictions driven by the correlation of the magnetic field and the flow; the data are in better accord with the CME related prediction.
[1] S. A. Voloshin, Phys.Rev. C70 (2004) 057901 hep-ph/0406311
[2] S. Schlichting, S. Pratt, Phys.Rev. C83 (2011) 014913
[3] S. Chatterjee, P. Tribedy, Phys.Rev. C92 (2015) 1,011902
Charge-dependent azimuthal correlations relative to the event plane in AA collisions have been suggested as providing evidence for the chiral magnetic effect (CME) caused by local strong parity violation. However, the observation of the CME remains inconclusive because of several possible sources of background correlations that may account for part or all of the observed signals. This talk will present the first application of three-particle, charge-dependent azimuthal correlation analysis in proton-nucleus collisions, using pPb data collected with the CMS experiment at the LHC at $\sqrt{s_{NN}}$ = 5.02 TeV. The differences found in comparing same and opposite sign correlations are studied as a function of event multiplicity and the pseudorapidity gap between two of the particles detected in the CMS tracker detector. After selecting events with comparable charge-particle multiplicities, the results for pPb collisions are found to be similar to those for PbPb collisions collected at the same collision energy. With a reduced magnetic field strength and a random field orientation in high multiplicity pPb events, the CME contribution to any charge separation signal is expected to be much smaller than found in peripheral PbPb events. These results pose a challenge for the interpretation of charge-dependent azimuthal correlations in heavy ion collisions in terms of the chiral magnetic effect.
Abstract:
Chiral Magnetic Effect (CME) is the macroscopic manifestation of the fundamental chiral anomaly in a many-body system of chiral fermions, and emerges as anomalous transport current in hydrodynamic framework. Experimental observation of CME is of great interest and significant efforts have been made to look for signals of CME in heavy ion collisions. Encouraging evidence of CME-induced charge separation has been reported from both RHIC and LHC, albeit with ambiguity due to potential background contributions. Crucial for addressing such issue, is need of quantitative predictions of CME signal with sophisticated modeling tool.
In this talk we report a recently developed Anomalous Viscous Fluid Dynamics (AVFD) framework, which simulates the evolution of fermion currents in QGP on top of the data-validated VISHNew bulk hydro evolution. With realistic initial conditions and magnetic field lifetime, the predicted CME signal is quantitatively consistent with measured change separation data in 200GeV AuAu collisions. We further develop the event-by-event AVFD simulations that directly compute CME-induced two-particle correlations as well as the non-CME background. Finally we report predictions for the upcoming isobaric (RuRu v.s. ZrZr ) collisions that could provide the critical test of the CME in heavy ion collisions.
We present a first principles approach to study the dynamics of the Chiral Magnetic Effect and Chiral Magnetic Wave based on real-time lattice simulations with dynamical (Wilson and Overlap) fermions simultaneously coupled to color and electro-magnetic fields. We discuss how these techniques can be used to study the Chiral Magnetic Effect during the pre-equilibrium stage of a heavy-ion collision and present first results obtained within a simplified setup. While for light quarks we observe a dissipationless transport of charges as in anomalous hydrodynamics, we demonstrate that for heavier quarks the effects of explicit chiral symmetry breaking lead to a significant reduction of the associated currents. Within our microscopic approach, we also extract the spectral properties of the fermion fields to establish an intuitive picture of what flows in the Chiral Magnetic Effect.
[1] N. Mueller, S. Schlichting, S. Sharma, arXiv:1606.00342 (accepted for publication in Phys. Rev. Lett.)
[2] M. Mace, N. Mueller, S. Schlichting, S. Sharma, in preparation
Quarkonium production is an important probe to study the properties of the Quark Gluon Plasma (QGP) formed in relativistic heavy-ion collisions. The suppression of J/ψ due to the color-screening effect in the medium was initially proposed as direct evidence of the QGP formation. However, the interpretation of J/ψ suppression is still challenging due to the regeneration contribution from the coalescence of uncorrelated $c\bar{c}$ pairs in the medium and the cold nuclear matter effects. By comparing productions of different charmonium states in p+p, p+Au, and Au+Au collisions, the cold and hot nuclear matter effects can be systematically studied in detail.
In the 2014 and 2015 RHIC runs, the STAR experiment recorded a large amount of data in p+p, p+Au, and Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV for charmonium studies via both the dielectron and dimuon channels. In this talk, we present precise measurements of nuclear modification factors for J/ψ production over a broad kinematic range in both p+Au and Au+Au collisions. We will also present the first measurements of the double ratio of ψ(2s) and J/ψ production rates at mid-rapidity in p+p and p+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV. We will compare these results with model calculations and discuss physics implications of the measured cold and hot nuclear matter effects for extracting the QGP properties.
ALICE at the Large Hadron Collider (LHC) provides unique capabilities
to study charmonium production at low transverse momenta. In the early
and hottest phase of nucleus-nucleus collisions the formation of a
Quark-Gluon Plasma (QGP) is expected. Several QGP induced effects,
such as the melting of charmonium states due to color screening and/or
a (re)combination of uncorrelated charm and anti-charm quarks, can
play a role. While a suppression of J/ψ with respect to pp
collisions was indeed observed in heavy-ion collisions at all
energies, recent measurements in Pb-Pb collisions at √sNN = 2.76
TeV indicate that (re)combination does seem to play an important role
in the low pT region at LHC energies.
At central rapidity, corresponding to the range |y| < 0.9, J/ψ
are reconstructed via their decay into two electrons down to zero
pT. We will present new results on the inclusive J/ψ nuclear
modification factor RAA as a function of centrality and transverse
momentum in Pb-Pb collisions at √sNN = 5.02 TeV. Due to the now
available higher event statistics these data allow a more differential
investigation of the evolution of RAA than previous measurements.
They provide, in combination with results from lower energies and
theoretical predictions, important information on the different
mechanisms related to the presence of the hot medium produced in
heavy-ion collisions.
Charmonium production in PbPb collisions requires the inclusion of many phenomena to be understood, such as melting in the quark gluon plasma and statistical recombination, on top of cold nuclear matter effects (modifications of nPDFs, initial-state energy loss, nuclear break-up), better probed in pPb collisions. Final results on the relative J/$\psi$ and $\psi$(2S) modification, based on the pp and PbPb data collected at $\sqrt{s_{NN}}$ = 5.02 TeV by CMS in 2015, will be reported. In addition, new prompt J/$\psi$ results in PbPb collisions at $\sqrt{s_{NN}}$ = 5.02 TeV, including the $R_{AA}$, will be presented over a wide kinematic and centrality range ($3 < p_{T} <30$ GeV/c, $|y|<2.4$, and fine event-centrality intervals). The results are compared to those obtained at $\sqrt{s_{NN}}$ = 2.76 TeV over the same kinematic range, considering also the J/$\psi$ $v_2$ obtained at the latter energy. Final prompt J/$\Psi$ results in pPb collisions at 5.02 TeV will also be presented, including the new measurement of the $R_{pA}$ using the 2015 pp data taken at the same energy. At last, new results will be reported regarding prompt $\Psi$(2S) meson production in pPb collisions at $\sqrt{s_{NN}}$ = 5.02 TeV, as a function of transverse momentum and rapidity and down to $p_{T}$ = 4 GeV.
The suppression of heavy charmonia states in heavy-ion collisions is a phenomenon understood as a consequence of QGP formation in the hot, dense system formed in heavy ion collisions at the LHC. In addition to hot matter effects in heavy-ion collisions , cold nuclear effects may also affect heavy charmonia production . Therefore, a full assessment requires detailed studies on the effects present in both A-A and p+A collisions. Based on p+Pb data collected in 2013 and pp and Pb+Pb data collected in 2015 at the LHC, the ATLAS experiment has studied prompt and non-prompt J/psi and psi(2S)productions via the di-muon decay final states. The production and excited-to-ground state ratios of heavy charmonia measured in both p+Pb and Pb+Pb collision data with respect to that measured in pp collision data will be presented in intervals of transverse momentum, rapidity and centrality.
ALICE is the LHC experiment dedicated to the study of ultra relativistic
heavy-ion collisions where the formation of a hot and dense
strongly-interacting medium, a Quark-Gluon Plasma (QGP), is expected.
Considerable theoretical and experimental efforts have been invested in
the last 30 years to study the properties of the QGP. One of the signals
of QGP formation is the charmonium suppression. Measurements from Pb-Pb
collisions at $\sqrt{s_{\rm NN}} = 2.76$ TeV revealed a suppression of
charmonium yields in central collisions, compared to binary-scaled pp
collisions. However, the magnitude of the suppression is smaller than
what was observed at lower energies at the SPS and RHIC, indicating that
J/$\psi$ regeneration via recombination of charm and anti-charm quarks
plays an important role at LHC energies.
In this contribution, charmonium ($\psi(2S)$ and J$/\psi$) measurements
at forward rapidity ($2.5 < y < 4$) in Pb-Pb collisions at
$\sqrt{s_{\rm NN}} = 5.02$ TeV with ALICE will be presented.
The analyses are performed in the dimuon decay channel down to zero
transverse momentum ($\it{p}_T$) with the data sample collected in 2015
(about 7 times more statistics than that collected in Pb-Pb collisions
at $\sqrt{s_{\rm NN}} = 2.76$ TeV). Together with results on the
J$/\psi$ nuclear modification factor $R_{\mathrm{AA}}$ as a function of
centrality, transverse momentum and rapidity, new multi-differential
measurements will be presented. First results on the J/$\psi$ $\langle
p_{\rm T} \rangle$ and $\langle p_{\rm T}^2 \rangle$ as a function of
centrality will be discussed. Preliminary results on the
($\psi(2S)$/J$/\psi$) ratio as a function of centrality and transverse
momentum will also be shown. The results will be compared to various
theoretical models as well as to other experimental results.
For theoretical understanding of quarkonium production in heavy ion collisions
it is important to know the potential between the heavy quark and anti-quark
at non-zero temperature. This potential is complex and provides an efficient
way to calculate quarkonium spectral functions at non-zero temperature and
an important input for dynamical models aiming to describe quarkonium
production in heavy ion collisions (see e.g. [1]).
I report on the lattice calculations of the heavy quark potential at T>0
in 2+1 flavor QCD at physical quark masses using Highly Improved Staggered
Quark formulation. The gauge configurations needed for the high statistics
study of the heavy quark potential have been generated by HotQCD and TUMQCD
collaborations [2]. In this study lattices with temporal extent $N_t=12$ and $16$
are used and the real and imaginary part of the potential are obtained using the
moments of the temporal Wilson loops. I study in detail the systematic effects in the determination of the real and the imaginary parts of the potential when using the moment method. It turns out that below the transition temperature the imaginary part is consistent
with zero, while above the transition temperature it increases with increasing temperature and separation between the quark and anti-quark till the signal diminishes.
The real part of the potential is similar to the zero temperature one for distances smaller than 0.8fm and temperatures smaller than 250 MeV. This analysis significantly extends the preliminary work presented in [3].
I will also discuss the implications of these findings for existence of heavy quark bound
state in QGP by calculating the corresponding meson spectral functions with the newly determined potential.
References:
[1] P. Petreczky and C. Young, Sequential bottomonium production at high temperatures, arXiv:1606.08421 [nucl-th]
[2] A. Bazavov et al (HotQCD), Phys. Rev. D90 (2014) 094503
[3] A. Bazavov, Y. Burnier and P. Petreczky, Nucl. Phys. A932 (2014) 117
We present recent results on measurements of jet substructures using grooming techniques with pp and PbPb data collected with the CMS detector at a center-of-mass energy of 5.02 TeV per nucleon pair. The grooming technique is used to focus on the hard structure of the jet by extracting the two subjets which correspond to the hardest parton splitting. This allows us to study medium-induced gluon emission properties and the evolution of partons through dense QCD matter. The hard jet structure is sensitive to the virtuality evolution of a parton in the medium, as well as the role of (de)coherent gluon emitters. Results and prospects on the transverse momentum balance, mass and angular difference of the two hard subjets over a wide range of jet transverse momentum and various collision centrality selections are discussed.
Within the overwhelming majority of models, light quark and gluon jet quenching in heavy ion collisions is described as resulting predominantly from pQCD-type gluon radiation, but details of the underlying mechanisms differ greatly. One key difference lies in the treatment of the Altarelli-Parisi, AP, splitting functions. While in some models, such as Q-PYTHIA, the splitting functions are directly modified in the medium, this core component remains unchanged in others (e.g. YaJEM). The shared momentum fraction $z_g$ was shown to be a Sudakov-safe measurement of the splitting function [1].
This quantity measures the $p_T$ ratio between the two dominant branches
as determined by the SoftDrop grooming process.
An inclusive measurement of $z_g$ in p+p collisions at top RHIC energy will be presented.
The focus of our Au+Au results will be on a comparative study to p+p
using the specific di-jet selection introduced in our previous momentum imbalance measurement,
i.e. jets geometrically matched to "hard core" jets found using only constituents above 2 GeV/$c$ and with a high tower above 5.5 GeV. Such di-jet pairs were found to be significantly imbalanced with respect to p+p, yet regained balance when all soft constituents were included. Individual examination of the splitting behavior of leading and recoil jet adds a new dimension to this observation, and new input to energy loss models.
[1] A. J. Larkoski, S. Marzani and J. Thaler, Phys. Rev. D 91, 111501 (2015)
The measurement of jet substructure provides important detailed information on the dynamics of the jet-QGP interaction. However, our ability to reliably extract such information is contingent on understanding the sensitivity of any given substructure observable to specific features of in-medium jet dynamics. Monte Carlo event generators with transparent physics content and that have been validated for a wide set of observables, e.g. JEWEL, are powerful tools to establish the sensitivity of observables to specifics of jet-QGP interaction. Using the generic procedure we put forward in Eur.Phys.J. C76 (2016) no.5, 288 (arXiv:1512.08107 [hep-ph]) — where we applied it to establish the origin of the excess dijet asymmetry observed in AA collisions as due to fluctuations of the jet fragmentation pattern rather than, as widely believed in the community, to the difference in the amount of matter traversed by the two jets in the pair — we examine the z_g substructure observable recently measured by CMS and STAR. We find straightforward interpretations of the z_g measurement as indicating a QGP-induced modification of the QCD splitting function to be over-simplistic and confounded by the observable sensitivity to fluctuations of the jet-medium interaction pattern. We propose several complementary measurements that can further elucidate the potential of this, and related observables, to give information on in-medium jet dynamics.
The modifications of jets in heavy-ion collisions are manifest in many measurements. However, inclusive observables are generally susceptible to the quantum mechanical nature of interactions of the jet fragments with the medium. We argue that contemporary jet substructure techniques facilitate a more direct measurement of the radiative mechanism caused by medium interactions. As a concrete example, we focus on jet grooming using the “soft drop” procedure that singles out the two leading jet substructures with largest angular separation inside an energetic jet. The interplay between hard, quasi-collinear vacuum or medium-induced radiation within the reconstructed cone and soft, large-angle emissions that are responsible for out-of-cone energy flow is studied. We find an enhancement of the splitting function at small energy-fractions which is attributed to rare, relatively hard medium-induced gluon radiation affected by LPM interference with the quark-gluon plasma.
In relativistic heavy ion collisions, a hot medium with a high density of unscreened color charges is produced. Jets are produced at the early stages of this collision and are known to become attenuated as they propagate through the hot matter. One manifestation of this energy loss is a lower yield of jets emerging from the medium than expected in the absence of medium effects. ATLAS has provided a quantification of jet suppression by measurements of jet R_AA in the LHC Run 1. A factor of two suppression was observed in central heavy ion collisions with respect to pp collisions. R_AA was also found to exhibit only a weak rapidity dependence, and a slow (but significant) rise with increasing jet momentum. The high-statistics run 2 data of Pb+Pb and pp collisions provide the opportunity to extend the jet R_AA measurement, to evaluate the center-of-mass-energy dependence of this quantity and to explore new techniques for the study of jet substructure using subjets. This talk will presents the Run 1 results on inclusive jet production and new Run 2 results on inclusive jet suppression. It will furthermore presents new results on the measurement of jet substructure from the Run 2 data.
The heavy-ion physics program at the LHC aims at characterizing the high energy density, high temperature, deconfined partonic state of matter called Quark-Gluon Plasma (QGP). High-momentum partons, that then hadronize in jets, are useful tools to study the QGP properties since they are produced via hard scattering processes and they probe the full evolution of the system, losing energy while passing through it.
In particular the characterization of the jet substructure can bring insight on the microscopic nature of modifications induced on these partons by the medium. These modifications can be studied using a set of jet shape observables like the first order radial moment, the jet momentum dispersion, the difference between the leading and sub-leading jet tracks. Moreover measurements of the jet mass will allow for direct experimental access to the virtuality evolution of partons interacting with the medium.
Measurements of these observables in pp or p-Pb collisions will be presented and compared with theoretical calculations, as well as with Monte Carlo generators, providing important tests of QCD in elementary collisions.
We will present final ALICE results on jet shapes in Pb-Pb collisions at
$\sqrt{s_{\rm NN}}=2.76~\rm{TeV}$, in order to investigate possible modifications of the distribution of particles within the jets. In addition, we present new results on jet mass in Pb-Pb collisions. These measurements have been performed using new techniques for background subtraction and a 2D unfolding procedure to correct the shapes to particle level.
As the density of matter increases, atomic nuclei disintegrate into nucleons and, eventually, the nucleons themselves disintegrate into quarks. The phase transitions (PTs) between these phases can vary from steep first order phase transitions to smooth crossovers, depending on certain conditions. First order phase transitions with more than one globally conserved charge, so-called non-congruent PTs, have characteristic differences compared to congruent PTs (e.g., dimensionality of phase diagrams, location and properties of critical points). I investigate the non-congruence of the quark deconfinement PT at high densities and/or temperatures in Coulomb-less models, relevant for heavy ion collisions, neutron stars, proto-neutron stars, supernova explosions and compact star mergers.
Significant progress has been made in the past few years in determining QGP properties, such as the temperature dependence of shear viscosity over entropy density ratio $\eta/s$. However, $\eta/s$ might depend also on the baryochemical potential $\mu_B$, as has been hinted at in a recent beam energy scan study [1].
It is generally difficult to determine the uncertainties associated with the extracted values of QGP properties, as the computational models used in the analysis typically have numerous interconnected parameters. We utilize novel optimization techniques based on Bayesian statistics and Markov Chain Monte Carlo (MCMC) methods to calibrate the computational model to data. The end result of such an analysis is a conditional probability distribution, which provides a set of data-calibrated parameter values with a full uncertainty quantification. Gaussian process emulators are also used in the analysis to overcome its significant computational expense and predict model results for all needed input parameter combinations. These statistical methods have already been applied with great success to Pb+Pb collisions at the LHC [2].
In this presentation we investigate the $\mu_B$ dependence of $\eta/s$, with collision energy as the control parameter, by performing a Bayesian analysis on RHIC beam energy scan data, applying the same UrQMD + viscous hydrodynamics hybrid model as in [1], to verify if the reported differences between the energies remain significant even when uncertainties are included. We determine the probability distributions for the model parameters for Au+Au collisions at $19.6$, $39$, and $62.4$ GeV; the results indicate that while the uncertainty on the optimal value of $\eta/s$ does increase at lower collision energies, the probability for a large value of the ratio is much higher at $19.6$ GeV with the median value $0.24$, compared to $62.4$ GeV with median value $0.07$ [3]. Moreover, we also find that multi-strange hadron yields provide significant constraints on the switching condition between the hydrodynamic evolution and the hadron transport afterburner and thus are essential for a proper model-to-data comparison [4].
