Probing color reconnection with underlying event observables at the LHC energies

Abstract

In this work we study the underlying event (UE) activity as a function of the highest jet transverse momentum ($p_{\mathrm{T}}^{\mathrm{jet}}$) in terms of the number and summed $p_{\mathrm{T}}$ densities of charged particles in the azimuthal region transverse to the $p_{\mathrm{T}}^{\mathrm{jet}}$ 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 $p_{\mathrm{T}}$ intervals in $pp$ collisions at $\sqrt{s}=7$\,TeV. We demonstrate that the saturations of both number and summed $p_{\mathrm{T}}$ densities, which are commonly claimed, are only observed for low-\pt charged particles ($0.5<p_{\mathrm{T}}<2$\,GeV/$c$). Moreover, for the $p_{\mathrm{T}}$-integrated case ($p_{\mathrm{T}}>0.5$\,GeV/$c$) the summed $p_{\mathrm{T}}$ 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-$p_{\mathrm{T}}$ ($2<p_{\mathrm{T}}<10$\,GeV/$c$). Finally, we show that CR produces flow-like behaviour only in the UE region and the effects are reduced with increasing $p_{\mathrm{T}}^{\mathrm{jet}}$ due to the hardering of UE. The outcomes encourage the measurement of inclusive and identified charged particle $p_{\mathrm{T}}$ spectra (over a wide range of $p_{\mathrm{T}}$) associated to UE aimed at better understanding the similarities between $pp$ and heavy-ion data discovered at the LHC.

arXiv commentAccepted for publication in Physical Review D

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