# A Baseline Study of the Event-shape and Multiplicity Dependence of Chemical Freeze-out Parameters in Proton-Proton Collisions at $\sqrt{s}$ = 13 TeV Using PYTHIA8

## TL;DR

This study investigates how event shape and multiplicity influence chemical freeze-out parameters in proton-proton collisions at 13 TeV using PYTHIA8, revealing dependencies and a possible thermodynamic limit at high multiplicities.

## Contribution

It provides a detailed analysis of the dependence of freeze-out parameters on event shape and multiplicity in pp collisions using PYTHIA8, highlighting the approach's potential to understand particle production mechanisms.

## Key findings

- Freeze-out parameters depend on multiplicity and event shape.
- At high multiplicity, freeze-out parameters become ensemble-independent.
- The study suggests a thermodynamic limit at N_ch ≥ 30.

## Abstract

The event-shape and multiplicity dependence of the chemical freeze-out temperature ($T_{\text{ch}}$), freeze-out radius ($R$), and strangeness saturation factor ($\gamma_{s}$) are obtained by studying the particle yields from the PYTHIA8 Monte Carlo event generator in proton-proton (pp) collisions at the centre-of-mass $\sqrt{s}$ = 13 TeV. Spherocity is one of the transverse event-shape techniques to distinguish jetty and isotropic events in high-energy collisions and helps in looking into various observables in a more differential manner. In this study, spherocity~classes are divided into three categories, namely (i) spherocity integrated, (ii) isotropic, and~(iii) jetty. The~chemical freeze-out parameters are extracted using a statistical thermal model as a function of the spherocity class and charged particle multiplicity in the canonical, strangeness canonical, and grand canonical ensembles. A clear observation of the multiplicity and spherocity class dependence of $T_{\text{ch}}$, $R$, and $\gamma_{s}$ is observed. A final state multiplicity, $N_{\rm ch}\geq$ 30 in the forward multiplicity acceptance of the ALICE detector appears to be a thermodynamic limit, where the freeze-out parameters become almost independent of the ensembles. This~study plays an important role in understanding the particle production mechanism in high-multiplicity pp collisions at the Large Hadron Collider (LHC) energies in view of a finite hadronic phase lifetime in small systems.