[1] Iu. Karpenko et al., PRC 91 6, 064901 (2015);
[2] J. E. Bernhard et al., PRC 94 2, 024907 (2016);
[3] S. A. Bass, talk at CPOD2016 (arXiv:1610.00590);
[4] J. Auvinen et al., talk at SQM 2016 (arXiv:1610.00589).
Light nuclei have much smaller binding energy compared to the temperature of the system. Consequently, their distributions can be used to probe the freeze-out properties, such as correlation volume and local baryon density of the medium created in high-energy nuclear collisions.
In this talk, we report the results of deuteron and anti-deuteron production in Au-Au collision at $\sqrt{s_{NN}}$ = 7.7$-$200 GeV, measured by STAR at RHIC. The collision energy, centrality and transverse momentum dependence of the coalescence parameter $B_2$ for deuteron and anti-deuteron production is discussed. We find the values of $B_2$ for anti-deuteron are systematically lower than those for deuterons. The difference in $B_2$ for deuteron and anti-deuteron indicate the residual isospin brought in at the beginning of the collisions. The values of $B_2$ are found to decrease with increasing collision energy. The rate of decreasing seems to change around $\sqrt{s_{NN}}$ = 20 GeV implying a dramatic change of the equation of state of the medium in these collisions.
Rapidity correlations in the RHIC Beam Energy Scan.
Sedigheh Jowzaee for the STAR Collaboration
ajowzaee@wayne.edu
A pair-normalized two-particle covariance versus the rapidity
of the two particles, called R$_2$, was originally studied
[1] in ISR and FNAL data in the 1970's and has recently seen
renewed interest [2] to study the dynamics of heavy-ion
collisions in the longitudinal direction. These rapidity
correlations can be decomposed onto a basis set of Legendre
polynomials with prefactors $\langle$a$_{\rm mn}$$\rangle$,
which can be considered the rapidity analog of the
decomposition of azimuthal anisotropies into a basis set of
cosine functions with prefactors v$_{\rm n}$. The
$\langle$a$_{\rm mn}$$\rangle$ values have been suggested [2]
to be sensitive to the number of sources, baryon stopping,
viscosities, and criticality. The rapidity correlations have
been measured by the STAR collaboration as a function of the
centrality and beam energy in the range of 7.7 to 200 GeV.
The experimental results and comparisons to those from the
UrQMD model will be presented.
[1] L. Fo`{a}, Phys. Lett. C22, 1 (1975); H. B\oggild, Ann. Rev. Nucl. Sci. 24, 451 (1974).
[2] T. Lappi and L. McLerran, Nucl. Phys. A 832, 330 (2010);
A. Bzdak and D. Teaney, Phys. Rev. C 87, 024906 (2013);
A. Monnai, B. Schenke, arXiv:1509.04103;
A. Bzdak, Volker Koch, and Nils Strodthoff, arXiv:1607.07375;
J. Jia, S. Radhakrishnan, and M. Zhou, Phys. Rev. C 93, 044905 (2016).
Using a hybrid (viscous hydrodynamics + hadronic cascade) framework, we model event-by-event bulk dynamics of relativistic heavy-ion collisions at the Relativistic Heavy Ion Collider (RHIC) Beam Energy Scan (BES) collision energies, including the effects from non-zero net baryon current and its dissipative diffusion during the evolution. This framework is in full (3+1)D, which allows us to study the non-trivial longitudinal structure and dynamics of the collision systems, for example, the baryon stopping and transport, as well as longitudinal fluctuations. We study hadronic chemistry, identified particle spectra, anisotropic flow, and HBT interferometry over the energy range relevant to the RHIC BES. For the first time, quantitative effects of boost-invariance breaking, net-baryon current/diffusion, and pre-equilibrium dynamics on these hadronic observables will be addressed. Finally, flow predictions for recent d+Au collisions at BES energies will be presented within the same framework. They shed new light on understanding the collective nature of small quark-gluon droplets.
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. The program will examine the
energy regime of interest determined from the results of BES-I. Some of
the key measurements anticipated 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 EOS, and the chiral
restoration in the dilepton channel. The measurements will be possible
with the order of magnitude better statistics provided by the electron
cooling upgrade of RHIC and with the detector upgrades planned to extend
STAR's experimental reach. The upgrades are: the replacement of the inner
TPC sectors (iTPC) that increases the rapidity coverage of identified
particles, the Event Plane Detector (EPD) that improves the triggering and
event plane resolution, and the end-cap TOF (eTOF) that extends
the PID capabilities to larger rapidities in one hemisphere of STAR. The
talk will highlight the physics opportunities enabled by these upgrades.
The chiral magnetic effect (CME) and the chiral vortical effect (CVE)
have been under intensive theoretical and experimental investigations in
recent years [1]. A three-point correlator, γ [2], has been used to measure electric/baryonic charge separations across the reaction plane as the
experimental manifestation of the CME/CVE. Considerable background
sources arising from charge/momentum conservation coupled with elliptic
flow anisotropy (v2) have been identified. Disentanglement of background
and the possible CME/CVE signal has been the central focal point of theoretical and experimental efforts. We report recent progresses from the
STAR experiment in searching for the CME/CVE with identified particles
using background suppression methods. The γ correlator measurements
of π−π, π−K, p−π, K−K, p−K, p−p, p−Λ pairs are presented as a
function of centrality and beam energy in Au+Au collisions from $\sqrt{s_{NN}}$
= 7.7 to 200 GeV. We explore the range of background variations to establish where a signal may exist. We use event-shape engineering [3] as well as mixed-event subtraction as a function of the event-by-event elliptic anisotropy [4,5] to reduce the flow background. In addition, preliminary results of small system 200 GeV d+Au collisions will also be discussed. These measurements will represent a major advance in our understanding of possible CME/CVE contributions to the three-point correlation.
[1] Kharzeev, D.E. et al. Prog. Part. Nucl. Phys. 88, 1 (2016).
[2] S. A. Voloshin, Phys. Rev. C 70, 057901 (2004).
[3] F. Wen, L. Wen, G. Wang, arXiv:1608.03205.
[4] L. Adamczyk, et al. (STAR Collaboration), Phys. Rev. C 89, 044908 (2014).
[5] F. Wang, J. Zhao, arXiv:1608.06610.
Non-central heavy-ion collisions have large ($\sim 10^{5} \hbar$) angular momentum which may be transferred, in part, to the quark-gluon plasma through shear forces that generate a vortical substructure in the hydrodynamic flow field. The vortical nature of the system is expected to polarize emitted hadrons along the direction of system angular momentum. $\Lambda$ and $\overline{\Lambda}$ hyperons, which reveal their polarization through decay topology, should be polarized similarly. The vorticity of the fluid is an important parameter for the generation of a Chiral Vortical Effect (CVE).
These same collisions are also characterized by dynamic magnetic fields with magnitude as large as $10^{14}$ Tesla. A splitting between $\Lambda$ and $\overline{\Lambda}$ polarization may signal a magnetic coupling and provide a quantitative estimate of the field strength at freeze out. Details of the magnetic field and its evolution are of particular interest to other novel phenomena, e.g. the Chiral Magnetic Effect (CME).
The STAR Collaboration has made the first observation of global hyperon polarization in non-central Au+Au collisions at Beam Energy Scan energies. A magnetic splitting is hinted at, but the improved statistics and resolution achievable with future runs are required to make a definitive measurement of the magnetic field.
The Quark Gluon Plasma formed in relativistic heavy ion collisions at finite impact parameter has a finite angular momentum perpendicular to the reaction plane and some fraction thereof may be converted into global polarization of final state hadrons along the angular momentum direction. The polarization can be calculated assuming that the spin degrees of freedom are at local thermodynamical equilibrium at the hadronization stage. The hydrodynamical quantity steering the polarization is the thermal vorticity, that is minus the antisymmetric part of the gradient of four-temperature field $\beta^\mu=u^\mu/T$ [1].
Based on this mechanism, we present a calculation of the global polarization of $\Lambda$ hyperons produced in relativistic Au-Au collisions at RHIC Beam Energy Scan range $\sqrt{s_{\rm NN}}=7.7 - 200$ GeV with a 3+1 dimensional cascade + viscous hydro + cascade model, vHLLE+UrQMD [2]. Within this model, the mean polarization of $\Lambda$ in the out-of-plane direction is predicted to decrease rapidly with collision energy from a top value of about 2% at the lowest energy examined. We explore the connection between the polarization signal and thermal vorticity and estimate the feed-down contribution to $\Lambda$ polarization due to the decay of higher mass hyperons.
[1] F. Becattini et al., Annals Phys. 338, 32-49 (2013);
[2] Iu. Karpenko et al., Phys.Rev. C 91, 064901 (2015).
Both strong magnetic field and sizable vorticity are present in the hot QCD matter created in non-central heavy-ion collisions. We report new phenomena that the interplay between the magnetic field and fluid vorticity induces the redistribution of the vector charge density and generates an axial current [1]. We show the role of the chiral anomaly underlying in these effects which, however, have not been captured by the conventional anomalous hydrodynamics. We discuss an imprint of these effects on the charged-particle spectrum measured in the experiment and argue that these effect should be implemented and quantitatively studied in anomalous magnetohydrodynamics.
[1] Koichi Hattori and Yi Yin. ``Charge redistribution from anomalous magneto-vorticity coupling,''
Accepted for publication in Phys. Rev. Lett. [arXiv:1607.01513 [hep-th]]
We computed the fermion spin distribution and correlations in vortical fluid employing event-by-event (3+1)D viscous hydrodynamics. Due to spin-vorticity coupling, the spin polarization density is proportional to the local fluid vorticity at the next-to-leading order of a gradient expansion in a quantum kinetic theory. As a result of strong collective flow, the spatial distribution of local vorticity on the freeze-out hypersurface converts to Lambda spins with intrinsic azimuthal angle distribution and correlation at RHIC and LHC energy. The azimuthal correlation of the transverse spin is shown to have a cosine form plus an offset due to a circular structure of the transverse vorticity around the beam direction and global angular momentum in non-central collisions. The longitudinal spin correlation shows a structure of vortex-pairing in the transverse plane due to the convective flow of hot spots in the radial direction. The dependence on colliding energy, rapidity, centrality and sensitivity to the shear viscosity are also investigated.
We employed a 3+1D anomalous hydro with initial condition generated by HIJING to calculate Chiral Vortical Effect and Chiral Magnetic Effect. This allowed us to compare these two effects at different collision energy and centrality. We calculated the charge dependent two-particle correlations with respect to the reaction plane, which is used to compare with current results and also can provide prediction for further experiments.
The production of heavy quarkonia is an important observable to study the
properties of the nuclear matter created in high-energy heavy-ion
collisions. Lattice QCD calculations predict a phase transition of the
hadronic matter to a deconfined medium of quarks and gluons, the Quark
Gluon Plasma (QGP), at extreme energy densities. The bottomonium bound
states while passing through the deconfined medium are dissociated
into quark-antiquark pair due to color screening. This is visible in data
as a suppression of $\Upsilon$ resonances with respect to the
proton-proton results scaled by the number of binary
collisions. However, the cold nuclear matter effects can also lead to the
suppression of $\Upsilon$ resonances in heavy-ion collisions. Cold
nuclear effects are studied in p-Pb collisions since the QGP is not
expected to be produced. ALICE measures the bottomonium down to zero
transverse momentum via the dimuon decay channel at forward rapidity
($2.5 \lt y \lt 4$).
In this presentation, the final results on the nuclear modification factor of $\Upsilon$
measured in Pb--Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV
will be shown as a function of centrality, transverse momentum and
rapidity. The results will be compared with the existing
theoretical models. In this context, the $\Upsilon$ measurements in
p-Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV will be
discussed as well. Finally, the ALICE results will be compared to results from other experiments.
Bottomonia are important probes of the quark-gluon plasma since they are produced at early times and propagate through the medium, mapping its evolution. They are also considered to be cleaner probes than charmonia due to the lack of regeneration even at the LHC energies. In Run 1 at the LHC, CMS was able to explore multiple measurements of the Y(nS) states in pp and PbPb collisions, down to $p_{T}$ = 0. In PbPb and pp, the production cross sections for all three Y(nS) states were measured at $\sqrt{s_{NN}}$ = 2.76 TeVwith the exception of the Y(3S) state, which was not observed in PbPb collisions. The suppression of the Y(1S) state was seen to depend on centrality, but not significantly on transverse momentum or rapidity. In Run 2, we have now measured the ground state to excited state ratios of Bottomonia in PbPb and pp collisions at $\sqrt{s_{NN}}$ = 5.02 TeV. The Y(2S)/Y(1S) ratiois found to be below 1 over the full centrality range and a weak dependence is found as a function of dimuon kinematics. For the Y(3S) state, an upper limit has been obtained as a function of centrality where a Y(3S)/Y(1S) ratio compatible with 0 is observed over the full centrality range. In this talk, we will present the final CMS results on bottomonium production from Run 1, together with new Run 2 results from the high statistics PbPb data at $\sqrt{s_{NN}}$ = 5.02 TeV collected in 2015.
The LHCb experiment has the unique property to study heavy-ion interactions in the forward region (2 < $\eta$ < 5), in a kinematic
region complementary to the general purpose detectors. The detector has excellent capabilities for reconstructing quarkonia and open charm states, including baryons, down to zero $p_T$. Notably, it can separate the prompt and displaced charm components. In pPb collisions, both forward and backward rapidities are covered thanks to the possibility of beam reversal. Results include measurements of the nuclear modification factor and forward-backward ratio for charmonia, open charm and bottomonia states. These quantities are sensitive probes for cold nuclear matter effects in heavy flavour producton. In 2015, LHCb also participated successfully for the first time in the Pb-Pb data-taking. The status of the forward prompt J/$\psi$ nuclear modification factor measurement for up to semi-central lead-lead collisions will be shown.
Quarkonium spectral functions have all information about in-medium properties of heavy quarkonia such as dissociation temperatures, which are important to understand suppression of quarkonium yields in relativistic heavy ion collision experiments at RHIC and LHC, where many interesting results on $J/\Psi$ and $\Upsilon$ suppression have been reported already. Since quarkonium suppression can occur through complicated processes not only related to the medium effect but also any other ones such as cold nuclear matter effects, good theoretical understanding of quarkonium behavior in the hot medium is required. Low frequency behavior of the quarkonium spectral functions for the vector channel also tells us transport properties of heavy quarks in quark-gluon plasma, which is important input for hydrodynamic models trying to explain collective phenomena in heavy-ion experiments. Therefore it is important to investigate the quarkonium spectral functions, especially using first-principle lattice QCD calculations.
In this talk we report our recent study on quarkonium spectral functions in lattice QCD at finite temperature. To get correlation functions with high data quality, which is important to extract reliable spectral functions, we performed simulations on very large and fine lattices with a couple of lattice cutoffs towards the continuum limit. Our previous studies on some of these lattices have been reported in [1,2]. At temperatures in a range between 0.75$T_c$ and 2.3$T_c$ we reconstruct quarkonium spectral functions from temporal Euclidean meson correlators with both charm and bottom quark masses, where to estimate systematic uncertainties we adopt the conventional maximum entropy method as well as two different stochastic methods: one is the stochastic analytical inference based on the Bayes' theorem and the other is the stochastic optimization method, which does not rely on any prior information (see our preliminary works in [3,4]). We discuss dissociation of quarkonium states from temperature and quark mass dependence of the spectral functions. We also estimate the heavy quark diffusion coefficient using low-frequency behavior of the spectral functions for the vector channel.
[1] H. Ohno, PoS LATTICE 2013, 172 (2014).
[2] H. Ohno, H.-T. Ding and O. Kaczmarek, PoS LATTICE 2014, 219 (2014).
[3] H.-T. Shu, H.-T. Ding, O. Kaczmarek, S. Mukherjee and H. Ohno, PoS LATTICE 2015, 180 (2016).
[4] H. Ohno, PoS LATTICE 2015, 175 (2016).
Measurements of quarkonium production have played an important role in understanding the properties of the Quark-Gluon Plasma (QGP) formed in relativistic heavy-ion collisions. The suppression of quarkonia in the medium due to color screening has been proposed as a direct signature of the QGP formation. However, other effects, such as regeneration of quarkonia by the coalescence of uncorrelated quark-antiquark pairs, co-mover absorption, and cold nuclear matter effects, add additional complications to the interpretation of the observed quarkonium suppression. Compared to charmonia, bottomonia suffer much less from regeneration contribution and co-mover absorption. Furthermore, different bottomonium states may dissociate at different temperatures, known as "sequential melting", which can be used to constrain the temperature of the medium.
Quarkonium measurements have been traditionally performed in the dielectron channel at STAR. In early 2014, the Muon Telescope Detector (MTD), which provides muon identification and triggering capabilities at mid-rapidity, was fully installed into the STAR experiment. It allows measurements of quarkonia via the di-muon channel with much smaller Bremsstrahlung radiation and thus much better invariant mass resolution than the dielectron channel. In this talk, we will present the measurements of Υ suppression in Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV via both the di-muon and dielectron channels. The centrality and transverse momentum dependences will be reported and compared to those at the LHC and theoretical calculations. We will also show the Υ measurements in p+p and p+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV via the dielectron channel using the data taken in year 2015. These measurements provide a significantly improved p+p reference and quantification of the cold nuclear matter effects for Υ measurements at RHIC.
The thermal suppression of heavy quark bound states represents an ideal observable for determining if one has produced a quark gluon plasma in ultrarelativistic heavy-ion collisions. In recent years, however, a paradigm shift has taken place in the theory of quarkonium suppression due to new first principles calculations of the thermal widths of these states. These thermal widths are large, eg O(20-100 MeV) for the Upsilon, and cause in-medium suppression of the states at temperatures below their traditionally defined disassociation temperatures. In order to apply the newly developed understanding to phenomenology, however, one must make detailed 3+1d dissipative hydrodynamical models of the plasma including the effects of finite shear viscosity. These effects include not only the modification of the time evolution of the temperature of the system, flow, etc., but also the emergence of potentially large local momentum-space anisotropies which can affect the in-medium properties of the states. I will discuss the setup for these model calculations and present comparisons of theory with data from RHIC 200 GeV/nucleon Au-Au collisions, LHC 2.76 TeV/nucleon Pb-Pb, and LHC 5.023 TeV/nucleon Pb-Pb collisions as a function of number of participants, rapidity, and transverse momentum.
The measurement of heavy flavour production is a powerful tool to study the properties of the high-density QCD medium created in heavy-ion collisions as heavy quarks are sensitive to the transport properties of the medium and may interact with the QCD matter differently from light quarks. In particular, the comparison between the nuclear modification factors ($R_{AA}$) of light- and heavy-flavour particles provides insights into the expected flavour dependence of in-medium parton energy loss. Furthermore, azimuthal anisotropy coefficient ($v_{n}$) of heavy-flavor particles provide insights into the degree of the thermalization of the bulk medium at low $p_{T}$, and unique information about the path length dependence of heavy quark energy loss at high $p_{T}$. Using the large statistics proton-proton and PbPb samples collected at 5.02 TeV during the 2015 LHC run, high precision open charm measurements are performed with the CMS detector in a wide transverse momentum range. This allows us to set an important milestone in our understanding of the interactions between charm quark and the medium. In this talk, the most recent results of $R_{AA}$, $v_{2}$ and $v_{3}$ of $D^{0}$ mesons in PbPb collisions at 5.02 TeV are presented and compared to the same results for charged hadrons at the same energy.
