The NA61/SHINE experiment at CERN SPS searches for the critical point of strongly interacting matter via scanning the phase diagram by changing beam momenta (13A-150A GeV/c) and system size (p + p, p + Pb, Be + Be, Ar + Sc, Xe + La). An observation of local proton-density fluctuations that scale as a power law of the appropriate universality class as a function of phase space bin size would signal the approach of the system to the vicinity of the possible critical point. An investigation of this phenomenon was performed in terms of the second-scaled factorial moments (SSFMs) of proton density in transverse momentum space with subtraction of a noncritical background. New NA61/SHINE preliminary analysis of Ar + Sc data at 150A GeV/c revealed a nontrivial intermittent behavior of proton moments. A similar effect was observed by NA49 in "Si" + Si data at 158A GeV/c. At the same time, no intermittency signal was detected in "C" + C and Pb + Pb events by NA49, as well as in Be + Be collisions by NA61/SHINE. EPOS1.99 also fails to describe the power-law scaling of SSFMs in Ar + Sc. Qualitatively, the effect is more pronounced with the increase of collision-peripherality and proton-purity thresholds, but a quantitative estimate is to be properly done via power-law exponent fit using the bootstrap method and compared to intermittency critical index φ 2 , derived from 3D-Ising effective action.
Recent results from proton intermittency analysis in nucleus-nucleus collisions from NA61 at CERN SPS Nikolaos Davis * for the NA61/SHINE collaboration
The search for experimental signatures of the critical point (CP) of strongly interacting matter [1] is one of the main objectives of the NA61/SHINE experiment at the CERN SPS. In the course of the experiment, an energy (beam momentum 13A -150/158A GeV/c) and system size (p+p, p+Pb, Be+Be, Ar+Sc, Xe+La) scan is performed. Proposed observables include nonmonotonic fluctuations of integrated quantities, as well as local critical fluctuations connected to the critical behavior of the order parameter in the neighborhood of the CP , which scale according to universal power-laws. We investigate proton density fluctuations as a possible order parameter of the phase transition in the neighborhood of the CP. To this end, we perform an intermittency analysis of the proton second scaled factorial moments (SSFMs) in transverse momentum space. A previous analysis of this sort [2] revealed significant power-law fluctuations in the NA49 heavy ion collision experiment for the "Si"+Si system at 158A GeV/c. The fitted power-law exponent was consistent with the theoretically expected critical value, within errors, a result suggesting a baryochemical potential for the critical point in the vicinity of 250 MeV. We now extend the analysis to NA61 systems of similar size, Be+Be and Ar+Sc, at 150A GeV/c. We adapt statistical techniques for the calculation of scaled factorial moments, in order to subtract non-critical background and enhance the signal in cases of low statistics. Our analysis is supplemented by both critical and non-critical Monte Carlo simulations, through which we estimate non-critical background effects on the quality and magnitude of uncertainties of the intermittency power-law fit, as well as explore the possibility of non-critical effects producing an intermittency signal. : 24.10.Lx, 25.75.Gz, 25.75.Nq Critical Point and Onset of Deconfinement PACS
Abstract. The existence of the QCD critical point is a controversial issue, which is expected to be clarified in current ion collision experiments. In the vicinity of the critical point, according to recent theoretical investigations, the chiral condensate (chiral order parameter) should exhibit critical fluctuations (self-similarity and scale-invariance). These fluctuations should also be present in the transverse momentum distributions of protons produced in ion collision experiments due to the coupling of the chiral condensate with the net-proton density. We use intermittency analysis to calculate the 2nd scaled factorial moment (SSFM) of transverse momenta of protons produced around mid-rapidity in central A+A collisions at the NA49 experiment (SPS,CERN). Three different systems are analysed (C+C, Si+Si and Pb+Pb). We develop techniques in order to improve the traditional intermittency approach, estimating the level of non-critical background and correcting for systematic methodological errors that arise in the case of low statistics. We find evidence of powerlaw dependence of the SSFM (intermittency) in the case of Si+Si and Pb+Pb systems. Furthermore, we perform a suitable correlation analysis in order to test and rule out the possibility that the observed behaviour is influenced by misidentified split tracks (spurious correlations) in the analysed data sets.
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