Effective field theory flavored paths to beyond the Standard Model dynamics

The aim of this thesis is to partially address some of the current physical unknowns by studying the effects of new physics on the predictions of the Standard Model for several observables. The thesis is organized in 3 parts. In Part I we introduce the two frameworks that serve as the building blocks of the material presented in the rest of the text. Part II and III are then devoted to present the core of the work done during the PhD. Each of the aforementioned parts contains 2 chapters. Hence, the thesis consists of a total of 6 chapters, a summary and a list of appendices that complement some of the calculations presented in the main text. In Chapter 1 the Standard Model of particle physics is reviewed. The basic QFT description of its particle content is given, followed by a detailed description of the strong and electroweak sectors. The spontaneous symmetry breaking mechanism is recalled at the end of the chapter. Chapter 2 focuses on the framework of effective field theories. First, their theoretical underpinnings are explained. Then, Chiral Perturbation Theory, the dual theory of QCD at low energies, is presented together with its extension Resonance Chiral Theory. This is followed by the description of the effective field theory of the Standard Model and its low-energy realization. Part II collects two strongly related works. Chapter 3 is dedicated to our phenomenological analysis on charged-lepton-flavor violation on tau leptons. After a historical and theoretical introduction of flavor is given, the techniques to compute the observables of interest are presented and the statistical tool used in the analysis is introduced. The corresponding results are given in several sections. These results are later used in Chapter 4 to set bounds on a general leptoquark framework. Such a framework is presented and the relevant interactions contributing to the observables studied in Chapter 3 are highlighted. Then the results are presented for scalar and vector leptoquars separately. Part III contains two unrelated chapters that deal with neutrino physics. In Chapter 5 the full effective field theory description of the COHERENT experiment with its coherent elastic neutrino-nucleus scattering detection process is provided. The oscillatory behaviour of the neutrinos involved in the experiment is characterized within the QFT formalism. Finally, some preliminary results are shown. The last Chapter 6 deals with a particular extension of the Standard Model addressing the generation of neutrino masses: the linear seesaw model. Within this model, the Quasi-Dirac nature of the extra heavy neutrinos is studied. The chapter ends with a phenomenological analysis that helps to constrain the parameters of the model. We close with a comprehensive summary of the thesis in Catalan and several appendices

Lepton-flavour violation in hadronic tau decays and μ-τ conversion in nuclei

Within the Standard Model Effective Field Theory framework, with operators up to dimension 6, we perform a model-independent analysis of the lepton-flavour-violating processes involving tau leptons. Namely, we study hadronic tau decays and ℓ-τ conversion in nuclei, with ℓ = e, μ. Based on available experimental limits, we establish constraints on the Wilson coefficients of the operators contributing to these processes. Our work paves the way to extract the related information from Belle II and foreseen future experiments

Snowmass 2021 White Paper: Charged lepton flavor violation in the tau sector

Charged lepton flavor violation has long been recognized as unambiguous signature of New Physics. Here we describe the physics capabilities and discovery potential of New Physics models with charged lepton flavor violation in the tau sector as its experimental signature. Current experimental status from the B-Factory experiments BaBar, Belle and Belle II, and future prospects at Super Tau Charm Factory, LHC, EIC and FCC-ee experiments to discover New Physics via charged lepton flavor violation in the tau sector are discussed in detail. Submitted to the Proceedings of the US Community Study on the Future of Particle Physics (Snowmass 2021)Comment: Contribution to Snowmass 202

Constraints on leptoquarks from lepton-flavour-violating tau-lepton processes

Leptoquarks are ubiquitous in several extensions of the Standard Model and seem to be able to accommodate the universality-violation-driven B-meson-decay anomalies and the (g −2)μ discrepancy interpreted as deviations from the Standard Model predictions. In addition, the search for lepton-flavour violation in the charged sector is, at present, a major research program that could also be facilitated by the dynamics generated by leptoquarks. In this article, we consider a rather wide framework of both scalar and vector leptoquarks as the generators of lepton-flavour violation in processes involving the tau lepton. We single out its couplings to leptoquarks, thus breaking universality in the lepton sector, and we integrate out leptoquarks at tree level, generating the corresponding dimension-6 operators of the Standard Model Effective Field Theory. In ref. [1] we obtained model-independent bounds on the Wilson coefficients of those operators contributing to lepton-flavour-violating hadron tau decays and ℓ–τ conversion in nuclei, with ℓ = e, μ. Hence, we use those results to translate the bounds into the couplings of leptoquarks to the Standard Model fermions

EFT analysis of New Physics at COHERENT

International audienceUsing an effective field theory approach, we study coherent neutrino scattering on nuclei, in the setup pertinent to the COHERENT experiment. We include non-standard effects both in neutrino production and detection, with an arbitrary flavor structure, with all leading Wilson coefficients simultaneously present, and without assuming factorization in flux times cross section. A concise description of the COHERENT event rate is obtained by introducing three generalized weak charges, which can be associated (in a certain sense) to the production and scattering of $\nu_e$, $\nu_\mu$ and $\bar{\nu}_\mu$ on the nuclear target. Our results are presented in a convenient form that can be trivially applied to specific New Physics scenarios. In particular, we find that existing COHERENT measurements provide percent level constraints on two combinations of Wilson coefficients. These constraints have a visible impact on the global SMEFT fit, even in the constrained flavor-blind setup. The improvement, which affects certain 4-fermion LLQQ operators, is significantly more important in a flavor-general SMEFT. Our work shows that COHERENT data should be included in electroweak precision studies from now on

Snowmass 2021 White Paper: Charged lepton flavor violation in the tau sector

International audienceCharged lepton flavor violation has long been recognized as unambiguous signature of New Physics. Here we describe the physics capabilities and discovery potential of New Physics models with charged lepton flavor violation in the tau sector as its experimental signature. Current experimental status from the B-Factory experiments BaBar, Belle and Belle II, and future prospects at Super Tau Charm Factory, LHC, EIC and FCC-ee experiments to discover New Physics via charged lepton flavor violation in the tau sector are discussed in detail. Submitted to the Proceedings of the US Community Study on the Future of Particle Physics (Snowmass 2021