Study of B⁰ → K*⁰γ, B⁰_s → φγ and B⁰_s → K*⁰γ decays using converted photons with the LHCb detector

The purpose of the work presented in this thesis is to study the radiative decays of B mesons. These processes correspond to the decay of a b-quark into a s-quark through the weak interaction with a photon in the final state. This kind of decay is not allowed at tree level in the quantum field theory but only through virtual loops. These processes have small branching fractions of the order of 10^-5. The study of these decays with the LHCb detector provides precise measurements of related observable such as branching fractions, CP asymmetries or angular variables such as the photon polarisation. These results can be compared with the theoretical prediction of the Standard Model of Particle Physics. Therefore, it is possible to test this model and to constrain the existence of new physics behind the Standard Model. The analysis presented in this thesis focuses on the B⁰ → K*⁰γ, B⁰_s → φγ and B⁰_s → K*⁰γ decays. Only events where the photon has converted into a dielectron pair by interacting with the detector are used. This allows to take advantage of the good resolution on the charged tracks momentum obtained with the LHCb's tracking system. The measurements of the ratio of branching fractions of the B⁰ → K*⁰γ and B⁰_s → φγ decays and of the direct CP asymmetry in the B⁰ → K*⁰γ process are performed. The good resolution obtained on the B meson mass allows to set the first upper limit on the branching ratio of the Cabibbo suppressed B⁰ → K*⁰γ decay. (author)