Correlated stopping, proton clusters and higher order proton cumulants

Adam Bzdak,Volker Koch,Vladimir Skokov

doi:10.1140/epjc/s10052-017-4847-0

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).

Highlights

The structure of the phase diagram is one of the fundamental problems of the theory of strong interactions, quantum chromodynamics (QCD); a variety of solutions to this problems are pursued in both theoretical and experimental studies
In the lattice QCD (LQCD) calculations, it was demonstrated that the transition between hadrons and quark/gluon degrees of freedom is of crossover type [25]
The twoparticle correlations obtained in our model of independent nucleon stopping together with baryon-number conservation and fast isospin equilibration are of the same magnitude as in the preliminary STAR data

Summary

Introduction

The structure of the phase diagram is one of the fundamental problems of the theory of strong interactions, quantum chromodynamics (QCD); a variety of solutions to this problems are pursued in both theoretical and experimental studies. It was predicted that the higher order cumulants of baryon/charge number fluctuations are very sensitive to the correlation length, ξ , and convey the information about the underlying behaviour of the critical mode [32,33,34,35]. −3 where β, δ and ν are critical exponents of the three-dimensional Ising universality class This sensitivity of the higher order cumulants to the critical dynamics does not come for free: they probe the tails of the probability distribution which are susceptible to various noncritical effects including baryon number conservation [36], volume or number of wounded nucleon fluctuations [37–. We will explore the influence of wounded nucleon [52] fluctuations as well as fluctuations due to multi-collision nucleon stopping, effectively resulting in proton clusters, on the correlation functions at low energies.

Notation

Correlations from fluctuations of the number of wounded nucleons

Monte Carlo calculation

Findings

Discussion and conclusions