Track Fitting in the ATLAS Experiment

ATLAS is one of the four experiments that will analyze the proton-proton collisions that the LHC will produce. ATLAS consists of several subsystems: the inner detector, the calorimeters, and the muon spectrometer. In this thesis, emphasis is placed on the analysis of the data from the inner tracker and the muon spectrometer. For each particle, the detector performs a number of position measurements. From this set of three-dimensional points, the trajectory of the particle can be reconstructed using computer calculations. In this thesis an algorithm is described that is based on the minimization of a global $\chi^2$ function. The algorithm corrects for energy loss and scattering in the detector material, by introducing extra fit parameters for these material effects. The propagation of the tracks through the magnetic field in the detector is performed using a fourth order Runge-Kutta procedure. This procedure is numerical, therefore it requires a significant amount of computation time. It turns out that the derivatives of the measurements with respect to the track parameters can be evaluated analytically, which reduces the amount of computation time required. The algorithm has been used to study data from the Combined Testbeam in 2004, as well as cosmic ray data recorded by the inner detector in 2006. In addition, the algorithm has been tested using simulated collision events in ATLAS. The performance of the algorithm is competitive with other solutions.