Study of b-hadron decays into two hadrons and a photon at LHCb and first observation of b-baryon radiative decays

The Standard Model (SM) of particle physics, a set of theories that were developed during the 20th century, aims to explain three of the four fundamental forces of nature: electromagnetism, strong and weak interactions. From a theoretical point of view, the SM was finished during the 1970s, but it was not until 2012 when its last piece, the Higgs boson, was experimentally confirmed. Despite having been proved to be a very successful theory with many experimental observations, the SM fails to explain crucial phenomena that would make it a complete theory. Its main shortcomings are the inclusion of gravity as described by general relativity, the existence and properties of dark matter and dark energy and neutrino oscillations. It also fails to explain the different abundances of matter and antimatter that are observed in the Universe. In particular, $CP$ violation is related to the last of these problems as it is a necessary condition to the asymmetrical matter-antimatter abundances to appear from a symmetrical initial state. However, the SM prediction for $CP$ violation is not enough to explain the large asymmetry observed. From the experimental point of view, $CP$ violation was first observed in neutral kaon decays. The Belle and BaBar experiments observed $CP$ violation in the $b$-sector in 2001. There are many physical observables in the $b$-sector related to $CP$ violation. These observables are predicted with high precision by the SM so any large deviation in these parameters would mean the existence of new physics. As a consequence, $b$-physics is an ideal place to test the SM predictions. The LHCb experiment, one of the experiments of the Large Hadron Collider, is specialized in the study of heavy flavour hadrons. One of the topics of interest is flavour-changing neutral currents, which can only happen via loop processes within the SM, making them a very sensitive place for the search of contributions from new particles entering the loop. Radiative decays (decays where there is a photon in the final state) are one of this kind of processes. This thesis will verse about the measurement of the ratio of branching fractions between the $B_{s} \to \phi\gamma$ and $\Lambda_{b}\to\Lambda^{*}\gamma$ decays with respect to the $B_{d} \to K^* \gamma$ decay as well as the direct $CP$ asymmetry for the $B_{d} \to K^* \gamma$ and $\Lambda_{b}\to\Lambda^{*}\gamma$ decays. In addition, the $\Lambda_{b}\to\Lambda^{*}\gamma$ decay corresponds to the first observation of $b$-baryon radiative decay. This work corresponds to the analysis of the whole LHCb Run 1 dataset, collected in the years 2011--2012, which amounts to a total of 3 fb$^{-1}$. The ratio of branching fractions between $B_{s} \to \phi\gamma$ and $B_{d} \to K^* \gamma$ and the $CP$ for the latter were already measured by the LHCb with data collected in 2011 and therefore this thesis superseeds those measurements. The photon reconstruction and identification is done with information from the Calorimeter detector. The SPD detector, the part of the calorimeter in charge of the discrimination of photons and electrons at the first level of trigger, suffered from ageing problems during the Run 1 period. This thesis includes detailed studies on the monitorization of the ageing effect on the SPD efficiencies as well as the description of the calibration process that followed. One of the main sources of background for radiative decays is the misidentification of neutral pions and photons. The thesis introduces an offline tool that improves the simulation description of the $\gamma/\pi^{0}$ separation variable.