Session JG: Heavy Ion Collisions: Phase Diagram, Energy Scan

Deconfinement and freeze-out in heavy ion collisions

Based on Lattice QCD calculations of fluctuations and correlations of various conserved charges we show that the deconfinement of strangeness takes place in the chiral crossover region of QCD; however, inside the quark-gluon plasma strange quarks remain strongly interacting at least up to temperatures twice the QCD crossover temperature. We also show that the freeze-out parameters of heavy-ion collisions can be determined model independently through direct comparisons between the Lattice QCD calculations and the experimentally measured higher order conserved charge cumulants. Utilizing the preliminary data for various higher order conserved charge cumulants measured by the STAR and PHENIX experiments we present a Lattice QCD based determination of the freeze-out parameters and show that, for moderately small baryonic densities, the freeze-out take place very close the chiral/denconfiment crossover region of QCD.   

Searching for the QCD Critical Point with the Energy Dependence of $p_t$ Fluctuations

If systems produced in relativistic heavy-ion collisions pass near the QCD critical point while cooling, the correlation length of the system may diverge due to the phenomena of critical opalescence. The transverse momentum distribution, being related to the state variable temperature, might be sensitive to this change in correlation length. Non-monotonic behavior with changing incident energy or centrality of any transverse momentum observable that is sensitive to the correlation length could thus be indicative of the QCD critical point [1]. Accordingly, we report measurements related to transverse momentum fluctuations such as $\left\langle {\Delta {p_{t,i}}\Delta {p_{t,j}}} \right\rangle$ as a function of event centrality and incident energy for Au+Au collisions at $\sqrt{s_{\rm NN}}$ = 7.7, 11.5, 19.6, 27, 39, 62.4, and 200 GeV using the STAR detector at RHIC. The results are compared to UrQMD model predictions and previous experimental measurements.\\[4pt] [1] H. Heiselberg, Phys.Rept. 351, 161 (2001).   

Beam energy dependence of the viscous damping of anisotropic flow

The flow harmonics $v_{2,3}$ for charged hadrons, are studied for a broad range of centrality selections and beam collision energies in Au+Au ($\sqrt{s_{NN}}= 7.7 - 200$ GeV) and Pb+Pb ($\sqrt{s_{NN}}= 2.76$ TeV) collisions. They validate the characteristic signature expected for the system size dependence of viscous damping at each collision energy studied. The extracted viscous coefficients, that encode the magnitude of the ratio of shear viscosity to entropy density $\eta/s$, are observed to decrease to an apparent minimum as the collision energy is increased from $\sqrt{s_{NN}}= 7.7$ to approximately 62.4~GeV; thereafter, they show a slow increase with $\sqrt{s_{NN}}$ up to 2.76 TeV. This pattern of viscous damping provides the first experimental constraint for $\eta/s$ in the temperature-baryon chemical potential ($T, \mu_B$) plane, and could be an initial indication for decay trajectories which lie close to the critical end point in the phase diagram for nuclear matter.   

Rapidity Dependent Pion Spectra from Fixed-Target Au$+$Al Collisions at STAR

By analyzing collisions between beam halo nuclei and the beam pipe, the STAR detector at RHIC can study fixed-target Au$+$Al collisions. As such, STAR can extend the reach of the beam energy scan to lower center-of-mass energies and higher baryon chemical potentials than previously considered. This allows for a more thorough search for the possible onset of deconfinement of the phase transition between hadronic and partonic matter. In this talk, we discuss details of the fixed-target acceptance and efficiency of the STAR detector. In addition, we present the fixed-target pion spectra as a function of rapidity from the three Au$+$Al datasets at center-of-mass energies of 4.5, 3.5, and 3.0 GeV.   

Multiplicity and pseudo-rapidity distributions of photons at forward rapidity in STAR at RHIC energies

The main goals of the STAR experiment at Relativistic Heavy Ion Collider (RHIC) is to study the properties of the QCD matter at extremely high energy and parton densities. The photon multiplicity is measured in the STAR experiment at RHIC energies by a Photon Multiplicity Detector (PMD) in pseudo-rapidity region $-3.7 \leq \eta \leq -2.3$. The variation of particle density in pseudo-rapidity ($\eta$) with collision centrality can shed light on the relative contribution of soft and hard (perturbative QCD jets) processes in particle production. The pseudo-rapidity distributions are found to be sensitive to the effects of re-scattering, hadronic final-state interactions, and longitudinal flow. We study the centrality and energy dependence of photon multiplicity away from mid-rapidity to explore if the longitudinal scaling observed for Au+Au collisions at $\sqrt{s_{_{NN}}}=$ 200 GeV and 62.4 GeV holds at lower energies also. For photons, this scaling was found to be independent of collision energy, event centrality as well as system size at $\sqrt{s_{_{NN}}}=$ 200 GeV and 62.4 GeV. Here we intend to measure the same for Au+Au collisions at $\sqrt{s_{_{NN}}}=$ 39, 27 and 19.6 GeV. We will compare these results with published results from 200 GeV and 62.4 GeV and also with models.   

Transverse Energy at Forward Rapidities at RHIC using the PHENIX Muon Piston Calorimeter

In 2010, RHIC produced Au+Au collisions at $\sqrt{s_{NN}}=200, 62.4, 39,$ and $7.7$~GeV. Progress in measuring transverse energy in the range $3.1<|\eta|<3.8$ using the PHENIX Muon Piston Calorimeter (MPC) will be reported. The status of the 2010 MPC calibrations, studies of the hadronic response (the MPC is an electromagnetic calorimeter) and inflow and outflow of energy will be discussed. Transverse energy has been used to estimate energy density in ultra-relativistic heavy ion collisions and to discriminate between competing models of hadronic interactions. Furthermore, fluctuations in transverse energy might signal the presence of a critical point in the phase diagram of nuclear matter.   

Intra-event correlations and the statistical moments of the identified particle multiplicity distributions in the RHIC beam energy scan data collected by STAR

Specific products of the statistical moments of the multiplicity distributions of identified particles can be directly compared to susceptibility ratios obtained from lattice QCD calculations. They may also diverge for nuclear systems formed close to a possible QCD critical point due to the phenomenon of critical opalescence. Of particular interest are the moments products for net-protons, net-kaons, and net-charge, as these are considered proxies for conserved quantum numbers. The moments products have been measured by the STAR experiment for Au+Au collisions at seven beam energies ranging from 7.7 to 200 GeV. In this presentation, the experimental results are compared to data-based calculations in which the intra-event correlations of the numbers of positive and negative particles are broken by construction. The importance of intra-event correlations to the moments products values for net-protons, net-kaons, and net-charge can thus be evaluated.   

Measurement of pion, kaon, and proton spectra in U+U collision at $\sqrt{s_{NN}}$ = 193 GeV with PHENIX

The Relativistic Heavy Ion Collider at Brookhaven National Lab allows nuclear matter to be studied at extremely high temperatures and energy densities. RHIC is uniquely versatile in it's ability to collide a wide range of species. In 2012 RHIC saw the first ever high energy collisions with the irregularly shaped uranium nuclei, providing the possibility to produce systems that have different initial energy density profiles for the same number of participating nucleons. This allows for systematic investigation of the effects of initial geometry and density on particle production. The work in progress for measurement of the identified pion, kaon, and proton spectra as a function of centrality will be presented. The nuclear modification factor R$_{AA}$ and particle ratios such as kaon/pion, proton/pion, and antiproton/proton will also be studied and compared with the ratios measured in Au+Au collisions.