Measurement of charged particle multiplicities in pp collisions at √s =7~TeV in the forward region

Charged particle production in proton-proton collisions is studied with the LHCb detector at a centre-of-mass energy of √s = 7~TeV in different intervals of pseudorapidity η. Charged particles are reconstructed close to the interaction region in the vertex detector, which provides high reconstruction efficiency in the η ranges −2.5<η<−2.0 and 2.0<η<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 η. In general, the models underestimate charged particle production.