Fast Track Reconstruction for the High Level Trigger of the LHCb Experiment

This work presents an algorithm for fast track reconstruction in the main tracking stations for the lowest level of the LHCb software trigger. True signals of the hardware trigger are confirmed by a track with an efficiency of larger than 95% within 1 ms. The tracking algorithm improves the momentum resolution of the trigger objects significantly and can thus be used to reduce the trigger rate by a factor of two with almost no loss in efficiency. The trigger sequence based on the fast track reconstruction in the main tracker developed within this thesis is a complimentary approach to the existing vertex detector based trigger sequence. It yields comparable efficiency and retention rates. The new approach significantly improves the overall robustness of the LHCb trigger system. A detailed evaluation of its performance is presented here. Additionally the complete software trigger for muons has been rewritten and optimized. It now allows to trigger events without affecting the B meson lifetime dependent acceptance nor the angular dependent acceptance of its decay products. This is crucial for the analysis of the CP violation in the decay Bs-->J/psi phi, one of the key measurements of the LHCb physics program.