Hadrons carrying heavy flavour (charm or beauty quarks) constitute an exceptional probe to study the properties of the Quark-Gluon Plasma (QGP) created in high-energy heavy-ion collisions. Heavy quarks are produced in initial hard parton-scattering processes of the nucleon-nucleon collisions and on short time scales compared to the QGP formation time. Therefore they experience the entire evolution of the medium interacting with its constituents.
The measurement of the nuclear modification factor ($R_{\rm AA}$) of charmed hadrons
allows us to gain insight into the colour-charge and parton-mass dependence of energy loss as well as into possible modifications of hadronization in presence of the medium.
Results from Pb--Pb collisions at $\sqrt{s}_{\rm NN} = 2.76$ TeV indicate that momentum distributions of charmed mesons are modified in Pb--Pb with respect to pp collisions,
owing to quenching of heavy quarks in the hot and dense medium. The D-meson $R_{\rm AA}$ exhibits a suppression of a factor 5-6 for $p_{\rm T}$ $\approx$ 10 GeV/$c$ in central collisions, with a hint of reduced suppression for D$_{\rm s}$ mesons as compared to non-strange D mesons.
Results from Run 2 will allow us to reduce uncertainties and draw firmer conclusions about possible charm recombination at low and intermediate $p_{\rm T}$ and about colour-charge and parton-mass dependence of energy loss.
The measurement of the elliptic flow ($v_2$) provides further insight into the deconfined phase. At low $p_{\rm T}$, D-meson $v_2$ offers the unique opportunity to test whether also charm quarks participate in the collective expansion dynamics and possibly thermalize in the medium. At low and intermediate $p_{\rm T}$, the elliptic flow is also expected to be sensitive to the hadronization mechanism, while at high $p_{\rm T}$, it can constrain the path-length dependence of parton energy loss. During the LHC Run 1, ALICE measured a positive $v_{2}$ for D$^{\rm 0}$, D$^{\rm +}$, D$^{\rm *}$ mesons in Pb--Pb collisions at $\sqrt{s}_{\rm NN} = 2.76$ TeV. The increased statistics of semi-central Pb--Pb events at $\sqrt{s}_{\rm NN} = 5.02$ TeV, obtained with the LHC Run 2, provides access to more precise results for non-strange D-meson $v_{2}$ as well as to the first measurement of the D$_{\rm s}$-meson elliptic flow at LHC energies.
In this talk, the elliptic flow of D mesons in Pb--Pb collisions at $\sqrt{s}_{\rm NN} = 5.02$ TeV will be presented, together with the status of the D-meson $R_{\rm AA}$ at the same energy.
Due to their large masses, heavy quarks are predominantly produced through initial hard scatterings in heavy-ion collisions and, as such, they experience the entire evolution of the hot and dense medium created in such collisions. Therefore, they can provide important insights into the properties of the strongly-coupled Quark Gluon Plasma (sQGP). For instance, the azimuthal anisotropy of charm quarks with respect to the reaction plane over a broad momentum range can provide information on the degree of thermalization for heavy flavor quarks in the medium and the bulk properties of the system. Specifically, at low transverse momenta we can examine the bulk properties in the strongly-coupled regime. Furthermore, several models have predicted that fluctuations in the initial conditions, together with strong charm-medium interactions, could lead to a finite triangular flow $v_3$ for the $D^0$ meson, providing another handle to study the early stages of the collisions.
In this talk we present the measurement of azimuthal anisotropy of $D^0$ mesons in Au+Au collisions at $\sqrt{s_{\rm{NN}}}= 200$ GeV with the Heavy Flavor Tracker at STAR. Compared to previously reported $D^0$ $v_2$ in minimum-bias collisions, the significance of the new result is improved by a factor of 2-4, allowing the study of the transverse momentum and centrality dependence of $D^0$ elliptic and triangular flow for the first time. The results will be compared with the measurements of other particle species and a series of model calculations. Charm quark dynamics in the sQGP medium will be discussed and the question of whether charm quarks are as thermalized as light quarks will be addressed.
Quark coalescence has been proposed as a new hadronization mechanism to explain the Number-of-Constituent-Quark scaling for meson/baryon elliptic flow as well as the enhancement in baryon-to-meson ratios in heavy ion collisions in the intermediate $p_{T}$ range (2$<$$p_{T}$$<$6 GeV/c) for both light and strange flavor hadrons. If the coalescence mechanism also plays a significant role for charm quark hadronization inside the hot and dense medium, one would expect enhancements in the $D_{s}^{+}$ and $\Lambda_{c}^{+}$ yields in heavy-ion collisions relative to p+p collisions. The magnitudes of the enhancements are sensitive to the QGP dynamics, e.g. the degree of thermalization for charm quarks, the amount of strangeness enhancement, etc. Knowledge of the yields for different charm hadrons is also critical for determining the total charm quark yield in heavy-ion collisions.
In this presentation, we will report the first measurement of $\Lambda_{c}^{+}$ baryon ($c\tau\sim60{\mu}m$) production in Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV from the STAR experiment. A significantly improved measurement of $D_{s}^{+}$ production with about a factor of 7 increase in signal significance compared to the previous results will also be reported. The $\Lambda_{c}^{+}$ and $D_{s}^{+}$ hadrons are reconstructed through their hadronic decay channels ($\Lambda_{c}^{+}{\rightarrow}p+K+\pi$, $D_{s}^{+}\rightarrow\phi(1020)+\pi$) using topological selections enabled by the STAR Heavy Flavor Tracker (HFT). The transverse-momentum spectra of $\Lambda_{c}^{+}$ and $D_{s}^{+}$ as well as their ratios to non-strange D meson will be presented and compared to theoretical calculations. In addition, nuclear modification factor and elliptic flow for $D_{s}^{+}$ will be presented. Physics implications on charm quark hadronization mechanisms in the QGP as well as the QGP medium properties will be discussed.
Open and hidden charm production in nucleus-nucleus collisions is
considered as a key signature of Quark Gluon Plasma (QGP) formation.
In the search of specific QGP effects, proton-nucleus collisions are used as
the reference as they account for the corresponding Cold Nuclear Matter
(CNM) effects.The LHCb experiment, thanks to its System for Measuring
Overlap with Gas (SMOG) can be operated in a fixed target mode with the LHC beams, at an intermediate center-of-mass energy between nominal SPS and RHIC energies. This allows for the required variety of beam- target combinations in a particularly interesting kinematical domain. In 2015, for the first time, reactions of incident LHC proton beams on noble gas targets have been recorded by the LHCb experiment at a center-of-mass energy of 110 GeV and within the center-of-mass rapidity range -2.3 < y* < 0.2. In this talk, we will present the first high resolution measurements on open and hidden charm production obtained under these conditions.
Hard hadrons, including heavy flavor and high $p_\mathrm{T}$ light flavor hadrons, serve as valuable probes of the quark-gluon plasma (QGP) matter produced in relativistic heavy-ion collisions. We establish a Linear Boltzmann Transport (LBT) coupled to (3+1)-D viscous hydrodynamic model that simultaneously describes the temporal evolution of both heavy and light partons inside QGP on the same footing [1]. Both quasi-elastic [2] and inelastic [3,4,5] processes are included in our LBT model for parton energy loss in the de-confined QCD medium. On the freeze-out hypersurface, the hadronization of hard partons into their corresponding color neutral bound states is calculated utilizing our hybrid fragmentation plus jet-thermal coalescence model [1,6].
Within this newly developed framework, we demonstrate that while quasi-elastic scattering leads to linear increase of parton energy loss with respect to time and is important at early time, inelastic scattering results in quadratic increase of energy loss at early time but then saturates to linear increase and dominates parton evolution at later time. With proper incorporation of the temperature and energy dependences of parton-medium interaction, we simultaneously describe heavy ($D$ and $B$ mesons) and light flavor (charged hadron) suppression, 2nd and 3rd order harmonic flows for all centrality bins and all collision energies as observed from RHIC to the LHC experiments. The temperature and momentum dependences of the jet transport coefficient ($\hat{q}$) extracted from our model to data comparison are consistent with the range previously constrained by the JET Collaboration. While $\Delta E_g > \Delta E_q > \Delta E_c > \Delta E_b$ holds in our framework, we show that such flavor hierarchy in $R_\mathrm{AA}$ at hadron level can be modified due to the combinatory effect of initial momentum spectra, parton energy loss and fragmentation functions.
We also perform a systematic comparison for the 2nd and 3rd order harmonic flows of heavy vs. light flavor hadrons, from which the degree of heavy quark thermalization inside QGP is investigated as functions of centrality and colliding energy. $D$-hadron and $e$-hadron correlation functions are studied for the first time as well and shown to be a good observable to quantify not only the thermalization degree of heavy quarks [7] but also the medium response to the energy deposited by hard probe particles. Comparisons between our predictions and future measurements are expected to provide better insights of the interaction dynamics between hard partons and the QGP.
[1] S. Cao, T. Luo, G.-Y. Qin, and X.-N. Wang, Phys. Rev. C94, 014909 (2016).
[2] Y. He, T. Luo, X.-N. Wang and Y. Zhu, Phys. Rev. C91 054908 (2015).
[3] S. Cao, G.-Y. Qin, and S. A. Bass, Phys. Rev. C92, 024907 (2015).
[4] S. Cao, G.-Y. Qin, and S. A. Bass, Phys. Rev. C88, 044907 (2013).
[5] X.-N. Wang and Y. Zhu, Phys. Rev. Lett. 111, 062301 (2013).
[6] K. C. Han, R. Fries and C. M. Ko, Phys. Rev. C93, 045207 (2016).
[7] S. Cao, G.-Y. Qin, and S. A. Bass, Phys. Rev. C92, 054909 (2015).
Theoretical calculations suggest that higher harmonic anisotropic flow vectors are superpositions of contributions from linear and non-linear hydrodynamic response, each reflecting different sensitivities to the fluctuating initial conditions and properties of the hot and dense matter created in heavy ion collisions [1].
In this talk, we present the first measurement on the non-linear hydrodynamic response of higher harmonic flow using multi-particle correlations in Pb-Pb collisions at $\sqrt{s_{_{\rm NN}}}=$ 2.76 and 5.02 TeV.
These measurements can be used to study the relation between the linear and non-linear response in different centrality classes.
In addition, the measured centrality dependence of symmetry plane correlations between lower- and higher- order flow vectors can be adequately explained by contributions from non-linear response.
Furthermore, the results of newly proposed non-linear response coefficients $\chi_{m,n}$ [1] will be presented. The measurements provide crucial information on freeze-out conditions, which are poorly constrained by previous flow measurements.
Last but not least, the symmetric cumulants, which probe the correlations between different order flow harmonics, are studied in different kinematic regions ($p_{\rm T}$ and $\eta$) and include higher harmonics $v_{n}$ ($n>3$). The derived approximate relation between symmetric cumulants and the symmetry plane correlations are investigated, and this allows a direct test of hydrodynamic behavior of the created matter.
[1] L. Yan and J. Y. Ollitrault, ``$\nu_4, \nu_5, \nu_6, \nu_7$: nonlinear hydrodynamic response versus LHC data'', Phys. Lett. B 744, 82 (2015)
Higher-order flow harmonics ($v_{n}$ with n $\gt$ 3) can be measured either with respect to the event plane of the same order, a lower order event plane, or a mixture of lower order planes (also called mixed higher-order harmonics). Studies of flow harmonics using the same order event plane have been used to extract the transport properties of the hot and dense medium produced in the collisions and to explore initial state effects. The mixed higher-order harmonics have been proposed to have strong sensitivity to the shear viscosity over entropy density ratio ($\eta$/s) of the medium. In this talk, for the first time, the $v_{5}$ and $v_{7}$ are measured with respect to the second and third order planes. With the nonlinear component of the flow harmonics extracted based on lower order event planes, the nonlinear response coefficients for n=4, 5, 6, 7 are also presented. The mixed harmonic coefficients are studied as a function of charged particle transverse momentum and collision centrality at $\sqrt{s_{NN}}$ = 2.76 and 5.02 TeV with the CMS detector. It is found that the nonlinear response coefficients for the odd flow harmonics are larger than for the even harmonics, reflecting a stronger contributions of the nonlinear part for odd harmonics. The results are compared to theoretical calculations with different $\eta$/s values and provide stringent constraints on the transport properties of the medium produced in heavy ion collisions.
Measurements of azimuthal anisotropic flow provide valuable information on the properties of the matter created in
heavy-ion collisions. In this talk we present the elliptic, triangular and quadrangular flow of inclusive and identified
charged particles measured in Pb-Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV recorded by the ALICE detector.
This center of mass energy is the highest attained in the laboratory for heavy-ion collisions. The measurements are
presented for a wide range of particle transverse momenta within the pseudo-rapidity region $|\eta|<0.8$. The results
are compared to the measurements at lower energy reported by the LHC experiments and also to theoretical predictions.
Recent measurements at the LHC involve the correlation of different azimuthal flow harmonics $v_n$. These new observables add constraints to theoretical models and probe aspects of the system that are independent of the traditional single-harmonic measurements such as 2-and multi-particle cumulants $v_n\{m\}$. Many of these new observables have not yet been measured at RHIC, leaving an opportunity to make predictions as a test of models across energies. We make predictions using NeXSPheRIO, a hydrodynamical model which has accurately reproduced a large set of single-harmonic correlations in a large range of transverse momenta and centralities at RHIC. Our predictions thus provide an important baseline for comparison to correlations of flow harmonics, which contain nontrivial information about the initial state as well as QGP transport properties. We also point out
significant biases that can appear when using wide centrality bins and non-trivial event weighting, necessitating care in performing experimental analyses and in comparing theoretical calculations to these measurements.
In this work we use our state of the art IP-Glasma+MUSIC+UrQMD framework to systematically study a wide range of hadronic flow observables at 2.76 TeV and to make predictions at 5.02 TeV \cite{1609.02958}. In addition to the single particle spectra and anisotropic flow coefficients $v_n$, we study event-plane correlations, non-linear response coefficients $\chi_n$, and flow factorization breaking ratios $r_n$, which were presented for the first time in the IP-Glasma framework. Furthermore, we investigate event shape engineering as well higher flow harmonics such as $v_5$, $v_6$, and $v_7$, which were recently measured at 5.02 TeV by the ATLAS collaboration. Taken together, these observables provide a wealth of insight into the collective behavior of the QGP and initial state fluctuations. These quantities shed light on flow correlations in different $p_{T}$ ranges, flow at fixed system size but different initial geometries, as well as the non-linear hydrodynamic response to the initial state energy anisotropy. By synthesizing this information we can gain further insight into the transport properties of the QGP as well as the fluctuation spectrum of the initial state. Finally, we examine the effect of pre-equilibrium longitudinal flow, which has previously been neglected in phenomenological studies, such as the hadron and direct photon spectra and $v_n$.
\begin{thebibliography}{}
\bibitem{1609.02958}
Scott McDonald, Chun Shen, Francois Fillion-Gourdeau, Sangyong Jeon and Charles Gale.
\newblock Hydrodynamic Predictions for Pb+Pb Collisions at 5.02 A TeV, 2016;
\newblock arXiv:1609.02958.
\end{thebibliography}
W and Z bosons are short lived and do not participate in the strong interaction. Thus their production yields, observed via dilepton decay channels in proton-lead and lead-lead collisions, provide direct tests of both binary collision scaling and the nuclear modification of parton distribution functions (nPDF). Proton-lead collisions further provide a relatively clean environment for benchmarking nPDFs. The ATLAS detector has a broad acceptance in the muon and electron channels, with excellent performance even in the high occupancy environment of central heavy-ion collisions. ATLAS has recorded 0.49 nb−1 of lead-lead data at a center-of-mass energy of 5.02 TeV per nucleon pair. W and Z production yields are expected to increase by a factor of eight relative to the available Run 1 data at 2.76 TeV. In addition the data can be compared directly to the 29 nb−1 of proton-lead data collected in Run 1. In this talk, W and Z yields, and lepton charge asymmetries from W decays, are presented differentially in rapidity and transverse momentum as a function of centrality in lead-lead and proton-lead collisions.
W and Z bosons are massive weakly interacting probes; insensitive to the strong interaction, they are clean observables of the initial state of heavy-ion collisions. Despite their low production rates, their relatively clean signatures in the leptonic decay channels allow their study in heavy-ion collisions at the LHC. Their measurement in p--Pb and Pb--Pb collisions provides constraints on the nuclear parton distribution functions (nPDFs). In particular, the W and Z rapidity-differential production cross sections and the decay lepton charge asymmetry as a function of rapidity provide stringent tests of nPDFs. Electroweak boson measurements in heavy-ion collisions also constitute a tool to validate the binary scaling of hard processes as well as a reference for medium-induced effects on other probes.
The measurement of electroweak boson production in p--Pb and Pb--Pb collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV with ALICE will be presented. The ALICE muon spectrometer capabilities to detect high $p_{\rm T}$ muons will be exploited to reconstruct electroweak bosons at large rapidity (2.5<$y_{\rm lab}$<4.0). These measurements are complementary to the ATLAS and CMS ones at central rapidity, and more precise than LHCb ones with a similar rapidity coverage. Rapidity-differential measurements of W and Z, as well as of the charge asymmetry of W-decay leptons, in p--Pb collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV will be discussed. First measurements of Z production cross section in Pb--Pb collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV will be shown. Results will be compared with model calculations including nPDFs. In addition, the centrality dependence of W yields in p--Pb collisions and of Z production in Pb--Pb collisions will be discussed as a test of binary scaling.
Our understanding of proton structure and of nuclear interactions at high energy would be advanced significantly with the definitive discovery of the gluon saturation regime. Forward particle production in hadron collisions at RHIC probes gluons at small x where gluon density is high and expected to reach the saturation regime. Until today the golden channel at RHIC to observe strong hints of saturation has been the azimuthal angular correlation between two back-to-back particles produced in p(d)+nucleus collisions. These correlations test the underlying QCD dynamics of the quark-gluon scattering that dominates at forward rapidity. During the 2015 RHIC run, STAR has collected data for di-hadron correlations of neutral pion production at forward pseudo-rapidity (eta=2.6 to 4.0) using its electromagnetic calorimeter in p+p, p+Au and p+Al collisions at √sNN=200GeV. New results from those data will be presented.
We review the recent results from the event-by-event NLO pQCD + saturation + viscous hydrodynamics (EbyE NLO EKRT) model [1,2,3], where we perform a simultaneous analysis of LHC and RHIC bulk observables to systematically constrain the temperature dependence of the QCD matter shear viscosity-to-entropy ratio eta/s(T), and to test the initial state computation. In particular, we study the centrality dependences of hadronic multiplicities, pT spectra, flow coefficients, probability distributions of relative elliptic flow fluctuations, and various flow-correlations in 2.76 and 5.02 TeV Pb+Pb collisions at the LHC and 200 GeV Au+Au collisions at RHIC [1,2]. Overall, our results match remarkably well with the LHC and RHIC measurements, and our predictions for the 5.02 TeV LHC run are in an excellent agreement with the latest data. We also explore the applicability of viscous hydrodynamics by quantifying the magnitude of delta-f corrections in the studied flow observables, and by charting the space-time evolution of the Knudsen number for the studied eta/s(T) parametrizations [3].
