School of Physics - Theses

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    A Taste of Flavour and Neutrino Physics with Scalar Leptoquarks
    Bigaran, Innes Elizabeth ( 2022)
    Flavour physics is the branch of particle physics that examines the structure of the flavour sector of the Standard Model. This sector, describing fermion masses and their mixing, involves a large number of free parameters which are determined via experimental input. Thus, understanding the nature of the flavour sector provides a key motivation for many theories beyond the Standard Model. The accidental lepton-flavour symmetry of the Standard Model need not be preserved in extended models. Although neutrinos are massless particles in the Standard Model, and thus their flavour is conserved, strong experimental evidence of neutrino flavour oscillations requires that neutrinos are actually massive. Since those masses, though nonzero, are constrained to be tiny, it is well motivated that they are generated in some exotic way. This observation highlights the need for new physics to explain lepton-flavour violation in the neutrino sector. In this thesis, we explore not only neutrinos and their flavour violation, but also how this violation could manifest in the charged-lepton sector. Extensions to the Standard Model discussed in this thesis centre around hypothetical particles called leptoquarks, which directly couple quarks and leptons. Their interactions naturally lead to violation of lepton flavour symmetries, and imbue a sense of linkage between these two classes of Standard Model fermions. Moreover, the simple nature of scalar leptoquark extensions motivate us to consider where these could fall within the larger framework of unified models of nature. In particular, these could explain the structure of the (presently semi-empirical) flavour sector. Chapter 1 provides background on the Standard Model of particle physics, and outlines the relevant conventions adopted in this thesis. Chapter 2 reviews the present landscape of the flavour and neutrino sectors, including an overview of experimental results to guide the ensuing work. Chapter 3 centres on understanding divergences in the Standard Model, and how one can extend this theory of nature within a framework of effective field theory. Chapters 4, 5 and 6 present a series of original studies that highlight the potential impact of scalar leptoquarks on the structure of the flavour and neutrino sectors. Chapter 4 explores the viability of scalar leptoquarks to generate large corrections to charged-lepton dipole moments. We identify the mixed-chiral scalar leptoquarks (S1 and R2) capable of generating chirally-enhanced and sign-dependent contributions to lepton magnetic moments (as favoured by present measurements). We find that TeV scale particles are capable of addressing present anomalies in the magnetic dipole moments of the electron and the muon. Moreover, signals of these models in the muon electric dipole moment are found to be within reach of future experimental programs. Chapter 5 presents a next-to-minimal scalar leptoquark model capable of reconciling recent experimental B-anomalies, and of radiatively generating neutrino masses. Building upon a single leptoquark model for addressing these B-anomalies, we combine two existing neutrino-mass models (containing the leptoquarks S1 and S3, and a vector-like quark) and find that this hybrid model is able to ameliorate the anomalies in b to s transitions, charged-current b decays, and the muon magnetic moment. Furthermore, it is capable of generating radiative neutrino masses consistent with experimental values. Chapter 6 involves the study of a discrete flavour-group model built around a scalar S1 leptoquark extension. Beginning with a GF = D17 x Z17 flavour group, we outline how this model is capable of generating the textures of charged-fermion masses and mixings, as well as the leptoquark couplings required to address anomalies in charged-current b decays and the muon magnetic moment.