Measurement of the charged particle density with the ATLAS detector: First data at $\sqrt{s}$ = 0.9, 2.36 and 7 TeV

In this thesis I describe the measurement of the charged particle density on early LHC data. The focus is on the measurement at sqrt{s} = 2.36 TeV where a novel technique to estimate the track reconstruction efficiency from data is used. The current Monte Carlo models that are used to describe soft QCD processes are also reviewed. Furthermore, an extensive study of the ATLAS tracking devices on cosmic ray data is presented and the track parameter resolutions are derived.