Probing color reconnection with underlying event observables at the LHC energies

TL;DR

This study investigates underlying event activity and color reconnection effects in proton-proton collisions at the LHC, revealing how these phenomena depend on particle transverse momentum and jet energy, with implications for understanding flow-like behaviors.

Contribution

It provides a detailed analysis of how color reconnection influences underlying event observables across different particle transverse momentum ranges using PYTHIA simulations.

Findings

UE activity normalizes to INEL multiplicity obeys KNO scaling

Color reconnection reduces low-pT UE activity and enhances intermediate-pT activity

CR induces flow-like behavior in UE region, diminishing at higher jet energies

Abstract

In this work we study the underlying event (UE) activity as a function of the highest jet transverse momentum ($p_{\rm T}^{\rm jet}$) in terms of the number and summed $p_{\rm T}$ densities of charged particles in the azimuthal region transverse to the $p_{\rm T}^{\rm jet}$ direction. The UE observables normalised to the INEL multiplicity obey an approximate Koba-Nielsen-Olesen (KNO) scaling. Based on PYTHIA~8.2 simulations of $pp$ collisions at LHC energies, we show that the remaining imperfection of the KNO scaling is due to the increasing importance of multiple partonic interactions (MPI) at higher $\sqrt{s}$. Motivated by this, we studied the UE activity considering charged particles within different $p_{\rm T}$ intervals in $pp$ collisions at $\sqrt{s}=7$\,TeV. We demonstrate that the saturations of both number and summed $p_{\rm T}$ densities, which are commonly claimed, are only observed for low-\pt charged particles ($0.5<p_{\rm T}<2$\,GeV/$c$). Moreover, for the $p_{\rm T}$-integrated case ($p_{\rm T}>0.5$\,GeV/$c$) the summed $p_{\rm T}$ density is not sensitive to the variation of color reconnection (CR), however at low-p_{\rm T} it is reduced with increasing CR, whereas an opposite behaviour is found at intermediate-$p_{\rm T}$ ($2<p_{\rm T}<10$\,GeV/$c$). Finally, we show that CR produces flow-like behaviour only in the UE region and the effects are reduced with increasing $p_{\rm T}^{\rm jet}$ due to the hardering of UE. The outcomes encourage the measurement of inclusive and identified charged particle $p_{\rm T}$ spectra (over a wide range of $p_{\rm T}$) associated to UE aimed at better understanding the similarities between $pp$ and heavy-ion data discovered at the LHC.