Bulletin of the American Physical Society
2011 Fall Meeting of the APS Division of Nuclear Physics
Volume 56, Number 12
Wednesday–Saturday, October 26–29, 2011; East Lansing, Michigan
Session JA: Mini-Symposium on From RHIC to LHC: Lessons Learned about the QGP II |
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Chair: Ulrich Heinz, The Ohio State University Room: 62 |
Friday, October 28, 2011 2:00PM - 2:12PM |
JA.00001: The QCD Equation of State and Implications for Heavy Ion Collisions Ron Soltz In the chiral limit, QCD exhibits a phase transition at finite temperature. The critical temperature for this transition and its implications for the crossover transition temperature have been calculated on a lattice using several formations of staggered fermion actions and there is now good agreement among them in the continuum limit. However, it is the Equation of State (EoS) that is inserted into hydrodynamic models used to study heavy ion collisions, and for the EoS calculations there remain significant discrepancies. We present the most recent Equation of State calculations from the Highly Improved Staggered Quark action (HISQ) and explore differences with other fermion actions. The lattice calculations will be compared to the EoS for a hadron resonance gas in the temperature region below the transition. Implications for hydrodynamic models of heavy ion collisions will be discussed and quantitative comparisons to experimental data will be presented. [Preview Abstract] |
Friday, October 28, 2011 2:12PM - 2:24PM |
JA.00002: Constraining QGP properties with CHIMERA Irakli Garishvili, Betty Abelev, Michael Cheng, Andrew Glenn, Ron Soltz Understanding essential properties of strongly interacting matter is arguably the most important goal of the relativistic heavy-ion programs both at RHIC and the LHC. In particular, constraining observables such as ratio of shear viscosity to entropy density, $\eta$/s, initial temperature, $T_{init}$, and energy density is of critical importance. For this purpose we have developed CHIMERA, Comprehensive Heavy Ion Model Reporting and Evaluation Algorithm. CHIMERA is designed to facilitate global statistical comparison of results from our multi-stage hydrodynamics/hadron cascade model of heavy ion collisions to the key soft observables (HBT, elliptic flow, spectra) measured at RHIC and the LHC. Within this framework the data representing multiple different measurements from different experiments are compiled into single format. One of the unique features of CHIMERA is, that in addition to taking into account statistical errors, it also treats different types of systematic uncertainties. The hydrodynamics/hadron cascade model used in the framework incorporates different initial state conditions, pre-equilibrium flow, the UVH2+1 viscous hydro model, Cooper-Frye freezeout, and the UrQMD hadronic cascade model. The sensitivity of the observables to the equation of state (EoS) is explored using several EoS's in the hydrodynamic evolution. The latest results from CHIMERA, including data from the LHC, will be presented. [Preview Abstract] |
Friday, October 28, 2011 2:24PM - 2:36PM |
JA.00003: (2+1)-d vs. (3+1)-d viscous hydrodynamics from RHIC and LHC Chun Shen, Bjoern Schenke, Ulrich Heinz Using (2+1)-d viscous hydrodynamics with a state-of-the-art equation of state (s95p-PCE), we present comparisons with recent ALICE measurements of the charged hadron spectra and elliptic flow, as well as successful predictions of the differential elliptic flow coefficient $v_2(p_T)$ for identified pions, kaons and protons from 2.76\,$A$\,GeV Pb+Pb collisions at the Large Hadron Collider (LHC) [1]. We also study how the ``universal'' curves describing the dependence of the eccentricity-scaled charged elliptic flow $v^{\mathrm{ch}}/\bar{\epsilon}$ on the charged multiplicity density per unit area $(1/S)(dN_{\mathrm{ch}}/dy)$ change from RHIC to LHC energies. In (2+1)-d viscous hydrodynamics we find a tendency of producing less $v^{\mathrm{ch}}/\bar{\epsilon}$ at higher collision energies, which contradicts the opposite tendency found by Hirano {\it et al.}[2] for (3+1)-d ideal hydrodynamics coupled to a hadron cascade. By comparing (2+1)-d with (3+1)-d viscous hydrodynamics we explore to what extend these different tendencies may indicate a collision energy dependent gradual breakdown of longitudinal boost-invariance near midrapidity when going from higher to lower collision energies. \\[4pt] [1] C. Shen, U. Heinz, P. Huovinen, and H. Song, arXiv:1105.3226 \\[0pt] [2] T. Hirano, P. Huovinen, Y. Nara, arXiv:1010.6222 [Preview Abstract] |
Friday, October 28, 2011 2:36PM - 2:48PM |
JA.00004: Early Flow from Matching Pre-Equilibrium Dynamics to Viscous Hydrodynamics Daniel White, Ulrich Heinz In the earliest stage of relativistic heavy-ion collisions, pre- equilibrium dynamics can generate collective flow before hydrodynamics becomes applicable. Most applications of hydrodynamics to heavy ion collisions use parametrized initial conditions, but this begs the question whether the parameters used are a realistic reflection of the (largely unknown) pre- equilibrium dynamics. We present an algorithm that allows to match any pre-equilibrium model for the energy-momentum tensor to relativistic viscous fluid dynamics and thus allows to study the effect of different pre-equilibrium scenarios on late-time observables. We explain how our algorithm differs from what was used in [1,2], and that this has important consequences for the starting values of the flow profile and shear stress tensor of the subsequent hydrodynamic evolution. By examining three specific models of early dynamics, we also find that the initial conditions for the viscous hydrodynamic evolution are sensitive to the specific mechanisms governing the pre equilibrium stage. \\[4pt] [1] Vredevoogd et al, Phys. Rev. C 79 (2009) 044915.\\[0pt] [2] Sinyukov et al, Acta Phys. Pol. B 40 (2009) 1109. [Preview Abstract] |
