Commissioning of the tracking system in the ATLAS detector

ATLAS is one of the four experiments that will analyze the p-p collisions at LHC. It consists of several subsystems: the Inner Detector is devoted to the measurement of the charged particle tracks in the interaction point region and the Pixel Detector is its innermost component. Both have been commissioned by using cosmic rays collected by the ATLAS detector in 2009. In the first part of the thesis, the spatial resolution of the Pixel Detector is studied and optimized. When a charged particle traverses the Pixel Detector, charges released in the sensors are collected by segmented electrodes, the pixels. The charge of each pixel is read out by the Time-over-Threshold technique and adjacent pixels are grouped into clusters. Cluster position can be computed by considering its geometrical center, but spatial resolution can be optimized if using charge information to improve position determination. In the second part of the thesis, the Inner Detector resolution in all track parameters has been studied by splitting each cosmic ray track into two halves. Since both halves stem from the same particle, they should be described by the same parameters. At the same time, the two tracks are fitted independently and can be compared to study the resolution of the tracking system. Resolution been studied as a function of track direction and distance from the beam axis. The multiple scattering contribution and several systematic effects due to residual misalignments have been evaluated.