Prospects of Observing the Decay B_c -> J/psi + pi and the Alignment Performance in the ATLAS Experiment

Sammanfattning: This thesis contains two main aspects of my research work towards physics in proton-proton collisions in the ATLAS experiment at the LHC. First, a Monte Carlo based analysis of the event yield and the mass shape for the ground state of the B_c meson in the hadronic decay channel B_c -> J/psi + pi was studied. The selection criteria have been optimised, the total number of signal events, and the significance of the observation have been estimated for the detector configuration and backgrounds expected at the centre-of-mass energy of 10 TeV with 1 fb^{-1} of integrated luminosity. As the momenta of these decay products are relatively low, compared with the main objectives of the ATLAS physics program, the performance of the inner detector is crucial for the low p_T tracking. As a result, the second part of the thesis is focused on the impact of the inner detector alignment on physics. Monte Carlo studies were performed by looking at the reconstructions of the well-known resonances, K_s^0, J/psi, Upsilon and Z^0 in misalignments, particularly the systematic misalignments. Then the performance of two different alignments that have been applied subsequently in ATLAS since the start up of the LHC were investigated using K_s^0 -> pi^+ pi^- events in the current 7 TeV collisions. Finally, I summarise the performance of the inner detector on the early J/psi ->µ^+ µ^- reconstruction in collisions, which is essential for the coming B physics measurements.