Searching for the QCD Critical Point through Fluctuations at RHIC

TL;DR

This paper investigates fluctuations of conserved quantities in heavy-ion collisions at RHIC to search for signs of a QCD critical point, using cumulant measurements and advanced correction techniques to improve reliability.

Contribution

It presents new measurements of cumulants of net-proton distributions at various energies and introduces an unfolding method to correct for detector effects in fluctuation analysis.

Findings

Cumulants up to sixth order were measured at multiple energies.

An unfolding technique was developed to correct for efficiency variations.

Comparison of correction methods guides reliable fluctuation measurements.

Abstract

Fluctuations and correlations of conserved quantities (baryon number, strangeness, and charge) can be used to probe phases of strongly interacting QCD matter and the possible existence of a critical point in the phase diagram. The cumulants of the multiplicity distributions related to these conserved quantities are expected to be sensitive to possible increased fluctuations near a critical point and ratios of the cumulants can be directly compared to the ratios of the susceptibilities from Lattice QCD calculations. In these proceedings, the measurements of the cumulants of net-proton multiplicity distributions from Au+Au collisions at $\sqrt{s_{NN}}$ = 7.7, 11.5, 14.5, 19.6, 27, 39, 62.4 (up to fourth order) and 200 GeV (up to sixth order) as measured by the STAR experiment at RHIC will be presented. Multi-particle correlation functions will also be presented. The measurement of higher-order cumulants are sensitive to experimental artifacts. Current efficiency correction methods are based on the assumption that the tracking efficiency is strictly binomial. An unfolding technique is used to account for multiplicity dependent detector responses and efficiency variations. A large sample of AMPT events will be used to check the validity of the unfolding technique. The comparison of the various correction approaches should provide important guidance towards a reliable experimental determination of the multiplicity cumulants.