[1] H. Niemi, K. J. Eskola and R. Paatelainen, Phys. Rev. C93 (2016) 2, 024907, arXiv:1505.02677 [hep-ph].
[2] H. Niemi, K. J. Eskola, R. Paatelainen and K. Tuominen, Phys. Rev. C93 (2016) 1, 014912, arXiv:1511.04296 [hep-ph].
[3] H. Niemi, K. J. Eskola and R. Paatelainen, work in progress
Exclusive vector meson production can be used to directly probe the gluon density of a hadron. Measuring the cross section differentially in transverse momentum transfer makes it possible to determine the transverse density profile (via coherent diffraction) and density fluctuations (incoherent diffraction) of the target hadron. This knowledge about the geometric fluctuations of the proton is particularly important for understanding collective phenomena observed in proton-nucleus collisions.
We calculate coherent and incoherent diffractive vector meson production in photon-proton scattering at high energy. We demonstrate that incoherent gamma-p scattering is sensitive to sub-nucleon scale fluctuations, and show that the effect of geometric fluctuations can be disentangled from saturation scale fluctuations.
The Bjorken-x (or energy) evolution of the fluctuations is studied by solving the JIMWLK evolution equation. In particular, we study the energy evolution of the diffractive cross section. This is particularly interesting, as the ALICE collaboration has recently observed the disappearance of the incoherent contribution to the diffractive cross section in ultraperipheral p+A collisions at high energies, which suggests that the proton gets smoother at small x.
The fluctuating proton, constrained by the HERA data, is then used as input for hydrodynamic calculations of azimuthal anisotropy coefficients in proton-nucleus collisions, which we show to be sensitive to initial state geometric fluctuations.
References:
H. Mäntysaari, B. Schenke, Phys. Rev. D94 (2016) no.3, 034042 , arXiv:1607.01711
H. Mäntysaari, B. Schenke, Phys. Rev. Lett. 117 (2016) no.5, 052301, arXiv:1603.04349
Chiral anomaly induces a new kind of macroscopic quantum behavior in relativistic magnetohydrodynamics, including the chiral magnetic effect. In this talk we will present two new quantum effects present in fluids that contain chiral fermions:
1) the turbulent inverse cascade driven by the chiral anomaly; 2) quantized chiral magnetic current induced by the reconnections of magnetic flux. The implications for the evolution of the quark-gluon plasma produced in heavy ion collisions will be discussed.
We discuss the role of tetraquarks in the phase transitions of QCD. For three very
light flavors, tetraquarks may generate a second chiral phase transition. In
the plane of temperature and chemical potential ($T$ and $\mu$), tetraquarks must
be included in order to use effective models to determine the position of the critical
endpoint. The tetraquark condensate is the (color invariant) square of the condensate for
color superconductivity. Hence it is natural that in the plane of $T$ and $\mu$, a crossover line for tetraquarks connects smoothly to the transition line for color superconductivity.
When high-energy partons traversing a quark-gluon plasma lose energy via bremsstrahlung or pair production, the quantum duration of that splitting process is known as the formation time. For high energy, the formation time exceeds the mean free time for collisions with the medium, leading to a significant reduction in the splitting rate: the LPM effect. But there are interesting and potentially important corrections to the usual treatment of the LPM effect that arise from situations where the formation times of two consecutive splittings overlap each other, and various attempts have been made over the years to account for these effects. I will summarize recent research on computing the effect of overlapping formation times, while avoiding soft-bremsstrahlung assumptions that have been used in some of the (very interesting) theoretical work of the last few years. I will also explain the bottom line of why finding the size of these effects has potentially important conceptual implications for the Monte Carlo treatment of in-medium splitting of high-energy partons.
I will discuss a novel solution the sign problem which prevents first principle Monte-Carlo computations of QCD at finite chemical potential (especially important for both the search for the critical point and neutron star physics) as well as real time quantities such as transport coefficients. The solution is based on deforming the region of integration in the path integral into a complex manifold where the sign problem can be mitigated substantially. I will explain the new Monte-Carlo algorithm based on this idea and give examples of interacting quantum field theories (bosonic and fermionic) with nonzero chemical potential as well as real time dynamics where this method successfully solves the sign problem. This approach generalizes the "Lefschetz thimble" method that received much attention lately. I will also compare/contrast with the complex Langevin method.
Electromagnetic fields are generated in high energy nuclear collisions by spectator valence protons. These fields are traditionally computed by integrating the Maxwell equations with point sources. One might expect that such an approach is valid at distances much larger than the proton size and thus such a classical approach should work well for almost the entire interaction region in the case of heavy nuclei. We argue that, in fact, the contrary is true: due to the quantum diffusion of the proton wave function, the classical approximation breaks down at distances of the order of the system size.
We compute the electromagnetic field created by a charged particle described initially as a Gaussian wave packet of width 1~fm and evolving in vacuum according to the Klein-Gordon equation. We completely neglect the medium effects. We show that the dynamics, magnitude and even sign of the electromagnetic field created by classical and quantum sources are different.
Nuclear collisions which produce a high transverse momentum (p_T) prompt photon offer a useful way to study the dynamics of the hot, dense medium produced in these events. Because photons do not carry color charge, they are unaffected by the hot, dense medium. Thus, the outgoing photon serves as a tag of the initial parton flavors, and measures the initial parton pT before they are quenched by their passage through the medium In 2015, ATLAS sampled 0.49 nb-1 and 26 pb-1 of Pb+Pb and pp data at 5.02 TeV, respectively, with a high-level photon trigger that selects p_T>25 GeV photons with high efficiency. The larger prompt photon cross-section and integrated luminosity with respect to 2.76 TeV data allow for new, differential studies of photon-jet correlations. In this talk, ATLAS results on photon-jet azimuthal and pT balance will be presented using pT > 60 GeV photons and R=0.4, pT > 30 GeV jets. Double-differential distributions of the jet-to-photon p_T ratio, x_Jg, and of the azimuthal difference, $\Delta\phi$, will be presented as a function of photon p_T and event centrality. The status of other photon-tagged jet observables will also be discussed.
We take a closer look at the single particle nuclear modification factor ($R_{AA}$) and azimuthal anisotropy ($v_{2}$) of
leading hadrons at high transverse momentum ($p_{T}$) at both RHIC and LHC collision energies. We focus on the
established reduction in the interaction measure $\hat{q}/T^{3}$ between RHIC and LHC, as discovered by the JET
collaboration. The centrality dependence of the $R_{AA}$ and $v_{2}$ at both these collision energies strongly suggests
that the reduction is not caused by a temperature dependence in the ratio of $\hat{q}/T^{3}$ but rather by an energy
dependence of $\hat{q}$.
We study this dependence by introducing an $x$ dependence in the distribution function that is integrated to obtain
$\hat{q}$. We conjecture on possible forms of a scale dependence by relating $\hat{q}$ to an object similar to a transverse
momentum dependent parton distribution function (TMDPDF). The ensuing operator product is then related to quantities
that may be estimated in lattice QCD.
A typical approach to study the medium produced in heavy ion collisions is to understand the passage of elementary particles through it. As Z bosons and photons do not participate in the strong interaction, their correlation with jets within the same event is a clean probe of the medium-induced energy loss of (predominantly) quark jets. In this analysis, Z+jet and photon+jet correlations are studied using the high statistics PbPb and pp data taken at a center-of-mass energy of $\sqrt{s_{NN}}$ = 5.02 TeV with the CMS detector. The evolution of azimuthal angular distributions and average momentum imbalance between the jet and Z or photon as a function of transverse momentum of the color neutral probe will be presented. In addition the jet $I_{AA}$, as a function of photon $p_{T}$ and collision centrality is studied.
Electroweak boson-tagged jet measurements provide a promising experimental channel to accurately study the physics of jet production and propagation in dense QCD medium. In this talk, we present theoretical predictions for the nuclear-induced attenuation $R^2_{AA}(V+J)$ of the differential cross section for photon-tagged and Z0-tagged jet production in heavy ion collisions, and provide theoretical interpretations to the recent LHC data. In particular, we identify the flavor origin of the vector boson tagged jet production and discuss its implications for the energy loss of the recoiling parton shower. By further using SCET with Glauber gluons improved energy loss model, we demonstrate quantitatively the significance of collisional and radiative energy loss, as revealed in the strong momentum asymmetry $d \sigma / d x_{VJ}$ in central lead-lead reactions. We show how the collective constraints form momentum imbalance shifts $\Delta x_{VJ}$, and tagged jet $I_{AA}$, combined with the absence of significant cold nuclear matter modification help constrain the transport properties of the QGP.
The 2015 US Nuclear Physics Long Range Plan calls for a state-of-the-art jet and upsilon detector at RHIC, called sPHENIX, to study the microscopic nature of the QGP, complementing similar studies at the CERN LHC. The sPHENIX detector will provide precision vertexing, tracking and full calorimetry over pseudorapidity |eta| < 1.1 and full azimuth at the full RHIC collision rate, delivering unprecedented data sets for jet and upsilon measurements at RHIC. This will enable the three pillars of the sPHENIX physics program, i.e.,
studies of jet structure modifications, measurements of heavy-flavor tagged jet production and precision upsilon spectroscopy. In this talk we will present an overview of the sPHENIX detector design, expected construction and running schedule and planned physics program.
Recent measurements of azimuthal particle correlations in small collision systems show striking simularities to flow signatures observed in gold-gold and lead-lead collisions, leading many to question if the origin of small system correlations is hydrodynamic in nature. The ensuing effort to construct a unified hydrodynamic model for small and large collision systems revealed new tension in the QGP initial conditions: heavy-ion collisions appear to prefer saturation based initial conditions [1209.6330, 1505.02677], while small system collectivity is currently best described using a Monte Carlo Glauber model [1502.04745].
It has since been suggested that adding subnucleonic structure to the QGP initial conditions could strongly affect flow in small collision systems and may explain apparent model discrepancies [1405.3605v1]. While practical implementations of subnucleonic structure are relatively straightforward—one simply replaces smooth protons with lumpy protons—there exist large theoretical uncertainties regarding the fluctuated shape of the proton, and corresponding theory predictions are highly model dependent.
In this work, we extend previous efforts to parametrize and constrain the QGP initial conditions using systematic Bayesian analysis and study the effects of subnucleonic structure predicted by a simple constituent parton model. We vary both subnucleonic degrees of freedom, e.g. the constituent parton number and effective parton width, as well as typical transport parameters such as the hydrodynamic starting time and QGP viscosity. The initial conditions are then embedded in an event-by-event hydrodynamic model and calibrated to simultaneously fit charged particle yields, flows and mean $p_T$ for light-heavy and heavy-heavy collisions at RHIC. We finally apply Bayesian parameter estimation methods to extract posterior distributions for the optimal initial condition parameters and comment on the implied viability of related theoretical frameworks.
Bose-Einstein correlations between identified charged pions are measured for $p$+Pb collisions at $\sqrt{s_{\mathrm{NN}}}$=5.02 TeV with the ATLAS detector with a total integrated luminosity of 28 $\mathrm{nb}^{−1}$. Pions are identified using ionization energy loss measured in the pixel detector. Two-particle correlation functions and the extracted three-dimensional source radii are presented as a function of average transverse pair momentum ($k_\mathrm{T}$) and rapidity ($y∗_{\pi\pi}$) as well as collision centrality. Pairs are selected with a rapidity $−2
Recent measurements in small collisions systems at LHC and RHIC indicate that the particles produced in high-multiplicity collisions exhibit collective behavior very similar to that observed in large systems where QGP is formed. In large systems, it is well established that the final-state particle correlations arise from anisotropic pressure gradients in the initial state of the collisions that drive a near-perfect fluid evolution. Whether QGP is also formed in small collision systems is presently under intense investigation. To study the origin of the collective behavior in small systems, the PHENIX experiment performed a series of geometry-controlled experiments using $p$+Al, $p$+Au, $d$+Au, and $^3$He+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV. The elliptic ($v_2$) and triangular ($v_3$) flow coefficients are measured as a function of $p_T$ for inclusive and identified charged hadrons. Mass dependence is observed in $v_2(p_T)$ indicative of hydrodynamic evolution. The relation of the $v_n$ strengths and the corresponding initial eccentricities in the different systems is studied and compared to several theoretical predictions that invoke different mechanisms for producing final-state particle correlations. The distinct initial geometries provide discriminating power against the models.
Observation of long-range ridge-like correlations in high-multiplicity pp collisions opened up new opportunities for exploring novel QCD dynamics in small collision systems. Based on data collected in 2015 and 2016 with the CMS detector at the LHC, the second-order ($v_{2}$) and third-order ($v_{3}$) azimuthal anisotropy harmonics of $K_{s}^{0}$, $\Lambda$ and inclusive charged particles are extracted from long-range two-particle correlations as functions of particle multiplicity and transverse momentum. For the first time in pp collisions, the $v_{2}$ signals are also extracted from multi-particle correlations, providing direct evidence of the collective nature of observed particle correlations. These results provide new insights on the origin of observed long-range correlations in pp collisions, and may shed light on how quantum fluctuations affect the proton structure at a very short time scale.
Current pQCD calculations for the energy loss of a hard parton moving through a medium of thermalized static scattering centres are inapplicable to the small colliding systems (such as p/d +A) that have, in recent years, hinted at the presence of tiny droplets of QGP through the presence of collective behaviour, strangeness enhancement and quarkonium suppression. The well-known DGLV, ASW, BDMPS-Z, AMY and HT calculations all exploit both the large separation distance (between scattering centre and radiation) and the large system approximations. We relax the large system assumption and recompute the energy loss in the DGLV formalism in order to address the glaring lack of theoretical control over small-system energy loss. Alarmingly, we find that the correction terms dominate at large energies, resulting in ~100% negative correction, calling into question the validity of the large formation time assumption used in all pQCD-based energy loss calculations. Our results demand a complete overhaul of pQCD-based energy-loss calculations for all system sizes.
Two- and multi-particle azimuthal correlations have proven to be an excellent tool to probe the properties of the Quark-Gluon Plasma created in Pb-Pb collisions.
Recently, the results obtained for multi-particle correlations has been interpreted as evidence for collectivity in the small pp and p-Pb collision systems providing new insights into the systems' fluctuating initial conditions.
In this talk, we present the first ALICE results of two- and multi-particle cumulants at midrapidity $|\eta| < 1.0$ as a function of multiplicity in pp collisions at $\sqrt{s}$ = 13 TeV.
Results will be compared to a broad range of collision systems and energies, including pp collisions at $\sqrt{s}$ = 7 TeV, p-Pb collisions at $\sqrt{s_{NN}}$ = 5.02 TeV, and Pb-Pb collisions at $\sqrt{s_{NN}}$ = 2.76 TeV and $\sqrt{s_{NN}}$ = 5.02 TeV. The azimuthal correlations obtained from Monte Carlo simulations will be presented for comparison. These results allow further insight into the matter created in pp collisions, and will broaden our knowledge about the initial conditions in such small collision systems.
Using ALICE's forward detectors, two-particle correlations with a very large $\Delta \eta$ range of $|\Delta \eta| < 8.5$ will be also discussed. This should shed new light into the nature of long-range correlations observed in small collision systems, and help probe the extent of the ridge in pp collisions.
The recent BES data of the energy dependent κσ^2 for net protons in Au+Au collisions presented large deviations from the statistical baselines at lower collision energies, and non-monotonic behavior at around 20 GeV, which indicates possible signals for the existence of the QCD critical point [1].
In our recent paper [2], we introduce a freeze-out scheme to the dynamical models near the QCD critical point. Our model calculations for the static critical fluctuations on the freeze-out surface shows that the C_4 and κσ^2 data of net protons can be roughly described. However, C_2 and C_3 are always over-predicted due to the positive static critical fluctuations. After solving the time evolution equations of the various cumulants of the sigma field, the BNL group found the Skewness and Kurtosis could change their sign after the evolution, which indicates that dynamical critical fluctuations could solve the sign problem of C_3 [3]. However, such BNL approach can not be easily combined with our freeze-out scheme to compared with the experimental data since only the zero mode of sigma field are considered there, which already erase the needed spatial information.
Within the framework of Langevin dynamics, we simulate the dynamical evolution of the fluctuating sigma field in position space and calculate the cumulants of the sigma field in the critical regime[4]. Our numerical simulations show that C_2 automatically increases as the system evolves in the critical regime, which represents the spontaneous increase of the chiral field’s correlation. Besides, for both C_3 and C_4, the sign in the earlier times can be remembered during the dynamical evolution due to the memory effects near the critical point[4]. Combined with the freeze-out scheme developed in [2], our calculation provides a possible way to qualitatively describe the different cumulants data of net protons.
[1] Xiaofeng Luo (for the STAR collaboration), PoS CPOD2014 (2014) 019. ArXiv:1503.02558v1.
[2] Lijia Jiang, Pengfei Li, Huichao Song, Phys. Rev. C 94, 024918 (2016).
[3] S. Mukherjee, R. Venugopalan and Y. Yin, Phys. Rev. C 92, 034912 (2015).
[4] Lijia Jiang and Huichao Song, in preparation.
We study elliptic and triangular flow and their dependence on rapidity using 3+1D hydrodynamic simulations with initial conditions (Nexus) that contain realistic fluctuations in all 3 dimensions. We compare to experimental data from STAR and find that long range, two particle $v_3$ correlations agree reasonably well with measurements. We find that an apparent decrease of $v_3$ with pseudorapidity in traditional measurements is not, in fact, a dependence on $\eta$, which is negligible within the TPC acceptance, but instead is a dependence on relative pseudorapidity due to a lack of perfect correlation between $v_3$ at different rapidities. We also observe short-range correlations, due to rapidity dependent fluctuations in the initial condition. While the short-range correlation is slightly smaller in both magnitude and range, it serves as a demonstration that short-range correlations are not necessarily generated only by non-flow sources such as jets, but can have a significant contribution from purely hydrodynamic effects.
Fluctuations have been playing an important role in understanding observables
in high-energy nuclear collisions.
It is well known that
higher harmonics of azimuthal angle distribution, 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 of the system in the intermediate stage
of the reactions.
These fluctuations are also known as hydrodynamic fluctuations and
are indispensable for the system to be stabilized
in a thermodynamic sense through
fluctuation-dissipation theorem [1].
We employ a cutting-edge integrated dynamical model [2,3]
which combines fully (3+1)-dimensional relativistic fluctuating hydrodynamics
with Monte-Carlo version of the Glauber model as an
event-by-event initialization model of the hydrodynamic fields
and the hadronic cascade model in the late stage.
By using this model, we first adjust initial parameters
and transport coefficeints to reproduce $dN_{\mathrm{ch}}/d\eta$
and centrality dependence of integrated $v_{2}$
in Pb+Pb collisions at the LHC energy.
We then analyze the event-plane correlations
between two different rapidity regions $r_{n}(\eta^a, \eta^b)$
and between two different $p_{T}$ regions $r_{n}(p^a_{\rm{T}},p^b_{\rm{T}})$.
