Searching for the critical point of strongly interacting matter in nucleus-nucleus collisions at CERN SPS

Abstract

One of the primary goals of the NA61/SHINE experiment at CERN SPS is the detection of the critical point (CP) of strongly interacting matter. In the interests of this search, an energy (beam momentum 13A - 150$A$ GeV/$c$) and system size (p+p, p+Pb, Be+Be, Ar+Sc, Xe+La) scan is being performed. A number of observables are being considered as possible signatures of the CP. Among these, local fluctuations of the proton density are especially suited for the task, being connected to the critical behavior of the chiral phase transition order parameter in the neighborhood of the CP. In particular, proton density fluctuations are probed by means of an intermittency analysis of the proton second scaled factorial moments (SSFMs) in transverse momentum space, expected by universality theory to scale as a power-law in the vicinity of the CP. A previous analysis of this sort probed a number of NA49 heavy ion collisions of different size [1]; significant power-law fluctuations were observed in "Si"+Si collisions at 158$A$ GeV/$c$, with a power-law exponent consistent with the theoretically expected critical value, within uncertainties. Recently, NA61/SHINE Be+Be collisions at 150$A$ GeV/$c$ were similarly probed, yielding a negative result. We now extend the analysis to NA61/SHINE Ar+Sc collisions at 150$A$ GeV/$c$. The system size and freeze-out baryochemical potential are similar to NA49 "Si"+Si, and preliminary analysis suggests the presence of intermittency. We employ statistical techniques in order to subtract non-critical background present in factorial moments and enhance the signal in cases of low statistics. Through combined use of critical and background Monte Carlo simulations, we assess the quality and statistical significance of the observed intermittency effect. [1] T. Anticic $\textit{et al}$, Eur. Phys. J. C $\textbf{75}$: 587 (2015).