Jet and underlying event properties as a function of charged-particle multiplicity in proton–proton collisions at √s= 7 TeV

Abstract

Characteristics of multi-particle production in proton-proton collisions at √s = 7 TeV are studied as a function of the charged-particle multiplicity, N ch . The produced particles are separated into two classes: those belonging to jets and those belonging to the underlying event. Charged particles are measured with pseudorapidity {pipe}η{pipe} < 2.4 and transverse momentum p T > 0.25 GeV/c. Jets are reconstructed from charged-particles only and required to have p T > 5 GeV/c. The distributions of jet p T , average p T of charged particles belonging to the underlying event or to jets, jet rates, and jet shapes are presented as functions of N ch and compared to the predictions of the pythia and herwig event generators. Predictions without multi-parton interactions fail completely to describe the N ch -dependence observed in the data. For increasing N ch , pythia systematically predicts higher jet rates and harder p T spectra than seen in the data, whereas herwig shows the opposite trends. At the highest multiplicity, the data-model agreement is worse for most observables, indicating the need for further tuning and/or new model ingredients.