Friday, October 28, 2011 2:48PM - 3:00PM |
JA.00005: Event-by-event viscous hydrodynamics for RHIC and LHC Zhi Qiu, Ulrich Heinz Heavy-ion collisions create deformed quark-gluon plasma (QGP) fireballs which explode anisotropically. The viscosity of the fireball matter determines its ability to convert the initial spatial deformation into momentum anisotropies of the final hadron spectra. The initial deformation fluctuates from event to event. Previous one-shot hydrodynamic calculations dealt with this problem by evolving a single ensemble-averaged smooth initial profile; we here perform event-by-event hydrodynamic simulations and study the spectrum of the initial geometry and final momentum fluctuations in shape and orientation. We study the difference of the resulting spectra and flows between event-by-event and one-shot hydrodynamic calculations, and how these differences are affected by non-zero shear viscosity. We find that the average elliptic flow from an ensemble of individually evolved fluctuating fireballs is smaller than that obtained from a single smooth initial profile with the same average eccentricity. This results in a smaller QGP viscosity extracted from experimental elliptic flow data. We also show results from event-by-event calculations for Pb+Pb collisions at LHC, and compare them with recently measured experimental data. [Preview Abstract] |
Friday, October 28, 2011 3:00PM - 3:12PM |
JA.00006: Triangular flow and nonflow by 2-, 4-, and 6-particle cumulants Li Yi Triangular flow ($v_3$) can arise from event-by-event fluctuations. Its connection to fluctuations in the initial state collision geometry may reveal hydrodynamic information of the collision system. Theoretical studies suggest its sensitivity to hydrodynamic evolution may even be stronger than elliptic flow ($v_2$). We present $v_3$ measurement by the 2-, 4-, and 6-particle cumulant method at $\sqrt{s_{NN}}$ = 200 GeV in Au+Au collisions by STAR. We compare our $v_3$ results to $v_2$, also from the multiparticle cumulant method. The 2- particle cumulant result contains nonflow contribution. We assess the nonflow effect by separating charges as well as applying a pseudo-rapidity gap. The 4-and 6-particle $v_3$ results are strongly affected (perhaps dominated) by $v_3$ fluctuations. Assuming Gaussian flow fluctuation, we further attempt to distinguish flow, flow fluctuation, and nonflow. [Preview Abstract] |
Friday, October 28, 2011 3:12PM - 3:24PM |
JA.00007: Measurement of higher order flow harmonics in PbPb collisions at 2.76 TeV by CMS Yuting Bai Collective flow is an important probe of the earliest stages in the expansion of the hot and dense matter created in relativistic heavy-ion collisions. Higher harmonics of the azimuthal distribution of emitted particles, in particular $v_3$ and $v_4$, complement $v_2$ measurements in elucidating the dynamical evolution of the bulk medium and providing constraints on its transport properties and initial conditions. With its large acceptance and broad rapidity coverage, the CMS detector is ideally suited to provide detailed analyses of higher order harmonic flow at the LHC. Measurements of higher order flow harmonics, $v_n$, in PbPb collisions at 2.76 TeV from the CMS detector will be presented as a function of $p_T$ and centrality. Connections of the results to the properties of the medium will be discussed. [Preview Abstract] |
Friday, October 28, 2011 3:24PM - 3:36PM |
JA.00008: Study of flow factorization with two particle azimuthal correlation Daniel Kikola, Fuqiang Wang Elliptic flow ($v_2$) provides information about initial expansion of the medium created in non-central heavy ion collisions. However non-flow effects, such as jet correlation, can contribute significantly to the measured $v_2$. In this talk we investigate the possibility of separating flow and non-flow components of $v_2$ ($v_n$ in general) measured via two particle azimuthal correlations. If the observed azimuthal anisotropy is due to global flow, then coefficients $v_{n,n}(p_{\rm{T}}^a, p_ {\rm{T}}^b)$ in Fourier decomposition of two particle correlation function $dN/d\Delta \phi$ factorize into product of single particle flow coefficients: $v_{n,n}(p_{\rm{T}}^a, p_{\rm {T}}^b)) = v_n(p_{\rm{T}}^a)v_n(p_{\rm{T}}^b)$. Deviation from $v_{n,n}$ factorization indicates a significant non-flow contribution. We investigate the flow and non-flow contributions to two particle azimuthal correlations with model of heavy ions dynamics which includes particles from hydro medium (with a given anisotropic flow) and jet correlations simulated with Pythia. We discuss the feasibility of separation of flow and nonflow in the real data based on the hypothesis of $v_{n,n}$ factorization for a global flow. [Preview Abstract] |
Friday, October 28, 2011 3:36PM - 3:48PM |
JA.00009: Higher-order flow harmonics: implications for initial-eccentricity models and the ``viscous horizon'' Roy Lacey Flow measurements continue to play an essential role for characterization of the transport properties of the quark gluon plasma produced in heavy ion collisions at both the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC). The scaling properties of recent higher-order flow measurements will be presented and shown to be compatible with the expected growth of viscous damping for sound propagation in the plasma produced in RHIC and LHC collisions. I will also show that these measurements provide a constraint for distinguishing between the two leading eccentricity models, as well as new estimates for the ratio of viscosity to entropy density $\eta/s$, the sound horizon and the ``viscous horizon'' or length-scale which characterizes the highest harmonic that survives viscous damping. [Preview Abstract] |
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