Ratios of Fluctuation Observables in the Search for the QCD Critical Point

TL;DR

This paper investigates how ratios of fluctuation observables in heavy ion collisions can signal the QCD critical point by analyzing higher moments of particle distributions and proposing parameter-independent ratios for detection.

Contribution

It introduces a method to use ratios of fluctuation observables that are independent of non-universal parameters to identify the QCD critical point.

Findings

Critical fluctuation contributions to third and fourth moments are quantified.

Parameter-independent ratios can signal the critical point.

Ratios can overconstrain non-universal parameters if the critical point is found.

Abstract

The QCD critical point can be found in heavy ion collision experiments via the non-monotonic 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 and proton, as well as estimates of various measures of pion-proton correlations, all as a function of the same five non-universal parameters. 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 non-universal parameters.