Search for Critical Behaviour of Strongly Interacting Matter at the CERN Super Proton Synchrotron

Abstract

History, status and plans of the search for critical behavior of strongly interacting matter created in nucleus-nucleus collisions at the CERN Super Proton Synchrotron is reviewed. In particular, it is expected that the search should answer the question whether the critical point of strongly interacting matter exists and, if it does, where it is located.First, the search strategies are presented and a short introduction is given to expected fluctuation signals and to the quantities used by experiments to detect them. The most important background effects are also discussed.Second, relevant experimental results are summarized and discussed. It is intriguing that both the fluctuations of quantities integrated over the full experimental acceptance (event multiplicity and transverse momentum) as well as the bin size dependence of the second factorial moment of pion and proton multiplicities in medium-sized Si+Si collisions at 158A GeV/c suggest critical behaviour of the created matter.These results provide strong motivation for the ongoing systematic scan of the phase diagram by the NA61/SHINE experiment at the SPS and the continuing search at the Brookhaven Relativistic Hadron Collider.

Equations of state for real gases on the nuclear scale

“…Refs. [58][59][60]), in particular, in the context of the search for the QCD critical point [61,62]. The higher-order (non-Gaussian) measures of the fluctuations of conserved charges were suggested to study the QCD phase structure.…”

Traces of the nuclear liquid-gas phase transition in the analytic properties of hot QCD

“…The nature of this transition at finite densities is not established yet. The experimental search for the hypothetical QCD chiral critical point (CP) [2] is performed at non-zero intermediate baryon densities using measurements of fluctuations in heavy-ion collisions [3][4][5][6] as well as indirect lattice gauge theory methods, such as a Taylor expansion around µ B = 0 [7,8] or analytic continuation from imaginary µ B [9,10]. Current high quality lattice QCD data at physical quark masses show no evidence or signatures of a chiral CP and disfavor the existence of a phase transition of first or second order at moderate baryon densities µ B /T π [11][12][13][14].…”

Fluctuations and correlation from NA61/SHINE

“…In the search for the critical point and the study of the onset of deconfinement these are unwanted effects [3]. However, the onset of fireball was discovered thanks to the sensitivity of fluctuations to the conservation laws [2].…”