By switching on and off hydrodynamic fluctuations,
we quantify the effect of them
on these observables.
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].
We study the gluon distribution produced via successive medium-induced branchings
by an energetic jet propagating through a weakly-coupled quark-gluon plasma. We show that
under suitable approximations, the jet evolution is a Markovian stochastic process,
which is exactly solvable. For this process, we construct exact analytic solutions
for all the n-point correlation functions describing the gluon distribution
in the space of energy [1,2]. Using these results, we study the event-by-event distribution of the energy lost by the jet at large angles and of the multiplicities
of the soft particles which carry this energy. We find that the event-by-event fluctuations are huge: the standard deviation in the energy loss is parametrically as large as its mean value [1]. This has important consequences for the phenomenology
of di-jet asymmetry in Pb+Pb collisions at the LHC: it implies that the fluctuations in the branching process can contribute to the measured asymmetry on an equal footing with the geometry of the di-jet event (i.e. as the difference between the in-medium path lengths of the two jets). We compute the higher moments of the multiplicity distribution and identify a remarkable regularity known as Koba-Nielsen-Olesen (KNO) scaling [2].
These predictions could be tested via event-by-event measurements of the di-jet asymmetry.
References
[1] Event-by-event fluctuations in the medium-induced jet evolution
M. Escobedo, E. Iancu, e-Print: arXiv:1601.03629 [hep-ph], JHEP 1605 (2016) 008.
[2] Multi-particle correlations and KNO scaling in the medium-induced jet evolution
M. Escobedo, E. Iancu, e-Print: arXiv:1609.06104 [hep-ph].
The shear and the bulk relaxation times are important ingredients of the second order hydrodynamics whose success in heavy ion phenomenology is unquestioned. Unlike viscosites themselves, field theoretical calculations of the relaxation times are hard to come by in literature, especially for the bulk relaxation time. In this work, we report two field-theoretical analyses involving the shear and the bulk relaxation time. First, by carefully examining the analytic structure of the stress-energy tensor response functions, we have been able to derive, for the first time, a Kubo formula involving both the shear and the bulk relaxation times. Second, by evaluating the Kubo formula within the massless scalar theory, we have so far been able to calculate the shear relaxation time in a simple form. We will then show how this calculation can be extended to calculate the bulk relaxation time as well.
Magnetic monopoles are suggested to play an important role in strongly coupled quark-gluon plasma (sQGP) near the deconfinement temperature. Lattice studies show that near the confinement temperature, $T_c$, quark-gluon plasma (QGP) contains both electric and magnetic quasiparticles. Further studies of the behavior of these quasiparticles at and above $T_c$, such as those by Liao and Shuryak (2006-2009) and D’Elia and D’Alessandro (2007-2009), found that the magnetic component of QGP forms a liquid, and that the magnetic monopoles form a Bose-Einstein condensate as the temperature approaches the critical temperature, creating the dual-superconductor proposed as a mechanism for confinement.
In this work, we conduct path-integral Monte Carlo (PIMC) simulations of magnetic monopoles, in order to study their behavior at temperatures at and above the confinement temperature. First, we sought to replicate the lattice results of D’Alessandro, D’Elia, and Shuryak (2009) through the study of the permutation cycles of one- and two-component plasmas of bosons interacting with a Coulomb potential. We found for the two-component plasma, as they did on the lattice, that as the system approaches $T_c$ from above the exponential suppression of permutation cycles of these bosons decreases before disappearing at $T_c$, indicating a phase transition.
We then study thermodynamics and physical distribution of magnetic monopoles, using the densities and coupling strengths given by D’Alessandro and D’Elia (2007); and Liao and Shuryak (2009). At low temperatures, in addition to the formation of the condensate, we see formation of “droplets” of the magnetic charges at strong coupling. This suggesting that the monopoles are indeed forming a liquid, as seen by Liao and Shuryak in their classical molecular dynamics simulations. At low temperatures and large couplings, there are also signs of crystallization in this system, an interesting analog to systems of extremely high density, such as helium white dwarf stars. We then simulate the system at higher temperatures — along lower densities and strong coupling — and see the breakup of these drops of monopole “liquid.” Finally, we draw conclusions about the contribution of the monopoles to the overall thermodynamics of the QGP above the confinement temperature.
We develop a set of kinetic equations for hydrodynamic fluctuations which are equivalent to nonlinear hydrodynamics with noise. The hydro-kinetic equations can be coupled to existing second order hydrodynamic codes to incorporate the physics of these fluctuations, which become dominant near the critical point.
We first show that the kinetic response precisely reproduces the renormalization of the shear viscosity and the fractional power ($\propto \omega^{3/2}$) which characterizes
equilibrium correlators of energy and momentum for a static fluid. Such fractonal powers are known as "long time tails", and were previously discussed by Kovtun, Moore and Romatschke.
Then we use the hydro-kinetic equations to analyze thermal fluctuations for a Bjorken expansion, evaluating the contribution of thermal noise from the earliest moments and at late times. In the Bjorken case, the solution to the kinetic equations precisely determines the coefficient of the first fractional power of the gradient expansion ($\propto 1/(\tau T)^{3/2}$) for the expanding system. Numerically, we find that the contribution to the longitudinal pressure from hydrodynamic fluctuations is larger than second order hydrodynamics for typical medium parameters used to simulate heavy ion collisions.
We extend hydrodynamics of a fluid with conserved charge to incorporate the phenomenon of critical slowing down -- an essential ingredient for describing the dynamics of QCD matter near the critical point. We develop the general formalism of hydrodynamics with additional critically slow mode extending the validity of hydrodynamics in critical regime. As an application we consider a simple Bjorken model of heavy-ion collision near the critical point.
We study QCD for temperatures up to about 500 MeV using the lattice
approach. We include two generations of dynamical quarks, with
physical strange and charm masses, which are known to be relevant in the explored
temperature range. Our lattice discretization - Wilson quarks with a twisted
mass term - has good chiral properties at a moderate computational
cost. The main focus is the measure of the topological susceptibility,
which on one side helps understanding fundamental properties of the quark-gluon plasma, on the other constrains properties of the axion,
one serious candidate for dark matter. We contrast and compare
the results from several methods for the measurements of the topological
susceptibility, and discuss the perspectives towards controlled
results in the continuum limit for physical quark masses, and implications
for the axions' search
A better understanding of the energy loss of partons in the quark-gluon plasma formed in the collisions of heavy ions can be gained by varying the collision system. Recent RHIC runs have provided Cu+Au and U+U collisions. Asymmetric Cu+Au collisions provide a system with similar energy density but different path lengths when compared to Au+Au with the same number of nucleon-nucleon collisions. Also, in the most central Cu+Au events the surface bias is reduced in the Cu-going direction. Similarly the non-spherical nuclear U+U collisions can produce different energy density and surface biases compared to Au+Au collisions. In this talk we present the results from $\pi^0$ and $\eta$ production in large systems. We discuss comparisons with Au+Au and how those comparisons further our understanding of parton energy loss in a quark-gluon plasma.
Single particle production has proven to be a valuable tool to study heavy ion collisions. The observation of collective behavior in $p$+Pb at the LHC and $d$+Au RHIC has spurred speculation that a plasma is formed in small collision systems. Jet production in the same collisions at the LHC and RHIC has an anomalous centrality dependence if centrality is determined the same way as in large ion-ion collisions. One interpretation could be that the nucleus probes the dynamical structure ("color fluctuations") of the projectile. Hints of gluon saturation effects have been observed at forward rapidities in $d$+Au collisions at RHIC. To systematically explore the physics using very asymmetric systems, RHIC has provided beams of $p$+Au, $p$+Al, $d$+Au and $^3$He+Au. Single particle production in these collisions should be sensitive to the physics of energy loss, modifications of the nuclear wavefunction, and the dynamics of the projectile wavefunction. PHENIX Central arms can measure pi0 at mid-rapidity. New MPC-EX detector allows $\pi^0$ measurement for $3.1 < \eta < 3.8$. In this talk we present the systematic study of $\pi^0$ production in several very asymmetric collision systems from PHENIX and discuss their impacts on our understanding of the physics of such systems.
We report measurements of two jet shapes, the ratio of 2-Subjettiness to 1-Subjettiness ($\tau_2/\tau_1$) and the opening angle between the two axes of the 2-Subjettiness jet shape which correspond to the axes of the hardest splitting in the jet. Comparison of these two jet shapes in Pb-Pb and pp collisions highlights the role of coherence effects on jet quenching, in the presence of the QGP medium, by separating two-pronged jets from the rest of the jet population. Coherence effects relate to the ability of the medium to resolve a jet's substructure, which has an impact on the energy loss magnitude and mechanism of the traversing jet. In both collision systems charged jets are found with the anti-$k_t$ algorithm, a resolution parameter of R = 0.4 and a constituent cut off of 0.15 GeV. The reclustering algorithm used to obtain the hardest splitting is the exclusive-$k_t$ algorithm. This analysis uses hadron-jet coincidence techniques in Pb-Pb collisions to reject the combinatorial background and further corrects for background effects by employing various jet shape subtraction techniques and unfolding. Measurements of the Nsubjettiness for jet momenta of $20 - 60$ GeV/c in Pb-Pb collisions at $\sqrt{s_{NN}}$ = 2.76 TeV and pp collisions at $\sqrt{s}$ = 7.00 TeV will be presented and compared to PYTHIA simulations.
A key feature of jet fragmentation in vacuum is colour coherence, which leads to angular ordering of the shower. Recent works have pointed out the importance of colour coherence for jets passing through QCD matter. The results are indicative of a reorganisation of the jet fragmentation in terms of resolved subjets each of which are affected independently by energy loss in the medium. We study this picture in detail for groomed jets in heavy-ion collisions using the “soft drop” procedure which singles out two hard jet substructures. As a direct measurement of colour (de)coherence, we show how wide-angle structures should be strongly suppressed compared to narrow ones. We also discuss the sizeable effects of colour (de)coherence on inclusive as well as jet substructure observables.
We study modification of full jet structures in the quark-gluon plasma (QGP) medium including effect of the hydrodynamic medium response. The structures and energies of jets in heavy ion collisions are significantly modified by the processes involving strong interactions during the propagation through the QGP medium, i.e., collisional energy loss, $p_T$-broadening, and induced parton radiation. The energy and momentum are deposited into the QGP medium by jets via the collisional energy loss and the $p_T$-broadening due to the energy-momentum conservation. The QGP medium is supposed to respond hydrodynamically to the deposited energy and momentum and flows propagating with the jets are caused. Particles originating from the jet-induced flows are observed as a part of the jets in the actual experiments, and contribute to the modification of the full jet structures. Studying this contribution is not only important for the precise interpretation of the experimental data but also provides a novel opportunity to investigate the collective response of the QGP.
We employ a full jet shower + QGP fluid model composed of transport equations for jet evolution and hydrodynamic equations with source terms for the QGP medium evolution. The transport equations describe the evolution of the three-dimensional momentum distributions of partons in the full jet [1]. In the transport equations, all the processes of the collisional energy loss, $p_T$-broadening and partonic splittings for all partons within the full showering jet are covered. The space-time evolution of the QGP medium is described by (3+1)-dimensional ideal hydrodynamic equations with source terms [2]. The source terms transfer the deposited energy and momentum to the QGP fluid and are constructed with the evolving distributions of the partons in jets obtained as solutions of the transport equations. In this work, we study the shape and energy loss of the full jet in Pb+Pb collisions at the LHC including the effect of the jet-induced flows. We find that the contribution of the particles originating from the jet-induced flows significantly modifies the full jet shape, and especially dominates it at large angles from the jet direction. We also find that this contribution increase the jet-cone size dependence of the full jet energy loss.
References
[1] N.-B. Chang and G.-Y. Qin, “Full jet evolution in quark-gluon plasma and nuclear modification of jet production and jet shape in Pb+Pb collisions at 2.76 ATeV at the LHC,” Phys. Rev. C 94, no. 2, 024902 (2016).
[2] Y. Tachibana and T. Hirano, “Momentum transport away from a jet in an expanding nuclear medium,” Phys. Rev. C 90, no. 2, 021902 (2014); Y. Tachibana and T. Hirano, “Interplay between Mach cone and radial expansion and its signal in $\gamma$-jet events,” Phys. Rev. C 93, no. 5, 054907 (2016)
Strangeness production in heavy-ion collision at energies below the free NN production threshold is an excellent tool to study medium properties of dense baryonic systems. For the first time, a nearly complete set of strange particles has been reconstructed in the 40% most central Au+Au collisions at $1.23 A$ GeV. The data sample includes multi-differential representations of charged and neutral Kaons, Lambdas and Phi-mesons. The multiplicities, together with those for non-strange hadrons, have been analyzed in the context of statistical hadronization models. Overall, a good fit is obtained if an additional parameter ($R_c$) is used to account for canonical strangeness suppression. We find that about 30 % of observed $K^-$ are produced through Phi-decay. If we correct the observed $K^-$ transverse momentum spectra for feed down from $\phi$ decay, all extracted slope parameters also support the assumption of a homogenous emission source for all particle types.
The RHIC Beam Energy Scan (BES) Program was proposed to look for the turn-off of signatures of the quark gluon plasma (QGP), search for a possible QCD critical point, and study the nature of the phase transition between hadronic and partonic matter. The results from the NA49 experiment at CERN have been used to claim that the onset of deconfinement occurs at √(s_NN ) ≈7 GeV, the low end of the BES range. Additionally, studies of several interesting observables during the BES, including v1 of protons and lambdas, v2 of identified hadrons, and net-proton higher moments, show interesting behavior below 20 GeV and could suggest a transition to a hadron dominated regime. Data from energies lower than 7 GeV could help determine whether these behaviors are indicative of phase transitions or criticality. The goal of the STAR Fixed-Target Program is to extend the collision energy range in BES-II with the same detector to lower energies than is feasible at RHIC with colliding beams.
In this talk we present results from STAR’s first dedicated fixed-target test run conducted in 2015 with Au + Au collisions at √(s_NN )=4.5 GeV. Direct flow of protons, elliptic flow of identified hadrons, HBT radii, as well as pion, proton, kaon, K_S^{0}, and lambda spectra are compared with previous results from the Alternating Gradient Synchrotron (AGS). These results demonstrate that STAR has good event reconstruction and particle identification capabilities for this fixed-target configuration even though it was optimized for colliding beams in the center of the detector. The implications of these results on future STAR fixed-target physics runs are discussed.
HADES provides a large acceptance combined with a high mass-resolution and therefore allows to study dielectron and hadron production in heavy-ion collisions with unprecedented precision. With the high statistics of seven billion Au-Au collisions at $1.23 A$ GeV recorded in 2012 the investigation of high-order flow harmonics is possible. Multi-particle azimuthal correlation techniques can be utilized to disentangle the contribution from collective and non-flow process involved in the dynamical evolution of heavy-ion reactions. At low energies v1 and v2, related to directed and elliptic flow, have been measured for pions, charged kaons, protons, neutrons and fragments at the BEVALAC and SIS18, but so far high-order harmonics have not been studied. They allow to characterize the properties of the dense hadronic medium produced in these collisions, such as its viscosity, and provide thus an important reference to measurements at higher energies.
Search for the conjectured QCD critical point is one of the major scientific goals for the Beam Energy Scan program at RHIC. The growth of the correlation length is a universal feature for systems near criticality, and observables which are most sensitive to the correlation length should be explored to identify signals of the QCD critical point.
Among all the first-order transport coefficients, bulk viscosity exhibits the strongest dependence on the correlation length. We investigate the effects of bulk viscosity near the QCD critical point on particle spectrum by solving relativistic viscous hydrodynamic equations at finite densities [1]. We find that rapidity distributions of charged particles and net baryon number are visibly modified if the fireball passes through the vicinity of the QCD critical point during its time evolution. We also discuss how critical modification of photon emission rate may leave imprints on thermal photon distributions.
[1] A. Monnai, S. Mukherjee, Y. Yin, arXiv:1606.00771[nucl-th]
Exploiting the universality between the QCD critical point and the three dimensional Ising model, closed form expressions derived [1] for non-equilibrium critical cumulants on the crossover side of the critical point reveal that they can differ both in magnitude and sign from equilibrium expectations. We demonstrate here that key elements of the Kibble-Zurek framework of non-equilibrium phase transitions can be employed to describe the dynamics of these critical cumulants [2]. Our results suggest that observables sensitive to critical dynamics in heavy-ion collisions should be expressible as universal scaling functions, thereby providing powerful model independent guidance in searches for the QCD critical point.
[1] Swagato Mukherjee, Raju Venugopalan, Yi Yin, Phys. Rev. C 92, 034912 (2015),
[2] Swagato Mukherjee, Raju Venugopalan, Yi Yin, arXiv:1605.09341 .
The search for the critical point of QCD is one of the main goals of the beam energy scan at RHIC and the CERN-SPS. In equilibrium, correlations diverge at the critical point leading to large event-by-event fluctuations in conserved quantities. For expanding systems like in heavy-ion collisions it is important to study the dynamical formation of long-range correlations in the critical region. The critical mode is the diffusive baryon current and can be described fluid dynamically. We include the propagation of fluctuations in the fluid dynamical equations. Using an equation of state with a critical point we study the evolution of critical fluctuations, Gaussian and non-Gaussian, in static systems to compare to known analytical results. The requirements for the emergence of non-Gaussian correlations from underlying white noise will be explored. We investigate both relativistic and nonrelativistic fluid dynamics. Finally, moving toward more realistic scenarios of heavy-ion collisions, we discuss the development of critical fluctuations in expanding systems.
Fluctuations of conserved charges are interesting probes of critical phenomena and freeze-out conditions in strongly interacting matter. In this context, experimental results will be presented on event-by-event analysis of net baryon fluctuation measurements in Pb-Pb collisions at $\sqrt{s_{NN}}$=2.76 TeV, recorded by the ALICE Collaboration at the CERN LHC. In addition to net-protons, used as a proxy for net-baryons, similar results for net-pions and net-kaons will be presented. The analysis will measure second moments of both net-particle and particle distributions. Furthermore, contributions from participant fluctuations and baryon number conservation will be discussed. Particular emphasis will be placed on the quantitative understanding of the centrality and rapidity width dependence of the obtained results. The data will be compared with recent predictions from the Hadron Resonance Gas model (HRG) and Lattice QCD (LQCD).
One of the main goals of the RHIC Beam Energy Scan program is to search for the QCD Critical Point (CP) and phase transition in heavy-ion collisions. Fluctuations of conserved quantities are highly sensitive to the correlation length, and are directly connected to the susceptibilities in the Lattice QCD. Therefore, they are ideal observables for finding the CP and phase transition signatures.
In this talk, we will present measurements of the cumulants of net-proton distributions from Au+Au collisions at $\sqrt{s_{NN}}$ = 7.7, 11.5, 14.5, 19.6, 27, 39, 62.4 (up to fourth order) and 200 GeV (up to sixth order) measured by the STAR experiment at RHIC. Multi-particle correlation functions are also extracted from the proton and anti-proton cumulants. It is observed that the four-particle correlations are positive. These provide additional insight into the non-monotonic energy dependence observed in fourth order proton fluctuations. Finally, we will discuss the corresponding physics implications.
Up to $6^{th}$ order cumulants of fluctuations of net baryon-number,
net electric charge and net strangeness as well as correlations among
these conserved charge fluctuations are now being calculated in lattice
QCD. These cumulants provide a wealth of information on the properties
of strong-interaction matter in the transition region from the low
temperature hadronic phase to the quark-gluon plasma phase.
