Searching for the QCD critical point via the rapidity dependence of cumulants

TL;DR

This paper proposes that analyzing the rapidity dependence of cumulants in heavy ion collisions can help identify the QCD critical point, especially with upcoming detector upgrades at RHIC.

Contribution

It introduces a method combining Ising universality and a freezeout model to predict distinctive rapidity-dependent signatures of the critical point in cumulant observables.

Findings

Rapidity dependence of kurtosis changes qualitatively near the critical point.

The method enhances the ability to detect the critical point in beam energy scans.

Distinctive rapidity-dependent signatures can be used to identify the critical point.

Abstract

The search for a possible critical point in the QCD phase diagram is ongoing in heavy ion collision experiments at RHIC which scan the phase diagram by scanning the beam energy; a coming upgrade will increase the luminosity and extend the rapidity acceptance of the STAR detector. In fireballs produced in RHIC collisions, the baryon density depends on rapidity. By employing Ising universality together with a phenomenologically motivated freezeout prescription, we show that the resulting rapidity dependence of cumulant observables sensitive to critical fluctuations is distinctive. The dependence of the kurtosis (of the event-by-event distribution of the number of protons) on rapidity near mid-rapidity will change qualitatively if a critical point is passed in the scan. Hence, measuring the rapidity dependence of cumulant observables can enhance the prospect of discovering a critical point, in particular if it lies between two energies in the beam energy scan.