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

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 || < 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 datamodel agreement is worse for most observables, indicating the need for further tuning and/or new model ingredients.