We use results from our $6^{th}$ order Taylor expansion of the QCD equation
of state to construct expansions for second and fourth order cumulants
of conserved charges and their correlations, e.g. the second order cumulants
can be calculated up to ${\cal O} (\mu_B^4)$ in the baryon chemical
potential. We show that these low order cumulants strongly constrain the
applicability range of hadron resonance gas model calculations. We point
out that the latter is inappropriate to describe equilibrium properties
of cumulants at finite $\mu_B$ already at $T\sim 155$ MeV.
For vanishingly small baryon chemical potential, we show that fourth
order cumulant ratios calculated in QCD start to deviate from hadron
resonance gas model calculations already at about 155 MeV, and
the sixth order cumulants differ from HRG model calculations even earlier.
Even some second order cumulants like the correlations between net-baryon
number and net strangeness or net electric charge differ significantly at
temperatures above 155 MeV in QCD and HRG model calculations. Since these
cumulants are calculated at vanishing chemical potential they can be compared
to measurements at the LHC.
We will discuss the relation between particle number cumulants and multi-particle correlation functions. It is argued that measuring couplings of the genuine correlation functions could provide cleaner information on possible non-trivial dynamics in heavy-ion collisions. We extract integrated multi-particle correlation functions from the presently available experimental data on proton cumulants. We find that the STAR data contain significant four-particle correlations, at least at the lower energies, with indication of changing dynamics in central collisions. We also find that these correlations are rather long-ranged in rapidity. Based on the signs of genuine correlation functions we provide exclusion plots for the QCD phase diagram.
Azimuthally differential femtoscopic measurements, being sensitive to spatio-temporal characteristics of the source as well as to the collective velocity fields at freeze-out, provide very important information on the nature and dynamics of the system evolution. While the HBT radii modulations with respect to the second harmonic event plane reflect mostly the spatial geometry of the source, the third harmonic results are mostly defined by the system dynamics. Radii variations with respect to the third harmonic event plane unambiguously signal a collective expansion and anisotropy in the flow fields. Strong fluctuations in the initial geometry of the system lead to fluctuations in the anisotropic flow as well as the shape of the pion source at freeze-out. Event shape engineering (ESE) is a technique proposed to select events corresponding to a particular shape. Azimuthally differential HBT combined with ESE allows for a detailed analysis of the relation between initial geometry, anisotropic flow and the deformation of source shape.
In this talk, we present azimuthally differential pion femtoscopy with respect to second and third harmonic event planes as a function of the pion transverse momentum for different collision centralities. Our results on the dependence of the side-, out-, and long-radii on the pion emission angle with respect to the second harmonic event plane qualitatively agree with theoretical calculations, but the details show significant deviations. The final-state source eccentricity, estimated via side radius oscillations is found to be significantly smaller than the initial state source eccentricity. While the final-state source eccentricity for the second harmonic event plane remains positive in all centralities, the third harmonic event plane eccentricity becomes negative. All these results are compared to existing models. The effect of the selection of the events with high/low elliptic and/or triangular flow is also presented.
Recently, theoretical and experimental studies of flow and multiplicity correlations in the longitudinal direction in heavy ion collision have revealed a rich dynamics not probed by traditional measurements that focused only on the transverse direction. Event-by-event longitudinal fluctuations in the initial conditions are expected to result in a strong asymmetry in particle multiplicities at forward and backward (FB) rapidities. In this talk, FB multiplicity correlations are measured in $pp$, $p$+Pb and Pb+Pb collisions, with a data-driven method utilized to separate long-range FB correlations (LRC) and short-range correlations (SRC). The magnitude of the LRC reveals a significant FB multiplicity asymmetry, quantified by its slope in pseudorapidity $a_1$, which decreases with increasing multiplicity and is similar in magnitude across the three collision systems. The measured correlation in $pp$ collisions is compared in detail with expectations from the PYTHIA 8 and EPOS-LHC models. Both models significantly underpredict the measured $a_1$ values (by a factor of two), and PYTHIA 8 is found to significantly overpredict the magnitude of the SRC, even though both of these models were tuned to reproduce the total charged particle multiplicity distributions out to $N_{ch}$~120. These findings imply that the measurement of FB multiplicity correlations provides supplementary constraints on multi-particle production mechanisms, including restricting the number of independent sources in $pp$, $p$+Pb and Pb+Pb collisions.
Studies of azimuthal anisotropies for very high $p_{T}$ particles in relativistic heavy ion collisions provide crucial information on the path length dependence of the parton energy loss mechanism in the quark-gluon plasma. Final high-precision data on the elliptic ($v_{2}$) and triangular ($v_{3}$) anisotropy harmonics of charged particles, obtained with the scalar product method, are presented up to $p_{T}$ $\sim$ 100 GeV/c in PbPb collisions at $\sqrt{s_{NN}}$ = 5.02 TeV, using data recorded during the LHC run 2 with the CMS detector. In particular, the $v_{3}$ harmonic is explored to a very high $p_{T}$ regime for the first time, allowing for an improved understanding of the effect of initial-state fluctuations on the parton energy loss. The $v_{2}$ values reaching up $p_{T}$ $\sim$ 100 GeV/c are also determined using 4-, 6- and 8-particle cumulants, shedding new light on the origin of the observed high-$p_{T}$ azimuthal anisotropies. These new results are compared to theoretical calculations and provide stringent constraints on the parton energy loss mechanisms and the influence of initial-state fluctuations.
Event-by-event participant geometry fluctuations are studied by measuring the distributions of event-by-event flow harmonics $p(v_{n})$. Insight as to the nature of these fluctuations is obtained by calculating from $p(v_{n})$ the cumulants and moments associated with the event shape. Flow harmonic distributions in PbPb collisions at $\sqrt{s_{NN}} = 5.02$ TeV are measured for the integrated $p_{T}$ ($\eta$) range $0.3 \leq p_{T} \leq 3.0$ GeV/c ($|\eta| \leq 2.4$) using the CMS detector at the LHC. The event-shape engineering technique is used to further divide events into classes based on their ellipticity, which allows for the study of detailed correlations between initial-shape components that would otherwise be destroyed by event-averaging techniques. Hydrodynamic models predict the $2^{nd}$ order participant eccentricity distributions to have a negative skewness, which is identified as the main source of non-Gaussian behavior in $p(v_{2})$ distributions. The skewness for $p(v_{2})$ distributions is measured with high precision over the full centrality range. In addition, $p(v_{n})$ distributions are fitted with an elliptic power law parameterization to infer the proportionality constant between the flow harmonics and the initial-state geometry. Furthermore, correlations between different order harmonics are measured using the event-shape selection technique.
The collisions of lead nuclei provided by the LHC in Run 2 provide new opportunities to study matter produced at unprecedented temperatures and densities. In particular, the study of the azimuthal anisotropy of produced charged particles not only constrains the initial state of the nuclear collisions and soft particle collective dynamics, but also sheds light on jet quenching via the measurement of flow harmonics at high transverse momenta. In this talk, new ATLAS measurements of flow harmonics from $v_2$ to $v_7$ in Pb+Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV, performed in a wide range of transverse momenta 0.5-40 GeV, pseudorapidity ($|\eta|$<2.5) and collision centrality are presented. This includes a first measurement of $v_6$ and $v_7$, as well as harmonics in ultra-central collisions. A procedure of removing correlations arising from back-to-back jets,
recently used in $pp$ collisions, is implemented in the Two-Particle Correlation method to evaluate $v_n$ without a jet bias. The scaling relations between the $v_n$ harmonics are also discussed.
Longitudinal dynamics has recently become a topic of great interest in the study of ultra-relativistic heavy ion collisions. Measurement of the longitudinal fluctuations of the flow harmonic coefficients $v_n$ and event-plane angles $\Psi_n$ can provide a more complete picture of space-time evolution of the hot, dense medium formed in heavy ion collisions. Longitudinal flow decorrelations can be modeled with two contributions: magnitude fluctuations and event plane twist. However, existing observables do not separate these two effects. In this analysis, a new 4-particle correlator is used to separate the event-plane twist from magnitude fluctuations in 2.76 and 5.02 Pb+Pb collisions. Results show both effects have a linear dependence on pseudorapidity separation for v_{2-5}, and show a small but measurable variation with collision energy. The correlation of $\Psi_n$ of different order are also expected to have longitudinal fluctuations due to the non-linear mixing effects between lower and higher order
flow harmonics. First measurement of such non-linear mode-mixing effects as a function of pseudorapidity is also presented. These result will help to constrain initial conditions along longitudinal direction and also help understand the longitudinal evolution of the fireball.
Measurements of the azimuthal anisotropy coefficients ($v_n$) of particle emission are an established method to characterize the quark gluon plasma (QGP) generated in the high-energy heavy-ions collisions. An important early finding at RHIC was that hydrodynamic calculations can account for the measured anisotropy at low $p_{T}$ and relate them to the collision geometry. At higher $p_{T}$, where hard processes are dominant, energy loss of partons traversing the QGP also creates an azimuthal anisotropy in the final-state hadrons which is related to the collision geometry. In order to better understand the interplay between the soft and hard processes, PHENIX measured the charged-hadron $v_{n}$ coefficients over a wide $p_{T}$ range (up to 10 GeV/$c$) as a function of centrality and beam energy.
In this talk, we will present new $v_{2}$ measurements for charged hadrons in 200 GeV Au+Au collisions as a function of $p_T$ and centrality and compare them to those from $\pi^{0}$ mesons. We will also report the $v_{2}$, $v_{3}$, and $v_{4}$ results for charged hadrons as function of the beam energy from 39 to 200 GeV.
We present a derivation of anisotropic dissipative fluid dynamics from the moments of the Boltzmann equation based on an expansion around an arbitrary anisotropic single-particle distribution function [1]. We construct such an expansion in terms of polynomials in energy and momentum in the direction of the anisotropy and of irreducible tensors in the two-dimensional momentum space orthogonal to both the fluid velocity and the direction of the anisotropy. From the Boltzmann equation we then derive the set of equations of motion for the irreducible moments of the deviation of the single-particle distribution function from the anisotropic distribution. Truncating this set via the 14-moment approximation, we obtain the equations of motion of anisotropic dissipative fluid dynamics. We further consider a particular choice for the anisotropic distribution function and the boost-invariant expansion of a fluid in one dimension, neglecting deviations from the chosen distribution function [2]. In order to close the conservation equations, we need to select in addition a particular moment of the Boltzmann equation. We discuss the influence of the choice of this moment on the time evolution of fluid-dynamical variables and identify the moment that provides the best match of anisotropic fluid dynamics to the solution of the Boltzmann equation in the relaxation-time approximation.
[1] E. Molnar, H. Niemi and D. H. Rischke, Phys. Rev. D 93, no. 11, 114025 (2016), [arXiv:1602.00573 [nucl-th]].
[2] E. Molnar, H. Niemi and D. H. Rischke, arXiv:1606.09019 [nucl-th].
In this work we describe the dynamics of a highly anisotropic system undergoing a boost-invariant longitudinal and azimuthally symmetric radial expansion (Gubser flow) for arbitrary shear viscosity to entropy density ratio. We derive the equations of motion of dissipative anisotropic hydrodynamics by considering the moments method recently derived by Molnar et al. (MNR), Phys. Rev. D 93, 114025 (2016) and arXiv:1606.09019, based on an expansion around an arbitrary anisotropic one-particle distribution function. In order to close the conservation laws, it is needed to choose an additional moment of the Boltzmann equation. This is achieved by selecting the relaxation equation for the longitudinal pressure with a suitable Landau matching condition. As a result one obtains two coupled differential equations for the energy density and the longitudinal pressure which respect the $SO(3)_q \otimes SO(1,1) \otimes Z_2$ symmetry of the Gubser flow in the deSitter space. These equations are solved numerically and compared with the predictions of the recently found exact solution of the relaxation-time-approximation Boltzmann equation subject to the same flow. We also compare our numerical results with other fluid dynamical models. We observe that the MNR description of anisotropic fluid dynamics describes better the space-time evolution of the system than all currently known hydrodynamical approaches.
Heavy flavor quarks have been suggested as excellent probes to study the Quark-Gluon Plasma (QGP) created in ultra-relativistic heavy-ion collisions. Significant suppression of open heavy flavor production at large tranverse momentum has been observed in Au+Au collisions relative to p+p collisions at $\sqrt{s_{NN}}$ = 200 GeV at RHIC. Such a suppression can be attributed to the energy losses of heavy flavor quarks due to their interactions with the QGP, which are expected to be different for bottom and charm quarks because of their different masses. In order to fully understand the parton-QGP interactions and thus the QGP properties, it is essential to measure open bottom and charm hadron suppressions separately in Au+Au collisions. Moreover, Cold Nuclear Matter (CNM) effects due to the different initial states in p+p and Au+Au collisions also need to be taken into account when interpreting these results.
In this talk, we will report measurements of open bottom and charm hadron production through multiple decay channels in p+p, p+Au and Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV with the STAR experiment. We will show the first results on open bottom hadron production in Au+Au collisions, where electrons, $D^0$ and J/ψ from open bottom hadron decays are topologically identified utilizing the STAR Heavy Flavor Tracker. These results will be compared to those of open charm hadron production to study the mass dependence of parton interactions with the QGP at RHIC energies. Nuclear modification factor $R_{pA}$ for electrons from inclusive open heavy flavor hadron decays will also be shown to quantify the CNM effects on open heavy flavor production.
Heavy quarks, i.e. charm and beauty, are sensitive probes to study the properties of the strongly-interacting matter created in heavy-ion collisions at ultra-relativistic energies, since they are mainly produced in initial hard scattering processes and experience the entire evolution of the system.
The heavy quarks traversing the medium lose energy via collisional and radiative processes in the interaction with the medium constituents. The heavy-flavour energy loss can be investigated with measurements of the modification of the transverse momentum distribution of heavy-flavour particles in heavy-ion collisions with respect to binary-scaled pp collisions (nuclear modification factor $R_{\rm{AA}}$). An observable that complements the investigation of the interaction of heavy quarks with the medium is the elliptic flow of heavy-flavour particles, which is defined as the second harmonic ($v_{2}$) of the Fourier expansion of the particle azimuthal distribution in momentum space. The measurement of the heavy-flavour particle $v_{2}$ at low transverse momentum ($p_{\rm T}$) provides insight into the collective motion of heavy quarks in the medium, while the heavy-flavour particle $v_{2}$ at high $p_{\rm T}$ is sensitive to the path-length dependence of the energy loss of heavy quarks in the almond-shaped overlap area in non-central collisions.
The semi-leptonic decay channel of open heavy-flavour hadrons is well suited for heavy-flavour studies in ALICE, since the branching ratio is relatively large (10$\%$) and ALICE has an unique capability for identification of electrons at mid-rapidity ($|y| < 0.8$) and muons at forward rapidity ($2.5 < y < 4$) over a wide $p_{\rm T}$ range.
In this talk, we will present the ALICE results on the nuclear modification factor and elliptic flow of open heavy flavour hadrons via their semi-electronic decays at mid-rapidity and via their semi-muonic decay channel at forward rapidity in Pb–Pb collisions. Progress on the measurements of leptons from heavy-flavour and beauty-hadron decays in Pb–Pb collisions at $\sqrt{s_{\rm{NN}}} = 2.76$ TeV will be discussed. The first results on the $R_{\rm{AA}}$ and $v_{2}$ of electrons from heavy-flavour hadron decays in Pb–Pb collisions at $\sqrt{s_{\rm{NN}}} = 5.02$ TeV and prospects for measurements of electrons from beauty-hadron decays will be presented. The latest results concerning the measurements of the production cross section and nuclear modification factor of muons from heavy-flavour hadron decays as a function of $p_{\rm T}$ and collision centrality in Pb–Pb collisions at $\sqrt{s_{\rm{NN}}} = 5.02$ TeV will also be shown. The results will be compared with model calculations including the interaction of heavy quarks with the medium.
The energy loss of jets in heavy-ion collisions is expected to depend on the mass and flavor of the initiating parton. Thus, measurements of jet quenching with identified partons place powerful constraints on the thermodynamic and transport properties of the hot and dense medium. We present recent result on heavy flavor jet spectra and nuclear modification factors of jets associated to charm and bottom quarks in both pPb and PbPb collisions. New measurements to be presented include the dijet asymmetry of pairs of b-jets in PbPb collisions and a finalized c-jet measurement in pPb collisions based on new data collected during the 2015 heavy-ion run period at the LHC.
ATLAS measurements are presented on the production of muons from heavy-flavor decays in $\sqrt{s_{\mathrm{NN}}}$ = 2.76 TeV Pb+Pb collisions and $\sqrt{s}$ = 2.76 TeV $pp$ collisions at the LHC. The measurements are performed over the transverse momentum range 4 <$p_{\mathrm {T}}$< 14 GeV and over the 0-60% centrality interval. Backgrounds arising from in-flight pion and kaon decays, hadronic showers, and mis-reconstructed muons are removed using a template-fit procedure. The heavy-flavor muon differential cross-sections and per-event yields are measured in $pp$ and Pb+Pb collisions, respectively. The nuclear modification factor $R_{\mathrm{AA}}$ is observed to be independent of $p_{\mathrm{T}}$ within uncertainties and to be less than unity, which indicates suppressed production of heavy flavor muons in Pb+Pb collisions. The heavy-flavor muon yield is also measured as a function of the azimuthal angle difference, $\phi-Psi_{2}$, relative to the second-order event plane angle. Fourier coefficients associated with the $\cos(2(\phi-\Psi_{2}))$ modulation, $v_2$ , are measured as a function of $p_{\mathrm{T}}$ and centrality. They vary slowly with $p_\mathrm{T}$ and show a systematic variation with centrality that is characteristic of other $v_2$ measurements. The higher-order harmonics $v_3$ and $v_4$ are also measured. These measurements provide insight into the energy loss mechanism of heavy quarks as they propagate through the hot, dense medium produced in heavy ion collisions.
Hadrons carrying heavy flavor (charm and bottom quarks) are a sensitive probe of the hot, dense medium created in high-energy nuclear collisions at RHIC because they are generated early in the reaction and subsequently propagate through the created matter.
The PHENIX experiment has measured inclusive open heavy flavor via the measurement of electrons from semi-leptonic decays of hadrons carrying charm or bottom quarks in a variety of Collision systems. After the addition of the silicon vertex tracker, VTX, independent measurements of charm and beauty meson are now possible via off-vertex decays. Using Bayesian unfolding techniques applied simultaneously to the heavy flavor electron yield and the distance of closest approach for heavy flavor electrons, PHENIX measured heavy-quark production of charm and bottom separately using data sets taken in 2011, 2014 and 2015 Au+Au and $p+p$ collisions.
In this talk, we will present the single electrons, from $b$ and $c$ decays separately, nuclear modification factors $R_{AA}$ and their interpretation in view of current theoretical understanding.
