The Charged Particle Multiplicity at Center of Mass Energies from 900 GeV to 7 TeV measured with the ATLAS Experiment at the Large Hadron Collider

The first measurements made by the ATLAS experiment at the LHC are presented. The charged particle multiplicity, its dependence on transverse momentum and pseudorapidity, and the relationship between mean transverse momentum and pseudorapidity are measured for events with at least one charged particle in the kinematic range |\eta| < 2.5 and pT > 500 MeV. The charged particle multiplicity distributions are measured at the three centre of mass energies at which protons have been collided in the LHC: 900 GeV, 2.36 TeV and 7 TeV. The results are compared to predictions from Monte Carlo models of proton-proton collisions. All models predicted a multiplicity at least 10% lower than was measured. They also failed to predict a sufficient increase in the multiplicity when the centre of mass energy increased from 900 GeV to 7 TeV. Updated models have already been produced using these data, which provide a significantly better description of the properties of proton-proton collisions at LHC energies.