Charged-Particle Multiplicity Measurement in Proton-Proton Collisions at √ S = 7 TeV with ALICE at LHC

The pseudorapidity density and multiplicity distribution of charged particles produced in proton-proton collisions at the LHC, at a centre-of-mass energy √s = 7 TeV, were measured in the central pseudorapidity region |η| < 1. Comparisons are made with previous measurements at √ = 0.9 TeV and 2.36 TeV. At √s = 7 TeV, for events with at least one charged particle in |η| < 1, we obtain d Nch/dη = 6.01 ± 0.01(stat.)-0.12+0.20(syst.). This corresponds to an increase of 57.6% ± 0.4%(stat.)-1.8+3.6%(syst.) relative to collisions at 0.9 TeV, significantly higher than calculations from commonly used models. The multiplicity distribution at 7 TeV is described fairly well by the negative binomial distribution.