B-Meson Decays as Probes of New Physics

Understanding the elementary particles and their interactions has been a longstanding pursuit in science. The Standard Model of Particle Physics has provided a particularly successful framework to describe these fundamental particles and forces. However, substantial open questions remain, such as the nature of dark matter and energy, the unification of the Standard Model with General Relativity, and the asymmetry between matter and antimatter in the universe. This thesis considers the realm of flavour physics, focusing on the study of B-meson decays to probe the Standard Model at high precision. We explore various B-meson decay channels, leveraging observables like branching fractions and CP asymmetries to enhance our understanding of the Standard Model and to potentially uncover New Physics. First of all, we scrutinise rare leptonic decays of neutral B mesons, such as decays into muon--antimuon and electron--positron pairs. Within the Standard Model, they suffer from an effect known as helicity suppression, which is one of the reasons for their rarity. The electron--positron final state has a chance of 10^{-13} or even 10^{-15} of being produced through Standard Model processes, depending on the type of neutral B-meson. However, particular types of New Physics could lift the helicity suppression and bring this decay to a level where it could be observed at current experiments. Secondly, we consider decays of B mesons through a charged-current process into a purely leptonic final state, as well as final states consisting of both leptons and mesons, in a combined analysis. The underlying weak-interaction process is the same in both cases, so that New Physics would contribute to all these channels. We obtain results in accordance with the SM, constraining New Physics effects. Here, we find as well that some decay modes suffer form helicity suppression which can be lifted by certain new contributions. Even within the constraints from current experimental data, the decay of a charged B meson into a final state consisting of an electron and an anti-neutrino could be enhanced to up to ten thousand times its Standard Model value. Finally, we explore the use of flavour symmetry in the analysis of neutral B-meson decays into final states consisting of pions and kaons to extract fundamental parameters related to CP violation. In view of increasing experimental precision, we minimise the flavour-symmetry-breaking effects in order to improve the precision of the theoretical analysis, providing insights into potential deviations from Standard Model predictions and alternative measurements. This research highlights the potential of B-meson physics in searching for new phenomena. The quest for improved experimental precision and theoretical understanding in this domain is crucial for unraveling the mysteries that persist beyond the Standard Model. We provide novel insights and lay the groundwork for future experimental endeavours that will hopefully shed light on what lies beyond the Standard Model.