Measurement of the photon polarization using ${B_s^0\to \phi\gamma}$ at LHCb.

This thesis is dedicated to the study of the photon polarization in ${B_s^0\to \phi\gamma}$ decays at LHCb. At the quark level, such decays proceed via a $b \to s \gamma$ penguin transition and are sensitive to possible virtual contributions from New Physics. The measurement of the photon polarization stands also as a test of the $V-A$ structure of the Standard Model coupling in the processes mediated by loop penguin diagrams. The measurement of the photon polarization can be done through a study of the time-dependent decay rate of the $B$ meson. A delicate treatment has been done to understand the proper time distribution and the selection acceptance affecting it. To control the proper time acceptance, data driven control methods have been developed. Several possible strategies to measure the photon polarization are introduced and preliminary blinded results are presented. A study of some of the systematic effects is discussed. In the context of studying radiative decays, the author has developed a new photon identification procedure and has provided a tool to calibrate the performance of the photon/neutral pion separation variable on simulation. Those tools are of general interest for the LHCb collaboration and are widely used