Universality of the underlying event in pp collisions

TL;DR

This study analyzes ATLAS data on the underlying event in proton-proton collisions at various energies, revealing universal scaling behaviors and KNO scaling, consistent with PYTHIA simulations, indicating underlying event universality at high energies.

Contribution

It demonstrates the energy independence of underlying event properties when scaled by multiplicity, supporting the universality hypothesis in high-energy hadronic interactions.

Findings

Underlying event particle production scales with multiplicity change.

No significant energy dependence in scaled particle production.

Underlying event multiplicity distributions exhibit KNO scaling.

Abstract

In this paper we study ATLAS results on underlying event in pp collisions at $\sqrt{s}$=0.9, 7 and 13 TeV. We show that the center-of-mass energy dependences of the charged-particle production sensitive to the underlying event ("transverse" region) and to the hardest partonic interaction ("towards" and "away" regions) in pp collisions can be both understood in terms of the change of the inclusive average multiplicity. Within uncertainties, the corresponding particle production as a function of the leading charged particle shows no significant $\sqrt{s}$-dependence for the three regions once they are scaled according to the relative change in multiplicity. The scaling properties reported here are well reproduced by PYTHIA 8.212 tune Monash 2013 and suggest an universality of the underlying event in hadronic interactions at high $\sqrt{s}$. Based on the simulations, we observed that the same scaling properties are also present in the average number of multi-partonic interactions as a function of the leading charged particle. Moreover, the multiplicity distributions associated to the underlying event exhibit a KNO scaling.