Abstract
A measurement of the underlying event (UE) activity in proton-proton collisions at a center-of-mass energy of 7 TeV is performed using Drell--Yan events in a data sample corresponding to an integrated luminosity of 2.2 inverse femtobarns, collected by the CMS experiment at the LHC. The activity measured in the muonic final state (q q-bar to opposite-sign muons) is corrected to the particle level and compared with the predictions of various Monte Carlo generators and hadronization models. The dependence of the UE activity on the dimuon invariant mass is well described by PYTHIA and HERWIG++ tunes derived from the leading jet/track approach, illustrating the universality of the UE activity. The UE activity is observed to be independent of the dimuon invariant mass in the region above 40 GeV, while a slow increase is observed with increasing transverse momentum of the dimuon system. The dependence of the UE activity on the transverse momentum of the dimuon system is accurately described by MADGRAPH, which simulates multiple hard emissions.
Highlights
In hadron–hadron scattering, the “underlying event” (UE) is defined as any hadronic activity that cannot be attributed to the particles originating from the hard scattering, which is characterized by a large momentum transfer, or to the hadronization of initial- and final-state radiation
In this paper we investigate some aspects of the UE modeling in detail by measuring the invariant mass dependence of the UE activity for DY events with small transverse momentum of the DY system
The UE activity in the transverse region is compared with that measured in hadronic events using a leading trackjet
Summary
In hadron–hadron scattering, the “underlying event” (UE) is defined as any hadronic activity that cannot be attributed to the particles originating from the hard scattering, which is characterized by a large momentum transfer, or to the hadronization of initial- and final-state radiation. The UE activity is due to the hadronization of partonic constituents, not involved in the hard scattering, that have undergone multiple-parton interactions (MPIs) and to the hadronization of beam remnants that did not participate in other scatterings These semihard interactions cannot be completely described by perturbative quantum chromodynamics (QCD) and require a phenomenological description. The UE affects the estimation of the efficiency of isolation criteria applied to photons and charged leptons, and the energy scale in jet identification It affects the reconstruction efficiency for processes like H → γ γ , where the primary vertex is partly determined from the charged particles originating from the UE.
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