Tesis doctoral de Jorge Alda

 Title: A Glance into Flavour Physics with Effective Field Theories and

Machine Learning

Abstract:
In the last years there has been a growing interest in the study of
deviations from the predictions of the Standard Model of particle
physics in the context of Flavour Physics. This dissertation is
devoted to the study of these deviations, with special focus on those
involving B mesons.
The interest on deviations from the Standard Model predictions is
twofold: theoretical questions not yet solved and recent experimental
measurements showing this kind of deviations. In our research we study
two classes of these experimentally-interesting observables: the
semileptonic decays of B mesons into K or K* mesons and a pair of
charged leptons through flavour-changing neutral currents,
characterized by the RK(*) ratios between the muonic and the
electronic branching ratios; and the semileptonic decays of B mesons
into D or D* mesons, a charged lepton and a neutrino through
flavour-changing charged currents, characterized by the RD(*) ratios
between the tauonic and the light branching ratios. We use the
framework of Effective Field Theories, being able to obtain
constraints on New Physics contributions to the Wilson coefficients of
the effective Lagrangian from the experimental results. A global fit,
including observables from all affected sectors, is considered. The
sheer number of observables involved makes mandatory the use of
numerical calculations, and in the most extreme cases, even using
Machine Learning tools such as regression trees and SHAP (SHAPley
Additive exPlanation) values. We use for the first time in the flavour
context a Montecarlo analysis to extract the confidence intervals and
correlations between observables, showing that it constitutes a
suitable strategy to use in this kind of analysis. Although most of
the dissertation deals with the framework of Effective Field Theories,
we have also extrapolated our results to specific models of New
Physics; leptoquarks and W' and Z' bosons. We have also performed a
more in-deep analysis of a model for Axion-Like Particles,
pseudoscalars that could appear as pseudo-Nambu-Goldstone bosons for
new global U(1) symmetries. Unlike the traditional approaches, we have
examined the case where the Axion-Like Particles have a non-trivial
flavour structure in their couplings to quarks and leptons.

Miércoles 27. Sala de Grados. Facultad de Ciencias