Deciphering QCD dynamics in small collision systems using event shape and final state multiplicity at the Large Hadron Collider

TL;DR

This study uses event shape and multiplicity data, simulated with PYTHIA8, to explore the thermodynamics and QCD dynamics in high-multiplicity proton-proton collisions at the LHC, revealing thresholds for particle production and thermalization.

Contribution

It introduces an event topology and multiplicity dependent thermodynamic analysis of pp collisions at 13 TeV, highlighting thresholds for particle production and equilibrium formation.

Findings

Identification of a multiplicity threshold (~10-20) for light hadron production.

Observation of a higher multiplicity threshold (~40) for heavier hadron equilibrium.

Dependence of thermodynamic observables on event shape and multiplicity.

Abstract

The high-multiplicity pp collisions at the Large Hadron Collider energies with various heavy-ion-like signatures have warranted a deeper understanding of the underlying physics and particle production mechanisms. It is a common practice to use experimental data on the hadronic transverse momentum ($p_T$) spectra to extract thermodynamical properties of the system formed in heavy ion and high multiplicity pp collisions. The non-availability of event topology dependent experimental data for pp collisions at $\sqrt{s}$ = 13 TeV on the spectra of non-strange and strange hadrons constrains us to use the PYTHIA8 simulated numbers to extract temperature-like parameters to study the event shape and multiplicity dependence of specific heat capacity, conformal symmetry breaking measure (CSBM) and speed of sound. The observables show a clear dependence on event multiplicity and event topology. Thermodynamics of the system is largely governed by the light particles because of their relatively larger abundances. In this regards, a threshold in the particle production, $\rm N_{ch} \simeq$ (10-20) in the final state multiplicity emerges out from the present study, confirming some of the earlier findings in this direction. As for heavier hadrons with relatively small abundances, a similar threshold is observed for $\langle \rm N_{ch} \rangle \simeq$ 40 hinting towards formation of a thermal bath where all the heavier hadrons are in equilibrium.