Getting chirality right: single scalar leptoquark solutions to the (g−2)e,μ(g-2)_{e,\mu} puzzle

We identify the two scalar leptoquarks capable of generating sign-dependent contributions to leptonic magnetic moments, $R_2\sim (\mathbf{3}, \mathbf{2}, 7/6)$ and $S_1\sim (\mathbf{3}, \mathbf{1}, -1/3)$, as favoured by current measurements. We consider the case in which the electron and muon sectors are decoupled, and real-valued Yukawa couplings are specified using an up-type quark mass-diagonal basis. Contributions to $\Delta a_e$ arise from charm-containing loops and $\Delta a_\mu$ from top-containing loops -- hence avoiding dangerous LFV constraints, particularly from $\mu \to e \gamma$. The strongest constraints on these models arise from contributions to the Z leptonic decay widths, high-$p_T$ leptonic tails at the LHC, and from (semi)leptonic kaon decays. To be a comprehensive solution to the $(g-2)_{e/\mu}$ puzzle we find that the mass of either leptoquark must be $\lesssim 65$ TeV. This analysis can be embedded within broader flavour anomaly studies, including those of hierarchical leptoquark coupling structures. It can also be straightforwardly adapted to accommodate future measurements of leptonic magnetic moments, such as those expected from the Muon $g-2$ collaboration in the near future.Comment: 13 pages, 4 figures, matches published version. Error identified in initial calculation of $\mu \to e \gamma$ which, when corrected, invalidated the top-philic coupling ansatz. Models revised to represent texture with dominant charm-loop NP contribution to $\Delta a_e$, and top-loop to $\Delta a_\mu

Lepton-flavour-violating tau decays from triality

Motivated by flavour symmetry models, we construct theories based on a low-energy limit featuring lepton flavour triality that have the flavour-violating decays $\tau^\pm \to \mu^\pm \mu^\pm e^\mp$ and $\tau^\pm \to e^\pm e^\pm \mu^\mp$ as the main phenomenological signatures of physics beyond the standard model. These decay modes are expected to be probed in the near future with increased sensitivity by the Belle II experiment at the SuperKEKB collider. The simple standard model extensions featured have doubly-charged scalars as the mediators of the above decay processes. The phenomenology of these extensions is studied here in detail.Comment: 24 pages, 4 figures Additional references and neutrino mass discussion added. Matches version accepted for publication in Physical Review

A Taste of Flavour and Neutrino Physics with Scalar Leptoquarks

© 2022 Innes Elizabeth BigaranFlavour 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

Mutually elusive: Vectorlike antileptons and leptoquarks

We study the properties of vectorlike fermions that have the same gauge charges as the Standard Model lepton doublets, but opposite lepton number. These antileptons undergo decays mediated by heavier scalar leptoquarks, while the symmetries of this renormalizable model protect the vectorlike fermions and the leptoquarks from standard decays probed so far at colliders. We derive upper limits on the new Yukawa couplings imposed by flavor-changing processes, including B→Kνν¯ and Bs-B¯s mixing, and show that they are compatible with prompt antilepton decays at the LHC for wide parameter ranges. If the new particles couple predominantly to second-generation quarks, then their collider probes involve multiple jets and two taus or neutrinos, and are hampered by large backgrounds. If couplings to third-generation quarks are large, then the collider signals involve top quarks, and can be probed more efficiently at the LHC. Even in that case, both the vectorlike fermion doublet and the leptoquarks remain more elusive than in models with standard decays.ISSN:1550-7998ISSN:0556-2821ISSN:1550-236

Flavor anomalies meet flavor symmetry

We construct an extension of the Standard Model with a scalar leptoquark φ∼(3,1,-13) and the discrete flavor symmetry Gf=D17×Z17 to explain anomalies observed in charged-current semileptonic B meson decays and in the muon anomalous magnetic moment, together with the charged fermion masses and quark mixing. The symmetry Z17diag, contained in Gf, remains preserved by the leptoquark couplings, at leading order, and efficiently suppresses couplings of the leptoquark to the first generation of quarks and/or electrons, thus avoiding many stringent experimental bounds. The strongest constraints on the parameter space are imposed by the radiative charged lepton flavor violating decays τ→μγ and μ→eγ. A detailed analytical and numerical study demonstrates the feasibility to simultaneously explain the data on the lepton flavor universality ratios R(D) and R(D·) and the muon anomalous magnetic moment, while passing the experimental bounds from all other considered flavor observables