School of Physics - Theses
Permanent URI for this collection
Search Results
1 - 1 of 1
-
ItemModels of radiative neutrino mass and lepton flavour non-universality( 2020)This thesis presents a series of original studies exploring the space of neutrino-mass models, and the connection that a class of these models might have with the recently purported violations of lepton flavour universality measured in $B$-meson decays. We begin by describing and implementing an algorithm that systematises the process of building models of Majorana neutrino mass starting from effective operators that violate lepton number by two units. We use the algorithm to generate computational representations of all of the tree-level completions of the operators up to and including mass-dimension eleven, almost all of which correspond to models of radiative neutrino mass. Our study includes lepton-number-violating operators involving derivatives, updated estimates for the bounds on the new-physics scale associated with each operator, an analysis of various features of the models, and a look at some examples. Accompanying this work we also make available a searchable database containing the catalogue of neutrino-mass models, as well as the code used to find the completions. The anomalies in $B$-meson decays have known explanations through exotic scalar leptoquark fields. We add to this work by presenting a detailed phenomenological analysis of a particular scalar leptoquark model: that containing $S_{1} \sim (\mathbf{3}, \mathbf{1}, -\tfrac{1}{3})$. We find that the leptoquark can accommodate the persistent tension in the ratios $R_{D^{(*)}}$ as long as its mass is lower than approximately $\SI{10}{\TeV}$, and show that a sizeable Yukawa coupling to the right-chiral tau lepton is necessary for an acceptable explanation. Agreement with the measured $R_{D^{(*)}}$ values is mildly compromised for parameter choices addressing the tensions in the $b \to s$ transition. The leptoquark can also reconcile the predicted and measured value of the anomalous magnetic moment of the muon, and appears naturally in models of radiative neutrino mass. As a representative example, we incorporate the field into a two-loop neutrino mass model from our database. In this specific case, the structure of the neutrino-mass matrix provides enough freedom to explain the small masses of the neutrinos in the region of parameter space dictated by agreement with the anomalies in $R_{D^{(*)}}$, but not in the $b \to s$ transition. In order to address the shortcomings of the $S_{1}$ scenario, we construct a non-minimal model containing the scalar leptoquarks $S_{1}$ and $S_{3} \sim (\mathbf{3}, \mathbf{3}, -\tfrac{1}{3})$ along with a vector-like quark, necessary for lepton-number violation. We find that this new model permits a simultaneous explanation of all of the flavour anomalies in a region of parameter space that also reproduces the measured pattern of neutrino masses and mixing. A characteristic prediction of our model is a rate of muon--electron conversion in nuclei fixed by the $b \to s$ anomalies and the neutrino mass. The next generation of muon--electron conversion experiments will thus potentially discover or falsify our scenario. We also present a general overview from our model database of those minimal radiative neutrino-mass models that contain leptoquarks that are known to explain the anomalies in $R_{D^{(*)}}$ and the $b \to s$ transition. We hope that our model database can facilitate systematic analyses similar to this, perhaps on both the phenomenological and experimental fronts. We conclude by presenting a study of the diphoton decay of a scalar $\mathrm{SU}(N)$ bound state, motivated by the 2016 \SI{750}{\GeV} diphoton excess.