Jet properties in low and high multiplicity events in p-p collisions at 7 TeV

The characteristics of multi-particle production in p-p collisions at $\sqrt{s}=7$ TeV are studied as a function of the event charged particle multiplicity ($N_{\rm ch}$), by classifying the measured tracks into two distinct classes, those belonging to jets and those belonging to the underlying event (UE). Charged tracks are measured within pseudorapidity $|\eta|<$ 2.4 and transverse momenta above $p_{T}$~=~0.25~GeV/c, and charged-particle jets are reconstructed above $p_{T}$~=~5~GeV/c with track-only information. The distributions of jet $p_{T}$, average $p_{T}$ of UE tracks and jets, jet rates, and jet shapes are studied as a function of $N_{\rm ch}$ and are compared to the predictions of the \textsc{pythia} and \textsc{herwig} Monte Carlo (MC) event generators. As the event-multiplicity increases, \textsc{pythia} systematically predicts higher jet rates and harder $p_{T}$ spectra than seen in the data, whereas \textsc{herwig} shows the opposite trend. Predictions without multi-parton interactions fail completely to describe the data. In the lowest-multiplicity events, the data show narrower jets than predicted by both MC generators.