Thermodynamical string fragmentation and string density effects in jets

TL;DR

This study investigates how different string fragmentation models in PYTHIA~8 affect jet particle composition and potential collective effects, revealing hints of baryon enhancement and flow-like phenomena in proton-proton collisions.

Contribution

It compares thermodynamical and Lund string fragmentation models in PYTHIA~8, highlighting their impact on baryon production and collective-like effects in jets without MPI.

Findings

Thermodynamical string fragmentation predicts baryon enhancement in jets.

Light-flavor baryon-to-meson ratios show flow-like patterns across models.

Heavy-flavor baryon ratios increase with charged-constituent multiplicity.

Abstract

It has been proposed to search for thermal and collective properties arising from parton-fragmentation processes by examining high jet charged-constituent multiplicities ($N_{\rm j,ch}$) in proton-proton (pp) collisions. This proposal was initially tested using the PYTHIA~8 event generator with the Monash tune, which incorporates multiparton interactions (MPI) and the MPI-based colour reconnection (CR) model. These studies did not reveal any conclusive evidence for the presence of radial flow. In this paper, we expand upon the proposed Monte Carlo study by eliminating selection biases associated with triggering on charged particle multiplicities. Furthermore, MPI are disabled to focus exclusively on jet fragments. We analyse pp collisions at $\sqrt{s}$=13 TeV simulated with PYTHIA~8, exploring different implementations of the generator: thermodynamical string fragmentation and the standard Lund fragmentation model. The impact of colour-string juctions was also explored. Surprisingly, the thermodynamical string fragmentation together with the close-packing of strings mechanism predicts a hint of baryon enhancement in jets. Additionally, the light-flavor baryon-to-meson ratios as a function of $j_{\rm T}$ exhibit similarities across all PYTHIA~8 implementations, and hint at radial flow-like effects. In contrast, the ratio of heavy-flavor hadrons (\LcToDz) at low $j_{\rm T}$ as a function of $N_{\rm j,ch}$ shows a trend similar to that observed as a function of charged-particle multiplicity in minimum-bias data, suggesting that colour-string junctions may play a crucial role in heavy baryon production in jets. The same mechanism also predicts a $j_{\rm T}$-integrated \LcToDz ratio that increases with increasing $N_{\rm j,ch}$.