Experimental data from heavy ion collisions at Relativistic Heavy Ion Collider (RHIC) and Large Hadron Collider (LHC) as well as first-principle lattice simulations, have provided rich information about the properties of the QCD plasma phase in the $1\sim 3 T_c$ regime. In particular, extensive jet energy loss measurements have allowed unique opportunity for probing the “internal working” of such plasma and for understanding the nonperturbative dynamics that underlies the confinement transition at $T_c$. Significant progress has been made recently in constructing such a microscopic model — the semi-quark-gluon-monopole plasma (sQGMP) which integrates two essential elements of confinements, i.e. the Polyakov-loop suppression of quarks/gluons and emergent magnetic monopoles. Based on sQGMP, a new comprehensive framework for simulating jet energy loss has been developed, called CUJET3.0, which (1) treats the radiative energy loss in the DGLV formalism; (2) convolutes energy loss with a bulk evolution of heavy ion collisions from the state-of-the-art viscous hydrodynamic simulation (VISHNU hydro) that is data-validated; (3) constrains the thermodynamic contents of the plasma constituents by current lattice QCD data. The CUJET3.0 simulation results have successfully passed the test of seven sets of jet quenching (leading-hadron) observables, including light hadrons' $R_{AA}$ and $v_2$ at AuAu 200GeV and at PbPb 2.76TeV as well as D and B mesons $R_{AA}$ and $v_2$ at PbPb 2.76TeV. In this talk the newest results from CUJET3.0 will be reported, with systematic predictions for the light and heavy flavor $R_AA$ and $v_2$ at the LHC 5TeV PbPb collisions. Furthermore, quantification of potential final-state jet attenuation effects from CUJET3.0 for small colliding systems at RHIC and LHC will also be presented.
[Refs] J.Xu, J.Liao and M.Gyulassy: (1) in preparation; (2) JHEP1602(2016)169; (3)CPL32(2015)092501.
We calculate higher order fluctuations of baryon number and electric charge in
lattice QCD. The results at real chemical potentials are obtained through
analytical continuation of simulations at imaginary chemical potentials. We
compare to (or discuss) the STAR proton and electric charge fluctuation data to
characterize the chemical freeze-out in view of the new lattice findings.
We study fluctuations of the sigma field and the net-baryon number on the crossover side of the critical point within the model of nonequilibrium chiral fluid dynamics (N$\chi$FD). Herein, the sigma field as the chiral order parameter is propagated explicitly and coupled to a fluid of quarks. Before investigating these fluctuations in an expanding nonequilibrium medium, we scrutinize the N$\chi$FD model by comparing cumulants of the sigma and net-baryon number fluctuations in a thermalized box to (ratios of) susceptibilities as they are obtained from derivatives of the grand canonical potential. The dynamically determined cumulants follow the trend of the thermodynamic susceptibilities. After implementing a particlization procedure into this model, we study the behavior of the net-proton kurtosis in the critical region and find that it shows the typical shape around the pseudocritical temperature. This demonstrates how critical fluctuations are able to develop in a realistic heavy-ion collision scenario and, moreover, have observable consequences. Finally, we present results for different rapidity windows and transverse momentum cuts.
The strong rise towards lower collision energies of the fourth moment of the e-by-e net-baryon multiplicity distribution observed by the STAR collaboration has recently triggered high attention. In view of theoretical studies of critical phenomena in the QCD matter phase diagram, this could signal the existence of a critical point. To provide further experimental insight, an extension of the respective excitation function to even lower collision energies is of high importance. We have investigated higher moments of e-by-e proton distributions using data from our high-statistics measurement of Au+Au collisions. Systematic effects have been studied making use of our GEANT-based detector response simulation which includes sophisticated digitizers for all detector systems in use. The data is corrected for detector effects like finite acceptance and multiplicity-dependent reconstruction efficiency using different approaches proposed by Koch and co-workers (arXiv-1206-4286, arXiv-1603-09057, arXiv-1607-07375).
We study neutron star matter equations of state at finite temperature for neutron star mergers and supernovae, including not only thermal quark fluctuations but also the Nambu-Goldstone modes. Our description is based on 3-window modeling in which nuclear matter at nB < 2n0 (nB:baryon density, n_0:saturation density) is smoothly connected to strongly correlated quark matter at nB > 5n0. Our quark matter at zero temperature is in the color-flavor-locked (CFL) phase with the gap of 100-200 MeV. The latter is significantly constrained by the two-solar mass constraint. Applying a schematic quark model for the nB > 5n0 domain, we constrain the possible range of the model parameters, and then use them to calculate various quantities of NG modes. Our predictions differ from the weak coupling results for the CFL because we are treating the strongly correlated domain. The resulting equations of state are like neither gapless quark nor nuclear equations of state, due to different temperature dependence of thermal contributions. The difference affects the pattern of gravitational wave signals.
I will discuss the new state-of-the-art perturbative Equation of State of quark matter valid at all temperatures and chemical potentials. The new result is accurate to order g^5 in the gauge coupling, and is based on a novel framework for dealing with the infrared sensitive soft field modes of the theory. The zero Matsubara mode sector is treated using a dimensionally reduced effective theory, while the soft non-zero modes are resummed using the Hard Thermal Loop approximation.
We study the effect of the chiral symmetry restoration (CSR) on heavy-ion collisions observables in the energy range $\sqrt{s_{NN}}$=3--20\,GeV within the Parton-Hadron-String Dynamics (PHSD) transport approach. The PHSD includes the deconfinement phase transition as well as essential aspects of CSR in the dense and hot hadronic medium, which are incorporated in the Schwinger mechanism for particle production. Our systematic studies show that chiral symmetry restoration plays a crucial role in the description of heavy-ion collisions at $\sqrt{s_{NN}}$=3--20\,GeV, realizing an increase of the hadronic particle production in the strangeness sector with respect to the non-strange one. We identify particle abundances and rapidity spectra to be suitable probes in order to extract information about CSR, while transverse mass spectra are less sensitive. Our results provide a microscopic explanation for the "horn" structure in the excitation function of the $K^+/\pi^+$ ratio: the CSR in the hadronic phase produces the steep increase of this particle ratio up to $\sqrt{s_{NN}} \approx$ 7 GeV, while the drop at higher energies is associated to the appearance of a deconfined partonic medium. Furthermore, the appearance/disappearance of the 'horn' structure is investigated as a function of the system size and collision centrality. We additionally present an analysis of strangeness production in the ($T,\mu_B$)-plane (as extracted from the PHSD for central Au+Au collisions) and discuss the perspectives to identify a possible critical point in the phase diagram.
The project NICA (Nuclotron-based Ion Collider fAcility) is aimed to study hot and baryon rich QCD matter in heavy ion collisions in the energy range √s_NN = 4 - 11 GeV. The heavy ion program includes the study of collective phenomena, dilepton, hyperon and hypernuclei production under extreme conditions of highest baryonic density. This program will be performed with the MPD (MultiPurpose Detector) at the NICA collider with the average luminosity of L = 1⋅10^27 cm−2⋅s−1 (for gold-gold collisions).
This presentation will review the projected accelerator performance and the physics opportunities for a heavy-ion programme at FCC-hh [1]. In addition, the status of the FCC-hh detector design studies will be discussed.
The FCC-hh Design Study will assess the feasibility and potential of a hadron collider with a centre-of-mass of 100 TeV for pp collisions. The status of the project will be summarized.
Operating FCC-hh with heavy-ion beams would provide Pb-Pb and p-Pb collisions at $\sqrt{s_{NN}}$ of 39 and 63 TeV, respectively. Current estimates indicate that a luminosity of about 30/nb could be integrated during a one-month Pb-Pb run, that is more than one order of magnitude above the maximum projections for the LHC. The FCC-hh beams could also be used for fixed-target collisions, either with beam extraction or gaseous target techniques.
The Quark-Gluon Plasma state produced in Pb-Pb collisions at 39 TeV is expected to have initial temperature and energy density substantially larger than at LHC energy, a stronger flow field and freeze-out volume twice as large. The larger temperature could entail novel features, like e.g. abundant in-medium production of charm quarks. The latter could determine an increase in the number of degrees of freedom of the QGP and provide a new tool to study its temperature evolution. New, rarer, hard probes would be available, like boosted top quarks, which could give access to the time-evolution of the medium opacity.
The physics of high gluon densities at small Bjorken-$x$ and the onset of saturation can be studied using pA, AA, and $\gamma$A collisions. The FCC-hh will provide access to the region down to $x<10^{-6}$ with perturbative probes like heavy quarks and quarkonia and to the region of high $Q^2$ down to $x\sim 10^{-4}$ with W, Z and top. High-energy photon-photon interactions in ultraperipheral AA collisions will also enable the study of very rare processes such as light-by-light scattering and $\gamma\gamma\to W^+W^-$.
Detector design studies, focused on multipurpose pp experiments, and a survey of the possible technological solutions are ongoing and will be summarised as well in the presentation.
[1] A. Dainese et al., Heavy ions at the Future Circular Collider, arXiv:1605.01389
QCD matter physics at the future FAIR facility in Germany
Peter Senger (GSI) for the CBM Collaboration
Abstract
The Compressed Baryonic Matter (CBM) experiment will be one of the major scientific pillars of the future Facility for Antiproton and Ion Research (FAIR) in Darmstadt. The goal of the CBM research program is to explore the QCD phase diagram in the region of high baryon densities using high-energy nucleus-nucleus collisions. This includes the study of the equation-of-state of nuclear matter at neutron star core densities, and the search for the deconfinement and chiral phase transitions. The CBM detector is designed to measure rare diagnostic probes such as hadrons including multi-strange (anti-) hyperons, lepton pairs, and charmed particles with unprecedented precision and statistics. Most of these particles will be studied for the first time in the FAIR energy range. In order to achieve the required precision, the measurements will be performed at very high reaction rates of 1 to 10 MHz. This requires very fast and radiation-hard detectors, a novel data read-out and analysis concept based on free streaming front-end electronics, and a high-performance computing cluster for online event selection. The status of FAIR and the physics program of the proposed CBM experiment will be discussed.
The QCD phase diagram has been explored in the high temperature side at RHIC and LHC, while the high density side is barely explored. Systematic studies of the QCD matter from Bevalac to LHC energies have suggested that the highest density QCD matter can be reached around the AGS energies (sqrt(s_{NN})=~5-GeV) where a rich production of strange hadrons is expected.
The future heavy-ion program at J-PARC (J-PARC-HI) is focused to explore such a highest density QCD matter. The J-PARC-HI will accelerate ions up to Uranium with the cms energy of sqrt(s_{NN})=2-6.2-GeV at the beam rate up to 1.0e+11 ions per cycle, five orders of magnitude higher than that of AGS. We could reach as 8-10 times higher density as the normal nuclear matter with the Uranium ions. The heavy-ion acceleration scheme consists of a new linac and a booster as the injector, followed by the existing 3-GeV Rapid-Cycling Synchrotron (RCS) and 50-GeV Main Ring (MR). The booster design is much advanced since last year by a new charge exchange injection scheme and multi-charge state acceptance.
Taking advantage of the very high intensity beam, we introduce new event selection quantities, such as strangity (strange hadron fraction) and baryonity (net baryons), on top of the conventional centrality variable, which would exclusively select high-density matter events. We will then primarily measure the probes that were not measured at AGS, namely, electromagnetic probes (photons and lepton pairs), higher-order flow of particles and the fluctuation of conserved charges such as net-baryons. We will also perform systematic measurement of conventional hadronic observables.
A large acceptance heavy-ion spectrometer based on a Toroidal magnet has been designed for the high density QCD matter study. We will show the updated acceleration scheme as well as the detector performance and expected physics result.
A search for the exotic hadrons and nuclei such as dibaryons, kaonic nuclei, and measure hypernuclei is also possible at this cms energy. We will also discuss about this measurement.
The ALICE TPC will undergo a major upgrade during the next LHC long shutdown in preparation for the higher luminosity planned for LHC Run-3 to start in 2021. This upgrade will allow ALICE to access new levels of sensitivity for untriggered processes. The present TPC is limited to recording minimum bias lead-lead collisions at a rate of about 1000 Hz. The upgrade will allow recording the full expected lead-lead collision rate of 50 kHz.
The present ALICE TPC uses multi-wire proportional (MWPC) chambers for readout. A gating grid is used to block positive ions created at the anode wires from flowing back into the main drift volume creating track distortions. The gating grid has an intrinsic dead time that limits the maximum collision rate that can be recorded. The goal of this upgrade is to replace the MWPCs and gating grid with Gas Electron Multiplier (GEM) arranged in a configuration that allows one to maintain the spatial and energy resolution of the present TPC. The electronics will be replaced with continuous readout electronics based on a new purpose designed chip. The project involves building 80 quadruple-GEM chambers (72 installed in the TPC plus 4 spares) utilizing 640 GEM foils. In order to accomplish such a large project the design, construction, quality assurance and testing are divided across many institutions and countries.
The motivation for the technology choices, status of the project, construction methods, quality assurance and testing procedures for the GEM foils and new readout chambers will be presented. Results from testing GEM foils, first chambers and readout electronics also will be presented.
The ALICE experiment will undergo a major upgrade during the next LHC Long Shutdown (LS2) scheduled in 2019-20 that will allow to study in detail the QGP properties exploiting the increased Pb-Pb luminosity expected during Run 3 and Run 4.
The replacement of the existing Inner Tracking System (ITS) with a completely new ultra-light high-resolution detector is one of the cornerstones within this upgrade program. The main motivation of the ITS upgrade is to provide ALICE with an improved tracking capability and impact parameter resolution at very low transverse momentum, as well as to enable a substantial increase of the interaction rate readout.
The new ITS will consist of seven layers of an innovative Monolithic Active Pixel Sensors with the innermost layers sitting at only 22 mm from the interaction point. This talk will focus on the design and the physics performance of the new ITS, as well as the technology choices adopted. The status of the project and the results from the prototypes characterization will also be presented
Relativistic hydrodynamics is the main theoretical framework used to describe the quark-gluon plasma produced in ultra-relativistic heavy ion collisions and, possibly, proton-proton and proton-ion collisions. Therefore, understanding the physical assumptions that enter in the hydrodynamical modeling of heavy ion collisions is crucial. Especially, it is essential to elucidate the reason behind the onset of fluid-dynamical behavior at the very early stages of the collisions.
In this contribution, we investigate the onset of hydrodynamic behavior for a gas of massless particles undergoing Bjorken expansion, a proxy for the dynamical evolution of the quark-gluon plasma produced in heavy ion collisions. In this scheme, we demonstrate that the Chapman-Enskog series, i.e., a gradient expansion in the hydrodynamic variables, has zero radius of convergence and cannot be used to consistently derive relativistic hydrodynamics [1]. On the other hand, we show that the method of moments, traditionally employed to derive Israel and Stewart's theory of hydrodynamics, converges and can be used to systematically improve the applicability of fluid-dynamical theories [2]. We further discuss what are the extended theories of hydrodynamics that emerge from higher-order truncations of the method of moments and how they can help in the description of heavy ion collisions.
[1] G.S. Denicol and J. Noronha, arxiv:1608.07869
[2] G.S. Denicol and J. Noronha, to appear soon.
In late 2015, the ALICE collaboration recorded data from Pb-Pb collisions at the unprecedented energy of $\sqrt{s_{\rm{NN}}} = 5.02$ TeV as well as reference data from pp collisions at the same energy. The $p_{\rm T}$-spectra of unidentified charged hadrons as well as of pions, kaons, protons, $\Lambda$, $\Xi$, $\Omega$, resonances and light (anti-)nuclei are presented.
Hydrodynamic and recombination models are tested against the measured spectral shapes at low and intermediate transverse momenta. A systematic study of strangeness production is of fundamental importance for determining the thermal properties of the medium created in heavy-ion collisions. The $p_{\rm T}$-integrated particle yields are compared to predictions from thermal-statistical models and the evolution of the particle ratios as a function of collision energy and centrality is discussed.
For the study of energy loss mechanisms in the QCD medium at high transverse momenta, the nuclear modification factors $R_{\rm{AA}}$ are computed and compared with model expectations.
Quantum Chromodynamics predicts the occurrence of a phase
transition from the hadronic matter to a plasma of deconfined quarks and gluons
(Quark-Gluon Plasma) at extreme conditions of temperature and energy density.
Ultrarelativistic heavy-ion collisions provide the means to study this phase of
matter in the laboratory.\
Strangeness production is a key tool to understand the properties
of the medium formed in heavy-ion collisions: an enhanced production of strange particles was early
proposed as one of the signatures of the QGP. The $\phi$ meson, due to its $s \bar s$ valence quark content, provides insight
into strangeness production.\
The ALICE experiment has measured $\phi$ meson production in the dimuon channel in the forward rapidity region $2.5 < y < 4$ in Pb--Pb collisions at $\sqrt{s_\mathrm{NN}}$ = 5.02 TeV.\
The preliminary $\phi$ meson $p_\mathrm{T}$ spectra for different centrality classes and the yield as a function of the collision centrality in the transverse momentum range $2 < p_\mathrm{T} < 7$~GeV/\textit{c} are presented. These results are also compared with the ones previously obtained in Pb--Pb collisions at $\sqrt{s_\mathrm{NN}}$ = 2.76 TeV.
While small at very high temperature, the bulk viscosity of quantum chromodynamics is expected to grow in the confinement region. Although its precise magnitude and temperature-dependence in the cross-over region is not fully understood, recent theoretical and phenomenological studies [1-5] provided evidence that the bulk viscosity can be sufficiently large to have measurable consequences on the evolution of the quark-gluon plasma. In this work, a Bayesian statistical analysis is used to establish probabilistic constraints on the temperature-dependence of bulk viscosity using combined hadronic measurements from RHIC and the LHC. IP-Glasma initial conditions are used to provide realistic event-by-event fluctuations, which are understood to have an important interplay with bulk viscosity. The width of the peak of bulk viscosity, along with the position of the peak in the transition region, are investigated phenomenologically for the first time. A lower but wider peak than the parametrization used in [3-5] is found to be preferred. Constraints on the position of the peak are found to be limited, with tension observed between the values favoured by RHIC and LHC measurements. The relative effect of shear and bulk viscosities on hadronic observables, in particular momentum anisotropies, is investigated.
[1] Karsch, F., Kharzeev, D. and Tuchin, K. (2008) Phys. Lett. B 663:217
[2] Noronha-Hostler, J., Noronha, J. and Greiner, C. (2009) Phys. Rev. Lett. 103:172302
[3] Ryu, S., Paquet, J.-F., Shen, C., Denicol, G.S., Schenke, B., Jeon, S., and Gale, C. (2015) Phys. Rev. Lett. 115:132301
[4] Bernhard, J.E., Moreland, J.S., Bass, S.A., Liu, J. and Heinz, U. (2016) Phys. Rev. C 94:024907
[5] Denicol, G., Monnai, A., and Schenke, B. (2016) Phys. Rev. Lett. 116:212301
Local momentum anisotropies are large in the early stages of the quark-gluon plasma created in relativistic heavy-ion collisions, due to the extreme difference in the initial longitudinal and transverse expansion rates. In such situations, fluid dynamics derived from an expansion around an isotropic local equilibrium state is bound to break down. Instead, we resum the effects of the slowest nonhydrodynamic degree of freedom (associated with the deviation from momentum isotropy) and include it at leading order, defining a local anisotropic quasi-equilibrium state, thereby treating the longitudinal/transverse pressure anisotropy nonperturbatively. Perturbative transport equations are then derived to deal with the remaining residual momentum anisotropies [1]. This procedure yields a complete transient effective theory called viscous anisotropic hydrodynamics [1,2].
