Freeze-out conditions from net-proton and net-charge fluctuations at RHIC

Paolo Alba,Wanda Alberico,Rene Bellwied,Marcus Bluhm,Valentina Mantovani Sarti,Marlene Nahrgang,Claudia Ratti

doi:10.1016/j.physletb.2014.09.052

Abstract

We calculate ratios of higher-order susceptibilities quantifying fluctuations in the number of net-protons and in the net-electric charge using the Hadron Resonance Gas (HRG) model. We take into account the effect of resonance decays, the kinematic acceptance cuts in rapidity, pseudo-rapidity and transverse momentum used in the experimental analysis, as well as a randomization of the isospin of nucleons in the hadronic phase. By comparing these results to the latest experimental data from the STAR Collaboration, we determine the freeze-out conditions from net-electric charge and net-proton distributions and discuss their consistency.

Highlights

Significant theoretical activity, aimed at understanding the properties of matter under extreme conditions, has been triggered recently by the heavy-ion collision experiments conducted at RHIC and the LHC, in which the deconfined phase of QCD matter, the Quark–Gluon Plasma, is created
As a consequence of the increasing precision achieved in the numerical simulations, it is becoming possible to extract the chemical freeze-out parameters from first principles, by comparing the measured fluctuation observables to corresponding susceptibility ratios calculated in lattice QCD [13,14,15,16,17]
We find that it is possible to extract, for each collision energy, a freeze-out temperature and baryo-chemical potential, which allow to simultaneously reproduce the ratios of the lowest-order susceptibilities for net-protons and n√et-electric charge

Summary

Introduction

Significant theoretical activity, aimed at understanding the properties of matter under extreme conditions, has been triggered recently by the heavy-ion collision experiments conducted at RHIC and the LHC, in which the deconfined phase of QCD matter, the Quark–Gluon Plasma, is created. Event-by-event fluctuations of the net-electric charge and netbaryon number, which are conserved charges of QCD, are expected to become large near a critical point [3,4]: for this reason, they have been proposed as ideal observables to verify its existence and to determine its position in the QCD phase diagram [5,6,7,8] Experimental results for these measures were recently reported for several collision energies [9,10,11,12]. The model allows to expand the range of μB -values and to calculate ratios of higher order susceptibilities at finite μB , as well as to implement kinematic acceptance cuts in rapidity y, pseudo-rapidity η and transverse momentum pT , providing a valuable tool to extract the freeze-out conditions from the experimental data.

The HRG in partial chemical equilibrium

Results

Conclusions