Resumen de A glance into flavour physics with effective field theories and machine learning
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. On the theoretical side, the Standard Model does not attemp to explain why the flavour structure is what we observe, with three generations of matter particles which only differ on their masses. On the experimental side, flavour is a remarkable hunting ground for hints of new physics, since many processes are affected by suppressions that could be lifted by new interactions, leading to clear experimental signatures. In this regard, the most important development is the number of precise flavour experimental measurements in tension with the Standard Model in the last few years. 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.
These anomalous experimental results have spurred numerous proposals for physics beyond the Standard Model. Effective Field Theories offer a model-independent way to analyze those New Physics effects. The idea is to integrate out the heavy fields appearing only in the internal lines of the Feynman diagrams, leaving a set of non-renormalizable interactions including just the light fields and their symmetries. We have used in this thesis 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.
Quantum corrections contained in the Renormalization Group have the effect of mixing the non-renormalizable interactions. As a consequence, the deviations from the Standard Model introduced through Effective Field Theories tend to propagate also to physical observables different from the ones we are interested in. The solution is to determine the coefficients entering the Effective Field Theory through means of global fits including observables from all affected sectors. 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; in particular to leptoquarks, hypothetical particles that could turn quarks into leptons or vice-versa, and to W' and Z' bosons, hypothetical gauge mediators that could exhibit non-universal couplings to each fermion. 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.
