Cumulants and Correlation Functions vs the QCD phase diagram

TL;DR

This paper explores how particle number cumulants and correlation functions relate to the QCD phase diagram, proposing that genuine multi-particle correlations can reveal non-trivial dynamics in heavy-ion collisions.

Contribution

It demonstrates extracting multi-proton correlation functions from experimental data and shows their potential to inform about the QCD phase diagram near the critical point.

Findings

Significant four-proton correlations in STAR data at lower energies

Correlations are long-ranged in rapidity

Signs of correlations can exclude certain phase diagram regions

Abstract

In this paper we discuss the relation of particle number cumulants and correlation functions. It is argued that measuring couplings of the genuine multi-particle correlation functions could provide cleaner information on possible non-trivial dynamics in heavy-ion collisions. We extract integrated multi-proton correlation functions from the presently available experimental data on proton cumulants. We find that the STAR data contain significant four-proton correlations, at least at the lower energies, with indication of changing dynamics in central collisions. We also find that these correlations are rather long-ranged in rapidity. Finally, using the Ising model, we demonstrate how the signs of the multi-proton correlation functions may be used to exclude certain regions of the phase diagram close to the critical point.