Auvinen, J.

Bleicher, M.

Elfner, Hannah

Steinheimer, J.

Stöcker, H.

GSI Repository

Directed flow as a phase transition signal in relativistic heavy ion collisions ∗J. Steinheimer1,2, J. Auvinen†1, H. Petersen1,2,3, M. Bleicher1,2, and H. Sto¨cker1,2,31Frankfurt Institute for Advanced Studies, Frankfurt, Germany; 2Institut fu¨r Theoretische Physik, Goethe Universita¨tFrankfurt, Frankfurt, Germany; 3GSI Helmholtzzentrum fu¨r Schwerionenforschung GmbH, Darmstadt, GermanyRecent STAR data [1] from the RHIC beam energy scanshow that the slope of the net-proton directed flow v1 nearmidrapidity y = 0 changes sign twice within the colli-sion energy range √sNN = 7.7 - 39 GeV, which has beenpredicted by the earlier fluid calculations to be a signal ofa first-order phase transition between hadronic matter andquark-gluon plasma. We study this phenomenon utilizinga hybrid model where the non-equilibrium phases at thebeginning and in the end of a heavy-ion collision are de-scribed by the UrQMD transport model, while the interme-diate hot and dense stage is modeled with (3+1)-D idealhydrodynamics. In [2] we examine the sensitivity of thedirected flow v1 to the order of the phase transition by com-paring simulations with a first-order phase transition ”Bagmodel” equation of state (EoS) to calculations with a chiralmodel EoS, which has a cross-over phase transition.First, to emulate the earlier fluid calculations, we initial-ize the cold matter of two colliding nuclei as two distribu-tions of energy and baryon density. The UrQMD modelis still utilized for the final hadron gas phase. Figures 1aand 1b show the difference in dv1/dy between the twoequations of state for Au+Au collisions at impact param-eter b = 8 fm. While the predicted minimum in dv1/dywith a first-order phase transition is observed when usingisochronous hypersurface for the transition from hydrody-namics to transport (Fig. 1a), the difference between thetwo equations of state is considerably smaller when usingiso-energy density condition for fluid to particle transition(Fig. 1b).Figure 1c shows the energy dependence of the pro-ton and antiproton v1 slope at midrapidity, obtained fromthe full hybrid simulation with the initial non-equilibriumtransport phase. The directed flow was calculated usingevents with impact parameter b = 4.6− 9.4 fm, which ap-proximates the (10 − 40)% centrality range of the STARdata. The two EoS are completely indistinguishable in thehybrid simulations, questioning the usability of v1 as a sig-nal of the first-order phase transition. However, the hybridmodel results also deviate notably from experimental data,which makes further studies on the topic necessary.References[1] L. Adamczyk et al. [STAR Collaboration], “Beam-EnergyDependence of the Directed Flow of Protons, Antiprotons,∗Work supported by HIC for FAIR and Helmholtz-NachwuchsgruppeVH-NG-822. Computational resources have been provided by the Centerfor Scientific Computing (CSC) at Goethe Universita¨t Frankfurt.† auvinen@fias.uni-frankfurt.deFigure 1: a) and b) Slope of proton v1 at midrapidity|y| < 0.5, calculated for an equation of state with first-order phase transition (black) and cross-over phase transi-tion (red) for isochronous fluid-to-particles transition hy-persurface (a) and iso-energy density hypersurface (b) forAu+Au collisions at impact parameter b = 8 fm. c) v1 slopeat midrapidity for protons (solid symbols) and anti-protons(open symbols) for impact parameter range b = 4.6 − 9.4fm, extracted from the hybrid model calculations with afirst order (black) and crossover EoS (red). Results arecompared with the UrQMD model calculations (grey) andexperimental data from STAR, NA49 and E895 collabora-tions (green). Originally published in [2].and Pions in Au+Au Collisions”, Phys. Rev. Lett. 112 (2014)162301[2] J. Steinheimer, J. Auvinen, H. Petersen, M. Bleicher, H.Sto¨cker, “Examination of directed flow as a signal for a phasetransition in relativistic nuclear collisions”, Phys. Rev. C 89(2014) 054913GSI SCIENTIFIC REPORT 2014 MU-THEORY-25DOI:10.15120/GR-2015-1-MU-THEORY-25 361

Directed flow as a phase transition signal in relativistic heavy ion collisions

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Directed flow as a phase transition signal in relativistic heavy ion collisions

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