Contribution à la calibration du calorimètre électromagnétique du détecteur LHCb

LHCb has been designed in order to perform precise measurements of CP violation and rare decays with B mesons. At the LHC start-up, one of the first tasks will be to check the expected apparatus performances. This thesis looks to the first recorded data by the LHCb experiment for two of the subjects developed: the photon reconstruction in the electromagnetic calorimeter assuming different experimental situations, and the calibration of the calorimeter. A photon identification tool is developed based on a Fisher discriminant method using information from all the calorimeter sub-detectors (SPD, PS, ECAL, and HCAL). The reconstructed photons are used for $\pi^{0}$ mass reconstruction parameters, able to evaluate the overall calibration level. For individual cell calibration, the constraint from the reconstructed $\pi^{0}$ mass is used. A 1\% calibration can be achieved even starting from a 30\% spread cell to cell calibration within a few tenth hours at the LHCb nominal luminosity. Two physics channels, for which the calorimeter calibration and neutral mesons decaying into photon pair reconstruction are key points, are studied. The channel $B^{0}_{d} \to J/\psi\pi^{0}$ to measure $\beta_eff$ angle, where unexpected contributions beyond the standard model may occur and the channel $B^{0}_{s} \to J/\psi\eta$ measures the $\beta_{s}$ angle has been studied.