Etude des désintégrations radiatives des mésons B dans le détecteur ATLAS

This thesis is dedicated to the the study radiative B decays with the ATLAS detector at the LHC. Radiative decays belong to the rare decays family. Rare decays transitions involve flavor changing neutral currents (for example b--> s\gamma), which are forbidden at the lowest order in the Standard Model. Therefore these processes occur only at the next order, thus involving penguin or box diagrams, which are very sensitive to New Physics contributions. The main goal of our study is to show that it would be possible to develop an online selection strategy for radiative B decays with the ATLAS detector. To this end, we have studied the treatment of low energy photons by the ATLAS electromagnetic calorimeter (ECal). Our analysis shows that ATLAS ECal will be efficient with these particles. This property is extensively used in the next section, where a selection strategy for radiative B decays is proposed. Indeed, we look for a low energy region of interest in the ECal as soon as the level 1 of the trigger. Then, photon identification cuts are performed in this region at level 2. However, a large part of the proposed selection scheme is also based on the inner detector, particularly at level 2. The final results show that large amounts of signal events could be collected in only one year by ATLAS. A preliminary significance estimation is also presented. Encouraging results concerning the observability of exclusive radiative B decays are obtained.