Charged particle multiplicities in pp interactions at sqrt(s) = 7 TeV measured with the ATLAS detector at the LHC

The first measurements from proton-proton collisions at sqrt(s)=7 TeV recorded with the ATLAS detector at the LHC are presented. Data presented here were collected on 30th March 2010 using a minimum-bias trigger. The charged particle multiplicity, its dependence on transverse momentum and pseudorapidity, and the relationship between mean transverse momentum and charged particle multiplicity are measured for events with at least one charged particle in the kinematic range |eta|<2.5 and pT>500MeV. This analysis follows very closely the strategy used in the sqrt(s)=900 GeV data publication. The final sqrt(s)=7 TeV results are compared to these published values. Several improvements were made to both the track and the vertex reconstruction algorithms. The range in both the transverse momentum and the charged particle multiplicity is extended. This note concentrates on describing the aspects of the analysis that have changed. The charged particle multiplicity per event and unit of pseudorapidity for |eta| =0 is measured to be 2.418 +/- 0.004 (stat.) +/- 0.076 (syst.). The dependence of the charge particle multiplicity on the centre of mass energy is compared to Monte Carlo model predictions.