On nuclear coalescence in small interacting systems

TL;DR

This paper presents a coalescence model incorporating both momentum correlations and emission volume size, explaining light nuclei formation in small systems and aligning with recent LHC data.

Contribution

It introduces a per-event coalescence model based on the Wigner function that accounts for both effects, improving understanding of small system nuclear production.

Findings

Model reproduces source size for baryon emission.

Accurately predicts coalescence factor B2 in pp collisions.

Explains collective behavior signals in small systems.

Abstract

The formation of light nuclei can be described as the coalescence of clusters of nucleons into nuclei. In the case of small interacting systems, such as dark matter and $e^+e^-$ annihilations or $pp$ collisions, the coalescence condition is often imposed only in momentum space and hence the size of the interaction region is neglected. On the other hand, in most coalescence models used for heavy ion collisions, the coalescence probability is controlled mainly by the size of the interaction region, while two-nucleon momentum correlations are either neglected or treated as collective flow. Recent experimental data from $pp$ collisions at LHC have been interpreted as evidence for such collective behaviour, even in small interacting systems. We argue that these data are naturally explained in the framework of conventional QCD inspired event generators when both two-nucleon momentum correlations and the size of the hadronic emission volume are taken into account. To include both effects, we employ a per-event coalescence model based on the Wigner function representation of the produced nuclei states. This model reproduces well the source size for baryon emission and the coalescence factor $B_2$ measured recently by the ALICE collaboration in $pp$ collisions.