• Accepted Paper

Probing neutron skin through event-by-event pion asymmetry in heavy-ion collisions

Xu-Hua Tian and Long-Gang Pang

Phys. Rev. C - Accepted 9 October, 2026

DOI: https://doi.org/10.1103/bw3p-lpg6

Abstract

In this work, we propose a novel approach for probing the neutron skin thickness of gold (Au) by analyzing the event-by-event distribution of π− and π+ yield differences. This is achieved through SMASH simulations of ultra-peripheral Au+Au collisions at $\sqrt{s_{\rm NN}}=3$ GeV. Our results demonstrate that the mean value of Δnπ=nπ−−nπ+, along with the Pearson correlation and mutual information between (π−+π+) and (π−−π+), all scale linearly with the neutron skin thickness. Moreover, the slope of the line connecting two distinct Δnπ values in the event-by-event distribution also exhibits a linear dependence on the neutron skin thickness. The most sensitive Δnπ pairs are identified as (−2,−1), (−1,1), (0,1), and (0,2). These findings establish a new pathway for determining the neutron skin thickness. Finally, by comparing SMASH, JAM, and UrQMD simulations under identical initial conditions, we observed a certain degree of model dependence in the outcomes, yet the overall trends remained consistent. However, by extracting slopes from multiple Δnπ pairs in experimental event-by-event data and inferring the corresponding neutron skin thickness, one can assess which model better aligns with the true physical value.

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