On the sign of kurtosis near the QCD critical point

TL;DR

This paper reveals that near the QCD critical point, the kurtosis of order parameter fluctuations becomes universally negative on the crossover side, impacting observable fluctuations like proton multiplicity in heavy-ion collision experiments.

Contribution

It identifies a universal negative kurtosis behavior near the QCD critical point on the crossover side, providing a new diagnostic for experimental searches.

Findings

Kurtosis of order parameter fluctuations is negative near the critical point.

Observable kurtosis can fall below Poisson expectations.

Implications for interpreting fluctuation measurements in heavy-ion collisions.

Abstract

We point out that the quartic cumulant (and kurtosis) of the order parameter fluctuations is universally negative when the critical point is approached on the crossover side of the phase separation line. As a consequence, the kurtosis of a fluctuating observable, such as, e.g., proton multiplicity, may become smaller than the value given by independent Poisson statistics. We discuss implications for the Beam Energy Scan program at RHIC.