Building a Scenario of Physics Beyond the Standard Model in the Flavor Sector

Abstract

Recently lepton flavor universality tests in colliders have reported tensions with the Standard Model, providing an experimental hint at low energies of the effect of a new high energy theory. These tensions are known as flavor anomalies and that is why in this thesis we focus on the study of the impact of the flavor anomalies on the flavor sector of the Standard Model of particle physics. We introduce the Standard Model of particle physics and list the relevant tensions of the theory with the experiments in B decays, commenting in a brief manner the possible solutions involving new physics, as well as the main advantages and inconveniences of each new physics scenario.

We first focus in a model with new heavy scalars and fermions were we only account for left-handed couplings to the Standard Model particles. We consider two possible models: one with an additional scalar and two vector-like fermions and another with two additional scalars and one vector-like fermion. The purpose of this model is to solve the tensions in FCNC decays, together with the anomalous magnetic moment of the muon, which are induced at loop-level in both models. We list the relevant constraints and show that the models are able to solve the anomalies, albeit a relatively large coupling of the muons is required. In the case of semi-leptonic decays, stringent constraints arising from B oscillations can be relaxed if the new fermions are considered Majorana particles.

Then, since we cannot explain the muon magnetic moment with only left-handed couplings, we construct a model with new scalars and fermions allowing also for right- handed couplings, were we list the relevant Wilson coefficients for b quark decays to a strange quark and two muons, as well as the relevant observables acting as constraints for any number of new scalars and fermions. In order to illustrate this generic approach we supplement the Standard Model with a fourth generation of quarks and leptons. With this model we can explain FCNC anomalies and the magnetic moment of the muon avoiding all the constraints.

The second kind of model that we explore is an extension of the Standard Model with scalar leptoquarks. In this case, we compute the Z- and W-decays to leptons for each one of the scalar leptoquarks at next-to-leading-logarithm approximation, and show that the finite terms can account for 20% of the total contribution for leptoquark masses of below 1.5 TeV. We also show their phenomenological relevance in a model with a singlet and a triplet, where our computation pushes the fit towards a better explanation of data. Besides, we comment on the fact that the B mixing has to be implemented carefully as it is one of the main constraints that was missing in earlier studies of these kind of models, and we also illustrate its key role in a particular model model, since it spoils the pure left- handed scenario with 2018 data.

Finally, we scrutinize the FCNC process of b quark dcaying into an s quark and two leptons, in the framework of a two Higgs doublet model. We compute all the relevant Wilson coefficients performing the matching of the full theory with the low energy theory showing that it is necessary to keep the external momenta for the scalar and pseudo-scalar operators. This is the first time computation of a proper matching including all the relevant operators. We perform a phenomenological analysis of the model with the strange B meson decaying to two muons and the B meson decaying into two muons and a K meson at high momenta, where we have control of the hadronic uncertainties.

Des del 2012, una serie de mesures en experiments com BaBar, Belle i LHCb, han presentat tensions respecte el model estàndard en decaïments de mesons B. Aquestes desviacions són conegudes com les anomalies de sabor. En aquesta tesi interpretem les anomalies de sabor com a possibles efectes de nova física i proposem alguns models simples per poder acomodar les dades experimentals que difereixen del model estàndard.

En primer lloc, proposem un model amb nous escalars i fermions pesats que només s’acoblen als fermions tipus esquerra del model estàndard. En aquest escenari intentem explicar una part de les anomalies ensems amb el valor anòmal del moment magnètic del muó. El resultat són uns acoblaments relativament grans de valor 2 aproximadament. Pel que fa a resoldre part de les anomalies, tenim la possibilitat de considerar els nous fermions com fermions de Majorana, que permet reduir el valor dels acoblaments.

Per tal de poder relaxar el valor dels acoblaments d’una forma més general, en el següent capítol proposem un model semblant però amb acoblaments tipus dreta amb el model estàndard. En aquest model calculem totes les fórmules d’una manera genèrica i posteriorment ho particularitzem en un model de quarta generació. Amb la introducció d’aquests acoblaments tipus dreta combinats amb els esquerra podem explicar part de les anomalies de sabor i també el moment magnètic del muó.

Seguidament, construïm un model amb leptoquarks escalars on estudiem alguns dels lligams més importants, com els decaïments de Z o les oscil·lacions del mesó B. Posteriorment, proposem models fenomenològics de 2 leptoquarks escalars on veiem l’impacte dels lligams estudiats anteriorment. També discutim com els models quedarien obsolets si no fos perquè les dades experimentals van canviar subtilment després de Moriond 2019.

Per últim, fem un estudi de l’impacte de dos decaïments: del mesó B estrany a dos muons i del mesó B al mesó K i dos muons dins d’un model de dos doblets de Higgs. Aquí no ens centrem en les anomalies, sinó que mirem l’impacte d’aquests observables en el model i realitzem una comparació adequada entre la teoria efectiva i la teoria a altes enegies.