Measurement of charged particle multiplicities in $pp$ collisions at $\sqrt{s}=7$TeV in the forward region

Abstract

The charged particle production in proton-proton collisions is studied with the LHCb detector at a centre-of-mass energy of $\sqrt{s}=7$TeV in different intervals of pseudorapidity $\hat{I}\cdot$. The charged particles are reconstructed close to the interaction region in the vertex detector, which provides high reconstruction efficiency in the $\hat{I}\cdot$ ranges $-2.5<\hat{I}\cdot <-2.0$ and $2.0<\hat{I}\cdot <4.5$. The data were taken with a minimum bias trigger, only requiring one or more reconstructed tracks in the vertex detector. By selecting an event sample with at least one track with a transverse momentum greater than 1 GeV/c a hard QCD subsample is investigated. Several event generators are compared with the data; none are able to describe fully the multiplicity distributions or the charged particle density distribution as a function of $\hat{I}\cdot$. In general, the models underestimate the charged particle production.