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 or charged-particle transverse momentum (${p}_{\mathrm{T}}^{\text{jet}}$ or ${p}_{\mathrm{T}}^{\text{leading}}$) in terms of number and summed-${p}_{\mathrm{T}}$ densities of charged particles in the azimuthal region transverse to the leading jet or particle direction. Contrary to inelastic $pp$ LHC data, the UE observables normalized to the charged-particle density obey an approximate Koba-Nielsen-Olesen (KNO) scaling. Based on PYTHIA 8.2 simulations of $pp$ collisions at LHC energies, we show that the small breaking of the KNO scaling is due to the increasing importance of multiple partonic interactions (MPI) at higher $\sqrt{s}$. This in turn makes that with increasing energy, the ${p}_{\mathrm{T}}$ spectra in the UE get harder than in inelastic $pp$ collisions. Color reconnection (CR) models the interactions among outgoing partons just before the hadronization; therefore, it modifies the ${p}_{\mathrm{T}}$-spectral shape. Motivated by this, we studied the UE activity considering charged particles within different ${p}_{\mathrm{T}}$ intervals in $pp$ collisions at $\sqrt{s}=7\text{ }\text{ }\mathrm{TeV}$. Although MPI saturate for ${p}_{\mathrm{T}}^{\text{jet}}>10\text{ }\text{ }\mathrm{GeV}/c$, the UE still increases with increasing ${p}_{\mathrm{T}}^{\text{jet}}$. We demonstrate that the saturation of both number and summed-${p}_{\mathrm{T}}$ densities, which are commonly claimed, are only observed for low-${p}_{\mathrm{T}}$ charged particles ($0.5<{p}_{\mathrm{T}}<2\text{ }\text{ }\mathrm{GeV}/c$). Moreover, for the ${p}_{\mathrm{T}}$-integrated case (${p}_{\mathrm{T}}>0.5\text{ }\text{ }\mathrm{GeV}/c$), the summed-${p}_{\mathrm{T}}$ density is not sensitive to the variation of CR; however, at low-${p}_{\mathrm{T}}$, it is reduced with increasing CR, whereas an opposite behavior is found at intermediate ${p}_{\mathrm{T}}$ ($2<{p}_{\mathrm{T}}<10\text{ }\text{ }\mathrm{GeV}/c$). Finally, we show that CR produces flowlike behavior only in the UE region, and the effects are reduced with increasing ${p}_{\mathrm{T}}^{\text{jet}}$ due to the hardening 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.