Correlated stopping, proton clusters and higher order proton cumulants

TL;DR

This paper explores how nucleon correlations and clustering affect proton cumulants in low-energy heavy-ion collisions, finding clustering can qualitatively match experimental data and proposing further tests.

Contribution

It introduces the role of proton clustering in explaining large proton correlations, beyond fluctuations of wounded nucleons, at low energies.

Findings

Fluctuations of wounded nucleons have a small effect on correlations.

Proton clustering can qualitatively reproduce experimental data.

Proposes measuring mixed multi-particle correlations to test clustering hypotheses.

Abstract

We investigate possible effects of correlations between stopped nucleons on higher order proton cumulants at low energy heavy-ion collisions. We find that fluctuations of the number of wounded nucleons $N_{\mathrm{part}}$ lead to rather nontrivial dependence of the correlations on the centrality; however, this effect is too small to explain the large and positive four-proton correlations found in the preliminary data collected by the STAR collaboration at $\sqrt{s}=7.7$ GeV. We further demonstrate that, by taking into account additional proton clustering, we are able to qualitatively reproduce the preliminary experimental data. We speculate that this clustering may originate either from collective/multi-collision stopping which is expected to be effective at lower energies or from a possible first-order phase transition, or from (attractive) final state interactions. To test these ideas we propose to measure a mixed multi-particle correlation between stopped protons and a produced particle (e.g. pion, antiproton).