Charged particle multiplicities in pp interactions at sqrt(s) = 900 GeV and sqrt(s) = 7 TeV measured with the ATLAS detector at the LHC.

Inclusive charged particle distributions have been measured in pp and proton antiproton collisions at a range of different centre-of-mass energies in the past. Most of the previous charged particle multiplicity measurements are obtained by selecting data with a double-arm coincidence trigger, thus removing large fractions of diffractive events. The data are then further corrected to remove the remaining single-diffractive component. This selection is referred to as NSD. The selection of NSD or inelastic non-diffractive charged particle spectra involves model-dependent corrections for the diffractive components. The measurement presented in this thesis implements different strategy, which uses a single-arm trigger overlapping with the acceptance of the tracking volume. Results are presented as inclusive inelastic distributions, with minimal model-dependence, by requiring one charged particle within the acceptance of the measurement. The measurements from pp collisions at sqrt(s) = 900 GeV and sqrt(s) = 7 TeV recorded with the ATLAS detector at the LHC are performed in this thesis. The measurements are compared to MC models of pp collisions and to results from other experiments at the same centre-of-mass energy. The charged particle multiplicity per event and unit of pseudorapidity at eta = 0 is measured to be 1.333 ± 0.003 (stat.) ± 0.027 (syst.) at sqrt(s) = 900 GeV and 2.418 ± 0.004 (stat.) ± 0.076 (syst.) at sqrt(s) = 7 TeV .