Neutron-skin effect in direct-photon and charged hadron-production in Pb+Pb collisions at the LHC

TL;DR

This paper investigates how the neutron-skin effect influences high-$p_T$ particle production in Pb+Pb collisions at the LHC, proposing that charge-hadron ratios could better reveal this effect despite challenges with direct photons.

Contribution

It introduces a method to detect the neutron-skin effect using charge-hadron ratios, which cancel nuclear PDF effects and are sensitive to neutron distribution differences.

Findings

Charge-hadron ratios can reveal neutron-skin effects up to 20% in peripheral collisions.

Direct-photon measurements are less effective due to spatial dependence of nuclear PDFs.

Neutron-skin effects are challenging to observe with direct photons but accessible via charge-hadron ratios.

Abstract

A well-established observation in nuclear physics is that in neutron-rich spherical nuclei the distribution of neutrons extends farther than the distribution of protons. In this work, we scrutinize the influence of this so called neutron-skin effect on the centrality dependence of high-$p_{\rm T}$ direct-photon and charged-hadron production. We find that due to the estimated spatial dependence of the nuclear parton distribution functions, it will be demanding to unambiguously expose the neutron-skin effect with direct photons. However, when taking a ratio between the cross sections for negatively and positively charged high-$p_{\rm T}$ hadrons, even centrality-dependent nuclear-PDF effects cancel, making this observable a better handle on the neutron skin. Up to 20~\% effects can be expected for the most peripheral collisions.