Impact of the Coulomb field on charged-pion spectra in few-GeV heavy-ion collisions

TL;DR

This paper investigates how the Coulomb field influences charged pion spectra and correlations in heavy-ion collisions, revealing the importance of spectator effects and the connection to freeze-out conditions.

Contribution

It introduces a Coulomb-modified Boltzmann model that accounts for dynamic Coulomb effects and spectator contributions in analyzing pion spectra and correlations.

Findings

Low-mt pion spectra are well described by Coulomb-modified distributions.

Centrality dependence of Coulomb potential deviates from simple scaling, highlighting spectator effects.

Coulomb modifications of HBT radii align with potentials from single-pion spectra.

Abstract

In nuclear collisions the incident protons generate a Coulomb field which acts on produced charged particles. The impact of these interactions on charged pion transverse-mass and rapidity spectra, as well as on pion-pion momentum correlations is investigated in Au+Au collisions at $\sqrt{s_{NN}}$ = 2.4 GeV. We show that the low-mt part of the data ($m_t < 0.2$ GeV/c$^2$) can be well described with a Coulomb-modified Boltzmann distribution that also takes changes of the Coulomb field during the expansion of the fireball into account. The observed centrality dependence of the fitted mean Coulomb potential deviates strongly from a $A_{part}^{2/3}$ scaling, indicating that, next to the fireball, the non-interacting charged spectators have to be taken into account. For the most central collisions, the Coulomb modifications of the HBT source radii are found to be consistent with the potential extracted from the single-pion transverse-mass distributions. This finding suggests that the region of homogeneity obtained from two-pion correlations coincides with the region in which the pions freeze-out. Using the inferred mean-square radius of the charge distribution at freeze-out, we have deduced a baryon density, in fair agreement with values obtained from statistical hadronization model fits to the particle yields.