Charged-particle multiplicities inppinteractions measured with the ATLAS detector at the LHC

Measurements are presented from proton-proton collisions at centre-of-mass energies of √ s = 0.9, 2.36 and 7 TeV recorded with the ATLAS detector at the LHC.Events were collected using a single-arm minimumbias trigger.The charged-particle multiplicity, its dependence on transverse momentum and pseudorapidity and the relationship between the mean transverse momentum and charged-particle multiplicity are measured.Measurements in different regions of phase space are shown, providing diffraction-reduced measurements as well as more inclusive ones.The observed distributions are corrected to well-defined phase-space regions, using model-independent corrections.The results are compared to each other and to various Monte Carlo (MC) models, including a new AMBT1 pythia6 tune.In all the kinematic regions considered, the particle multiplicities are higher than predicted by the MC models.The central charged-particle multiplicity per event and unit of pseudorapidity, for tracks with p T > 100 MeV, is measured to be 3.483 ± 0.009 (stat) ± 0.106 (syst) at √ s = 0.9 TeV and 5.630 ± 0.003 (stat) ± 0.169 (syst) at √ s = 7 TeV.