Deuteron and Antideuteron Coalescence in Heavy-Ion Collisions: Energy Dependence of the Formation Geometry

TL;DR

This study explores how the formation and emission geometry of deuterons and antideuterons vary with collision energy in heavy-ion collisions, revealing a transition from bulk to surface emission due to annihilation effects.

Contribution

It provides a novel analysis of energy-dependent emission geometries of nuclei and antinuclei, supported by experimental data and transport model comparisons.

Findings

At low energies, nuclei freeze out over the entire fireball volume.

Antinuclei are mainly emitted from the surface due to annihilation.

Higher energies lead to similar freeze-out distributions for nucleons and antinucleons.

Abstract

We investigate the collision energy dependence of deuteron and antideuteron emission in the RHIC-BES low- to mid-energy range $\sqrt{s_{NN}} = 4.6-200$ GeV where the formation rate of antinuclei compared to nuclei is strongly suppressed. In the coalescence picture, this can be understood as bulk emission for nuclei in contrast to surface emission for antinuclei. By comparison with experimental data on the coalescence parameter $B_2$, we are able to extract the respective source geometries. This interpretation is further supported by results from the UrQMD transport model, and establishes the following picture: At low energies, nucleons freeze out over the total fireball volume, while antinucleons are annihilated inside the nucleon-rich fireball and can only freeze out on its surface. Towards higher energies, this annihilation effect becomes irrelevant (due to the decreasing baryochemical potential) and the system's freeze-out is driven by the mesons. Thus, the nucleon and antinucleon freeze-out distributions become similar with increasing energy.