Search for radiative B decays to orbitally excited mesons at LHCb

This document presents studies of radiative decays of B⁰ and B_s mesons to orbitally excited states, produced in proton collisions at the Large Hadron Collider (LHC) of CERN (European Council for Nuclear Research). It takes place in the context of the search for new physics processes beyond the standard model of particle physics. The work is based on the data collected at the LHCb experiment during the Run 1(2011-2012) and the Run 2 (2015-2018) of the LHC, which corresponds to an integrated luminosity of about 8.7 fb^-1. First, an optimised selection of the final states comprising of two charged tracks and a high energy photon is put in place, regrouping a large samples of B hadron radiative decays. Then, a mass model is fit to the data distribution, allowing to statistically subtract the background contributions in the data set. A first amplitude analysis is then developed to study the hadronic structure in the B_s → K⁺K^-γ decay mode. Resulting from this analysis is the first observation of the B_s → f'₂(1525)γ mode together with a relative branching ratio measurement: B(B_s → f'₂γ)/B(B_s → φγ) = (24.1 ± 0.9 (stat) ^+1.6_-0.8(syst) ± 0.6 (BR)) x 10^-2. It is the second radiative decay mode observed for the B_s meson. A similar analysis is performed on the B⁰ → K^±π^±γ mode, allowing a preliminary measurement of the relative branching ratio of B⁰ → K*₂(1430)γ: B(B⁰ → K*⁰₂(1430)γ)/B(B⁰ → K*⁰(892)γ) = (36.4 ± 0.4 (stat) ± 1.3(BR))%. However, a detailed systematics study is needed to assess its competitiveness with previous measurements of this decay mode. Making use of a similar selection strategy, a preliminary study of B → K_S⁰hh γ modes is also set up during the thesis to estimate the available statistics for these modes at LHCb, which are particularly interesting to measure the photon polarisation in the b → sγ transition. This work will be the starting point of an analysis comprising the future data that will be collected during the Run 3 of the LHC. (author)