Heavy Ion Tea

Search for the QCD Critical Point Through Kurtosis of
Event-by-Event Proton Multiplicity Distributions in STAR Experiment at RHIC
Bedanga Mohanty (VECC)

Abstract: Several experiments at RHIC, SPS and FAIR have planned to look for the existence of QCD phase boundary and the possible critical point by colliding heavy ions at varying incident beam energies. The characteristic signature of existence of a critical point is increase and divergence of fluctuations [1, 2]. For better sensitivity, it has been proposed to study higher moments (Kurtosis of net-baryons, for example) of event-by-event proton multiplicities [3] compared to the lower moments. The kurtosis (fourth moment) of these multiplicity distribution is expected to be proportional to seventh power of correlation length whereas the second moment is proportional to square of correlation length [3]. Further in Lattice calculations, which assumes the system to be in thermal equilibrium, the kurtosis of event-by-event net-proton is related to baryon susceptibilities. These susceptibilities shows large values or diverge at critical temperature in presence of QCD critical point [1, 4]. The measuring higher moments of event-by-event identified particle multiplicity distributions will provide the first direct connection between experimental observables and Lattice Gauge Theory calculations [4]. In this talk, we present the first results on energy and centrality dependence of fourth moments of event-by-event multiplicity distributions of protons and net-protons as measured in the STAR experiments Time Projection Chamber at RHIC. Results based on a systematic study of various moments of cleanly identified proton ($\bar{p}) multiplicity distribution measured at mid-rapidity (|y| < 0.5, pT<=0.8 GeV/c) from Au+Au and Cu+Cu collisions at $\sqrt{s_{NN}}$ = 62.4 and 200 GeV will be discussed. The benefits from the completion of STAR Time-Of-Flight in 2010, towards particle identification (pions and kaons) for analysis related to isospin susceptibilities and strangeness susceptibilities will be also briefly discussed. -------------- References : -------------- [1] V. Koch, arXiv:0810.2520 [nucl-th] [2] M. A. Stephanov, arXiv:0809.3450 [hep-ph] [3] M. Cheng et al., arXiv:0811.1006