Using higher moments of fluctuations and their ratios in the search for the QCD critical point

Abstract

The QCD critical point can be found in heavy-ion collision experiments via the nonmonotonic behavior of many fluctuation observables as a function of the collision energy. The event-by-event fluctuations of various particle multiplicities are enhanced in those collisions that freeze-out near the critical point. Higher, non-Gaussian, moments of the event-by-event distributions of such observables are particularly sensitive to critical fluctuations, since their magnitude depends on the critical correlation length to a high power. We present quantitative estimates of the contribution of critical fluctuations to the third and fourth moments of the pion, proton and net proton multiplicities, as well as estimates of various measures of pion-proton correlations, all as a function of the same five nonuniversal parameters, one of which is the correlation length that parametrizes proximity to the critical point. We show how to use nontrivial but parameter-independent ratios among these more than a dozen fluctuation observables to discover the critical point. We also construct ratios that, if the critical point is found, can be used to overconstrain the values of the nonuniversal parameters.