The anisotropic hydrodynamic approach, especially after perturbative inclusion all residual viscous terms, has been shown to dramatically outperform viscous hydrodynamics in several simplified situations for which exact solutions exist but which share with realistic expansion scenarios the problem of large dissipative currents [1,3,4]. We will discuss the present status of applying viscous anisotropic hydrodynamics to the phenomenological description of the quark-gluon plasma in realistic expansion scenarios. To satisfy the high-performance needs of the JETSCAPE Collaboration, standard [5] and anisotropic viscous hydrodynamics algorithms were implemented on graphical processing units (GPU), leading to a 100-fold speed-up. Results from these accelerated 3+1-dimensional viscous hydrodynamic simulations for event-by-event fluctuating initial conditions will be compared between the standard and anisotropic frameworks and with experimental data.
[1] D. Bazow, U. Heinz, M. Strickland, Second-order (2+1)-dimensional anisotropic hydrodynamics, Phys. Rev. C 90, 054910 (2014).
[2] D. Bazow, U. Heinz, M. Martinez, Nonconformal viscous anisotropic hydrodynamics, Phys. Rev. C 91, 064903 (2015).
[3] M. Nopoush, R. Ryblewski and M. Strickland, Anisotropic hydrodynamics for conformal Gubser flow, Phys. Rev. D91, 045007 (2015).
[4] G. S. Denicol, U. Heinz, M. Martinez, J. Noronha and M. Strickland, Studying the validity of relativistic hydrodynamics with a new exact solution of the Boltzmann equation, Phys. Rev. D90, 125026 (2014).
[5] D. Bazow, U. Heinz, M. Strickland, Massively parallel simulations of relativistic fluid dynamics on graphics processing units with CUDA, arXiv:1608.06577, in press.
Experimental results on azimuthal correlations in high energy nuclear collisions (nucleus-nucleus, proton-nucleus and proton-proton) seem to be well described by viscous hydrodynamics. It is often argued that this agreement implies either local thermal equilibrium or at least local isotropy. In this note, I present arguments why this is not the case. Neither local near-equilibrium nor near-isotropy are required in order for hydrodynamics to offer a successful and accurate description of experimental results. However, I predict the breakdown of hydrodynamics at momenta of order twenty times the temperature, corresponding to a smallest possible QCD liquid drop size of 0.05 fm.
Heavy quarks are useful probes of nuclear matter since at RHIC energies as they are produced only in initial parton collisions, and thus are sensitive to effects at all stages of the collisions. Because of their large mass, b quarks are expected to lose less energy through gluon radiation than lighter quarks. $J/\psi$ production from the $B \rightarrow J/\psi$ decay is a powerful observable to measure the nuclear modification of $B$ mesons in the relevant $p_T$ range ($p_T$ << $m_b$).
PHENIX has measured the production of non-prompt $J/\psi$ from $B \rightarrow J/\psi$ decays in the dimuon channel at forward and backward rapidities, by the analysis of displaced vertex muons from the $B$ meson decay with the Forward Silicon Vertex Detector (FVTX). Comparison of the measured yields in the asymmetric Cu+Au, $p$+Au systems and in $p+p$ collisions can provide insights into the contributions of hot and cold nuclear matter effects.
Beauty production and phenomena in heavy-ion collisions are considered to be one of the key measurements to address the flavour-dependence of in-medium energy loss in PbPb collisions at the LHC. The CMS experiment has excellent capabilities for measuring b-quark production thanks to the excellent performances of its muon and tracker system, allowing the measurement of $D^{0}$ and J/$\Psi$ mesons from B meson decays, separately from prompt production, as well as fully reconstructed B mesons. In this talk, CMS will present the first measurement of the $v_{2}$ Fourier harmonic and the $R_{AA}$ down to $p_{T} > 3$ GeV/c, for J/$\Psi$ produced in B meson decays, in PbPb collisions at $\sqrt{s_{NN}}$ = 2.76 TeV, as a function of transverse momentum, rapidity and event centrality. New measurements of the $R_{AA}$ of non-prompt J/$\Psi$ and $D^{0}$ from $B$ decay in PbPb collisions at $\sqrt{s_{NN}}$ = 5.02 TeV will also be reported. Finally, the measurement of $R_{AA}$ for fully reconstructed B mesons will be shown, for the first time, in PbPb collisions at 5.02 TeV. The results are compared to various model calculations.
Despite intense theoretical and experimental investigation, the physical mechanisms governing the suppression of bound quark-antiquark states in nuclear collisions are not yet fully understood. While color screening in a plasma phase is expected to play a role, there are numerous other possible suppression mechanisms that do not require deconfinement, as well as effects on the heavy quark initial state in the nucleus which can also play a role. To study these effects, the PHENIX collaboration has used the flexibility of the RHIC accelerator complex to observe the evolution of open heavy flavor and quarkonia dynamics as both the projectile and target nuclei size are varied. Open heavy flavor in small collision systems can serve as the baseline for interpreting quarkonia production in the nuclear environment, and comparisons of the $\psi(2S)$ with the $J/\psi$ show that in rapidity regions with relatively high hadron density, the larger $2S$ state is preferentially more suppressed than the more tightly bound $J/\psi$. This suggests that late-stage mechanisms may be at least partially responsible for quarkonia suppression in nuclear collisions. In this talk, we will present results on excited-state quarkonia in $p+p$, $p$+Al, and $p$/$d$/$^3$He+Au collisions and open heavy flavor in small systems, and discuss how these measurements impact our understanding of heavy quark behavior in the quark-gluon plasma.
It has recently been argued that event-by-event fluctuations, choice in initial conditions, and proper calculations of $v_2\{2\}$ are necessary to resolve the long-standing $R_\text{AA}$ to $v_2\{2\}$ puzzle for high $p_T$ identified hadrons [1,2]. Here we investigate the effects of full event-by-event fluctuating hydrodynamic backgrounds on the nuclear suppression factor and flow harmonics of heavy flavor mesons and non-photonic electrons. We obtain an $R_\text{AA}$ for D$^0$ and B$^0$ that is roughly equivalent, in line with recent CMS results, and find similar results for the elliptical flow, triangular flow as well as the multiparticle cumulants in PbPb collisions at both 2.76 TeV and 5.02 TeV. Finally, we propose new experimental observables in the heavy flavor sector that will provide greater insight into consequences of flow fluctuations as well as further constraints on energy loss models.
[1] J. Noronha-Hostler, B. Betz, J. Noronha and M. Gyulassy,
Phys. Rev. Lett. 116, no. 25, 252301 (2016) [arXiv:1602.03788 [nucl-th]].
[2] B. Betz, M. Gyulassy, M. Luzum, J. Noronha, J. Noronha-Hostler, I. Portillo and C. Ratti, arXiv:1609.05171 [nucl-th].
Heavy quarks are considered as valuable probes of the quark-gluon plasma (QGP) created in ultra-realistic heavy-ion collisions. However, the simultaneous description of the heavy meson nuclear modification factor $R_{AA}$ and the elliptic flow $v_2$ poses a significant challenge for most commonly used transport modes, especially those based on Langevin transport. We propose a generalized ansatz for the temperature and momentum dependence of the heavy quark diffusion coefficient and subsequently extract its functional form by calibrating against RHIC and LHC data utilizing a Bayesian model-to-data analysis. Using the extracted transport coefficient, our improved Langevin framework is able to simultaneously reproduce the measured $R_{AA}$ and $v_2$ at both RHIC and LHC energies.
The Bayesian analysis used to extract the transport coefficient is set up as follows: a set of input parameters, in which the temperature and momentum dependence of the transport coefficient $D_s$ is encapsulated, are evaluated via an event-by-event heavy flavor transport model. In a (2+1)-dimensional viscous hydrodynamical model describes the QCD medium, heavy quarks propagate according to an improved Langevin equation that incorporates both radiative and collisional energy loss. Hadronization of heavy quarks occurs via a hybrid model of fragmentation and recombination. Those model outputs are used to train Gaussian process emulators that mimic the behavior the heavy quark transport model, and act as a fast surrogate of the transport model to interpolate across the full model parameter space. We then calibrate the model parameters on experimental data via a Markov chain Monte Carlo (MCMC) using Bayes' Theorem. The final results of the analysis are the posterior probability distribution of all the model parameters that contain the high likelihood parameters range in which the model describes the data optimally. We find that the transport coefficient $D_s$ has a minimum value around critical temperature, and is comparable to lattice QCD calculation. A non-trivial momentum dependence of $D_s$ is observed as well. With the extracted functional form of the transport coefficients, the $R_{AA}$ and $v_2$ of heavy quarks in different centralities at 200 GeV AuAu collisions and 2.76/5.02 TeV PbPb collisions are calculated, and observed to be consistent with the experimental data. The result of p-Pb collisions at 5.02 TeV is calculated and compared with experimental data.
Heavy-flavor observables are excellent probes of the properties of the in-medium interactions, the medium properties and the degrees of freedom of the quark-gluon plasma created in heavy-ion collisions. Progressing toward a quantitative description, we describe, in EPOSHQ, the dynamics of heavy quark coupled systematically to the EPOS3 model: heavy-quarks are produced from the EPOS3 flux tube initial conditions both in momentum and in coordinate space and subsequently propagated in parallel to the fluid dynamical evolution of the viscous QGP. Hadronization of the heavy quarks via coalescence and fragmentation and particlization of the fluid enable us to investigate the importance of the final hadronic rescatterings on the heavy-flavor observables.
This global description allows us to draw conclusions from the comparison to a variety of heavy-quark observables in different systems and constrain important aspects in our underlying model for the in-medium interaction, such as the contributions stemming from elastic and inelastic energy loss, or the mass dependence by comparing charm and bottom quark dynamics. We present strategies to quantify the off-equilibrium dynamics of heavy flavor at lower momentum compared to the bulk flow by focussing on the higher-order flow harmonics of B and D mesons and the light, charged hadrons and it's centrality dependence. At higher momentum path length differences become the driving force of flow observables. Our sophisticated energy loss models and QGP-heavy quark coupling allow us in particular to obtain robust estimates of different contributions to the heavy flavor flow in the intermediate momentum range.
Ágnes Mócsy is a theoretical physicist and professor of physics and astronomy at Pratt Institute, Brooklyn, NY. “Smashing Matters: Behind the Science Scene” is her first documentary film, looking behind the scenes of the build up of big science discussed at Quark Matter, revealing relationships, friendships, mentorships that develop as essential parts in the lives of scientists in the high intensity pursuit of their passion. The screening of the 30 min film is followed by a short Q&A.
Meeting of the QM2017 International Advisory Committee
The measurement of resonances in ultra-relativistic heavy-ion collisions allows one to study the properties of the hadronic medium. Resonances with short lifetimes compared to the duration of the hadronic phase are good candidates to probe the interplay of particle re-scattering and regeneration in the hadronic phase, which result in a modification of the measured yield of resonances.
Measurements of $\Sigma$(1385)$^{\pm}$ and $\Xi$(1530)$^{0}$ have been performed with the ALICE detector at the LHC in pp, p--Pb and Pb--Pb collisions at different energies. We report on the transverse momentum ($p_{\mathrm{T}}$) spectra, their mean values and yields as a function of the event multiplicity. The $p_{\mathrm{T}}$-integrated yield ratios of excited to ground-state hyperons and to pions are discussed as a function of the mean charged-particle multiplicity densities and compared with models.
Longitudinal dynamics has recently become a topic of great interest in the study of ultra-relativistic heavy ion collisions. Measurement of the longitudinal fluctuations of the flow harmonic coefficients $v_n$ and event-plane angles $Psi_n$ can provide a more complete picture of space-time evolution of the hot, dense medium formed in heavy ion collisions. Longitudinal flow decorrelations can be modeled with two contributions: magnitude fluctuations and event plane twist. However, existing observables do not separate these two effects. In this analysis, a new 4-particle correlator is used to separate the event-plane twist from magnitude fluctuations in 2.76 and 5.02 Pb+Pb collisions. Results show both effects have a linear dependence on pseudorapidity separation for v_{2-5}, and show a small but measurable variation with collision energy. The correlation of $Psi_n$ of different order are also expected to have longitudinal fluctuations due to the non-linear mixing effects between lower and higher order flow harmonics. First measurement of such non-linear mode-mixing effects as a function of pseudorapidity is also presented. These result will help to constrain initial conditions along longitudinal direction and also help understand the longitudinal evolution of the fireball.
The study of beauty production in heavy-ion collisions is considered one of the key measurement to address the flavour-dependence of in-medium energy loss in PbPb collisions. In pPb collisions, studies of b-quark production can also provide insights into the relevance of cold nuclear matter effects in the heavy-flavour sector. The CMS experiment has excellent capabilities for measuring b-quark production thanks to the excellent performances of its muon and tracker system. In this talk, we will present the measurement of nuclear modification factors for fully reconstructed B mesons in pPb, and for the first time, pp and PbPb collisions at 5.02 TeV, as a function of transverse momentum.
Azimuthal anisotropies of particle production in high energy heavy ion collisions have proven to be an excellent tool for investigating the initial geometry and the bulk properties of the Quark Gluon Plasma (QGP). Azimuthal anisotropy, measured through Fourier coefficients v_n, have been measured at mid-rapidity and are used to constrain the initial geometry and viscosity-over-entropy ratio eta/s of the QGP. Although there are many experimental observables and theoretical models, there are still uncertainties of the initial geometry and the eta/s. Measurements of v_n at forward/backward rapidity provide further insight into initial geometry. It is interesting to measure the v_n coefficients at forward/backward rapidity in Cu+Au collisions, because of the asymmetry in number of participants and geometry in forward and backward direction. In this poster, we will present our work to measure forward/backward asymmetry of v_n coefficients at pseudorapidity 3<|eta|<4 in Cu+Au collisions in comparison to results from Au+Au and Cu+Cu collisions.
Charm quarks, predominantly produced in the early stage of heavy-ion collisions, are believed to provide unique information on the hot and dense medium created in such collisions. At RHIC, an enhancement in baryon-to-meson ratios for light hadrons and hadrons containing strange quarks has been observed in central heavy-ion collisions compared to p+p and peripheral heavy-ion collisions in the intermediate $p_T$ range (2 < $p_T$ < 6 GeV/c). This was explained by the hadronization mechanism involving multi-parton coalescence. $\Lambda_{c}^+$ is the lightest charmed baryon with the mass close to $D^0$ meson, and it has an extremely short life time ($c\tau\sim60$ $\mu m$). Different models predict different levels of enhancement in the $\Lambda_{c}^+$/$D^0$ ratio depending on the degree of charm quark thermalization in the medium and how the coalescence mechanism is implemented.
In this poster, we will report the first measurement of $\Lambda_{c}^+$ production in heavy-ion collisions using the recently installed Heavy Flavor Tracker at STAR. $\Lambda_{c}^+$ are reconstructed through the hadronic decay channel ($\Lambda_{c}^+$ $\rightarrow$ $pK\pi$) using topological cuts optimized by the Toolkit for Multivariate Data Analysis (TMVA). After correcting for the reconstruction efficiency and acceptance, the transverse-momentum spectrum of $\Lambda_{c}^+$ in Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV will be presented. The measured $\Lambda_{c}^+$/$D^0$ ratio will be compared with different model calculations, and the physics implications will be discussed.
We study bottomonium production at RHIC and the LHC using a transport model including both suppression and regeneration mechanisms. The transport model utilizes a kinetic rate equation [1] to calculate the centrality dependence of the production yields, and a Boltzmann equation for transverse-momentum ($p_T$) spectra. It has been successful in describing and predicting charmonium data at SPS, RHIC and the LHC. The bottomonium dissociation rates are improved over previous work [2] by using in-medium binding energies from an in-medium T-matrix approach, which, in turn, require to account for both gluo-dissociation (dominant for large binding) and inelastic parton-induced break-up (dominant for weak binding) including interference effects [3]. We also update the equation of state for the bulk medium using lattice-QCD results. For the calculation of the $p_T$-spectra and elliptic flow of the regeneration contribution we use a coalescence model [4] where the input bottom-quark spectra are taken from Langevin transport simulations of bottom quarks [5] to account for their non-equilibrium distributions. We then conduct a systematic analysis of bottomonium observables for the nuclear modification factor as a function of $N_{\rm part}$ and $p_T$ in comparison to ALICE, CMS and STAR data. The comparison suggests that the centrality dependence of the total yields is sensitive to different scenarios for the screening of binding energies. The off-equilibrium bottom-quark spectra are found to play an important role in both the bottomonium $p_T$ spectra and their predicted elliptic, which helps to disentangle the role of regeneration contributions.
Reference:
[1] X. Du, R. Rapp, J. Fox and M. He, in preparation
[2] L. Grandchamp et al., Phys. Rev. C 73 (2006) 064906; A. Emerick, X. Zhao and R. Rapp, Eur. Phys. J. A48 (2012) 72.
[3] M.Laine, O. Philipsen, P. Romatschke, M. Tassler, JHEP 0703 (2007) 054
[4] V. Greco, C. M. Ko, P. Levai, Phs. Rev. C68 (2003) 034904
[5] M. He, R.J. Fries and R. Rapp, Phys. Lett. B735 (2014) 445.
In this talk, based on arXiv:1605.09176, we present a microscopic realization of the hollowness effect observed in proton-proton scattering at $\sqrt s = 7$ TeV. The initial collision geometry proposed in our model could impact significantly the interpretation of data specially sensitive to it, like the eccentricities of proton-proton, proton-nucleus and nucleus-nucleus collisions.
The hollowness effect, not observed at lower energies, consists in a depletion of the inelasticity density at zero impact parameter of the collision. Counterintuitively, there is more inelasticity when the two protons are at about half a fermi transverse separation that for head-on collisions. Our analysis is based on three main ingredients: we rely gluonic hot spots inside the proton as effective degrees of freedom for the description of the scattering process. Next we assume that some non-trivial correlation between the transverse positions of the hot spots inside the proton exists. Finally we build the scattering amplitude from a multiple scattering, Glauber-like series of collisions between hot spots. In our approach, the onset of the hollowness effect is naturally explained as due to the diffusion or growth of the hot spots in the transverse plane with increasing collision energy. Furthermore, we will explore the impact of the non-trivial correlations between the transverse positions of the hot spots in the calculation of eccentricities in proton-proton collisions, a highly debated topic nowadays as there are suggestive signals of collective phenomena, associated to the formation of QGP in heavy ion collisions, in this smaller system that may be caused by the initial state geometry.
Currently at the Beam Energy Scan at RHIC experimental efforts are being made to find the QCD critical point. On the theoretical side, the behavior of higher-order susceptibilities of the net-baryon charge from Lattice QCD may allows us to estimate its position via Taylor expansion of the density of states at $\mu_B=0$. However, even if the series expansion continues to higher-orders, there is always the possibility to miss the critical point behavior due to truncation errors.
An alternative approach to exploring the QCD critical point is using black hole engineering. This method allow us to obtain susceptibilities fitting the lattice data at $\mu_B=0$ but also can be expanded out to extremely large baryonic chemical potentials as well. Additionally, in the black hold engineered EoS there is a clear critical point at $\mu_B=725$ MeV and $T=80$ MeV. In this talk, we obtain the freeze-out line and compare it with the hadron resonance gas model, lattice calculations, and experimental data. We also explore fluctuations at the lowest energies at the beam energy scan to see if there are signatures of the critical point.