Charged particle multiplicities in pp interactions for track pT > 100 MeV at sqrt{s} = 0.9 and 7 TeV measured with the ATLAS detector at the LHC

New measurements are presented from proton-proton collisions at centre-of-mass energies of sqrt(s)=0.9 and 7 TeV recorded with the ATLAS detector at the LHC. The 0.9 TeV data uses the same runs as the first publication, while the 7 TeV data sample is increased to a total of ten million events, corresponding to an integrated luminosity of ~190 µb-1. These events were all collected using a single-arm 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 two charged particles in the kinematic range |eta| < 2.5 and pT > 100 MeV. This analysis follows closely the strategy used in our earlier sqrt(s) = 0.9 TeV publication and the sqrt(s) = 7 TeV CONF note. The principal improvement with respect to previous analyses is the lowering of the track pT threshold from 500 MeV to 100 MeV. The results are compared to various Monte Carlo models, including the new ATLAS AMBT1 tune. The charged particle multiplicity per event and unit of pseudorapidity for eta = 0 is measured to be 5.635 +- 0.002(stat) +- 0.149(syst) at 7 TeV and 3.486 +- 0.008(stat) +- 0.077(syst) at 0.9 TeV.