ATLAS Alignment, tracking and physics performance results

The present contribution describes the track reconstruction performance of the ATLAS Detector, based on the first proton-proton collisions delivered by the LHC accelerator at CERN at centerof-mass energies of 900 GeV and 7 TeV. The nominal tracking performance is first outlined in terms of expected track parameter resolutions and then the influence of the level of residual misalignment and of the precise knowledge of the detector material is discussed, based on real data. Preliminary results of methods which are sensitive to the amount of material in the tracking detector, as the reconstruction of photon conversions or the momentum scale and resolution of short-lived particles, are presented. The tracking performance in data and in simulation is compared and a quantitative estimate of the transverse impact parameter resolution, after unfolding the effect of the primary vertex resolution, is presented. Some information are finally provided about the physics performance of the tracking detector, in particular about the present phase of commissioning of the simplest b-tagging algorithms available in ATLAS, which are meant to be used for the first top-quark observation at ps = 7 TeV.