On nuclear coalescence in small interacting systems

M Kachelrieß,J Tjemsland,S Ostapchenko

doi:10.1140/epja/s10050-021-00469-w

M Kachelrieß, J Tjemsland + Show 1 more

Open Access

https://doi.org/10.1140/epja/s10050-021-00469-w

Copy DOI

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.

Highlights

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
Annihilations or pp collisions, the production of light nuclei is usually described by the coalescence model in momentum space [6,7,8], where nucleons originating from a particle collision merge to form a nucleus if their invariant momentum difference is smaller than the coalescence momentum p0

Summary

Introduction

Annihilations or pp collisions, the production of light nuclei is usually described by the coalescence model in momentum space [6,7,8], where nucleons originating from a particle collision merge to form a nucleus if their invariant momentum difference is smaller than the coalescence momentum p0. We will use this model to describe the production of hadrons and nuclei in high energy pp collisions and compare it to recent experimental data by the ALICE collaboration on the size of the baryon emitting source [48] and on the multiplicity and transverse momentum dependence of the coalescence factor B2 [45,49,50] Both data sets have been interpreted as evidence of collective flows, but we will show that the same characteristics are described using QCD inspired event generators, like QGSJET II [51,52] and Pythia 8.2 [53,54].

The quantum mechanics of coalescence and the WiFunC model

Relation to the femtoscopy framework

Size of baryon-emitting source

Multiplicity dependence of coalescence in small interacting systems

Astrophysical applications

Conclusions