School of Physics - Theses
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ItemSemi-leptonic decays at the Belle and Belle II experiments( 2022)This thesis explores semi-leptonic decays of the type $B \to V L$ along two avenues; their contribution to high-precision tests and metrology of the Standard Model, and their potential for probing physics beyond the Standard Model. The magnitude of the Cabbibo-Kobayashi-Maskawa matrix element corresponding to $b\to c$ quark transitions, $|V_{cb}|$, has a long-standing discrepancy between its measurements from exclusive and inclusive decays, which presents a puzzling gap in our knowledge of the Standard Model. In this thesis, recent Lattice QCD calculations of form factors at nonzero hadronic recoil are incorporated into novel analysis methods to examine the phenomenology and analysis techniques behind the exclusive $|V_{cb}|$ measurement in $B \to D^{*}\ell^+ \nu_\ell$ decays. This results in a measurement of $|V_{cb}|=(38.49 \pm 0.54 \pm 0.92 \pm 0.91) \times 10^{-3}$, the least model-dependent result to date. The results of this analysis of the $B \to D^{*}\ell^+ \nu_\ell$ branching fraction and decay rate are used in a test of Standard Model QCD predictions of hadronic factorisation, with the first measurement of the parameter $|a_1|$ within a single experiment, based on a ratio that cancels most experimental systematic uncertainties, resulting in $|a_1| = 0.884 \pm 0.004 \pm 0.003 \pm 0.016$. This corresponds to an $8.9\sigma$ discrepancy from the Standard Model, suggesting the existence of large non-factorisable contributions in hadronic $B$-decays, or potential contributions from new physics amplitudes. A probe for new physics in photon-dipole interactions, described by the $\mathcal{O}_7$ term in an operator product expansion, is then performed to constrain non-Standard Model right-handed currents in flavour-changing neutral current semi-leptonic decays. New lepton identification techniques in the Belle experiment are developed for use in the first study of $B \to K^* e^+ e^-$ decays at the very low dilepton invariant mass region, $q^2 < 1.12 \gevccsq$, to constrain the Wilson coefficient $\mathcal{C}_7$, and its right-handed counterpart $\mathcal{C}_7^{\prime}$, expected to be zero in the Standard Model.
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ItemSearching for Dark Matter( 2022)The nature of Dark Matter (DM) is one of the most prominent unanswered questions in particle physics. The Standard Model (SM) has been remarkably successful in describing subatomic phenomena, however not all observations can be explained using this model including DM. The existence of DM is supported by a number of independent astrophysical observations, which when taken together, indicate DM is an elementary particle or particles, however their nature remains largely unknown. The focus of this thesis is on work towards experimental searches for particle DM under the Weakly Interacting Massive Particle (WIMP) paradigm using alter- native but complementary methods to the astrophysical observations in order to test the particle nature of DM. The first approach used is collider searches which test for DM production from the incident SM particles in particle colliders. The second approach is Direct Detection (DD) which aims to observe DM scattering off a SM particle. Using the motion of the Earth and Sun, some more unique features of the expected DM signal may be used to enhance experimental sensitivity. The orbit of the Earth around the Sun results in a time dependent signal with period of a year. The large dataset collected by A Toroidal LHC ApparatuS (ATLAS) allows searches for DM in many areas of phase space. These searches are limited by the ability to identify and discriminate the hypothesised DM signals from back- ground. As such the reconstruction and proper identification of objects in the detector over the largest possible range of momenta plays a key role in what is experimentally accessible. The use of track-jets to allow the identification of low momentum b-hadrons as well as the extension of this identification to lower momentum ranges will be described. The pioneering use of these detector objects to search for DM in regions of phase space previously thought to be inaccessible or too difficult will be described. Many experiments have failed to find DM using direct detection but only one (DArk MAtter (DAMA)) still maintains they have found it, appearing as a time dependent signal. This result is somewhat at odds with other results, however due to experimental differences, it is not completely incompatible. To properly test this an independent experiment using the same experimental approach as DAMA is needed to verify the results. This is the aim of the Sodium-iodide with Active Background REjection (SABRE) experiment, the creation of data acquisition and management systems will be described as well as simulation results used to inform the design and understand detector backgrounds.
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ItemA Taste of Flavour and Neutrino Physics with Scalar Leptoquarks( 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.
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ItemBelle II Silicon Vertex Detector and a measurement of B → D**lν decays at Belle( 2021)The Belle II Silicon Vertex Detector (SVD) is a silicon strip detector designed to possess a high irradiation tolerance and short shaping time, making the detector suitable for operation at the high luminosity SuperKEKB collider. In this thesis, the construction of the inner most layer of the detector ''Layer-3'' and subsequent electrical characterisation of the devices are described. Each of the 11 Layer-3 ladders produced were of a high electrical quality, with a strip failure rate of less than 0.2%, demonstrating each of the ladders to be a suitable candidate for installation into the Belle II detector. In the early stages of the detector commissioning phase, numerous high occupancy regions were discovered on the origami sensors. This problem was identified to be caused by crosstalk between control lines on the pitch adapters and the electrodes of the sensor beneath. An algorithm was developed to identify events in which these clusters were present, such that further studies into the affect of the crosstalk clusters could be performed. In particular, the impact on the track finding performance was studied in the search of an offline software approach to mitigating the crosstalk clusters. It was found that the signal-to-noise Ratio (SNR) of the crosstalk clusters were distinct from clusters deposited by signal tracks and an SNR cut based approach demonstrated an improvement to the tracking computation time of order 10%, and a slight improvement to the track parameter resolution. The hit occupancy in the SVD is expected to continually increase as the instantaneous luminosity of SuperKEKB increases over the course of the experiment. As a means of reducing the exponentially growing number of 2D hit candidates which are supplied to the track finding software, detector information was utilised to filter background events. Through exploiting cluster charge, cluster time, and cluster size correlations between each side of the strip detector, a quality index was assigned to each of the reconstructed 2D hits. The quality index of the 2D hits was included in the track candidate multivariate classifier (MVC), having the second highest impact of all the included variables. Through inclusion as a feature of the MVC, the purity of the global track quality ranking was improved. Additionally, a measurement of the semi-inclusive $B\rightarrow D^{**}\ell\nu$ rates, (where $\ell$ denotes either an electron or a muon) were obtained from the entire 711 $fb^{-1}$ Belle data-set. $B\rightarrow D^{**}\ell\nu$ decays are of particular interest due to the uncertainty in the branching fractions calculated by previous measurements. A more precise measurement is of importance for the difference between the inclusive charmed semileptonic decay rate and the sum of the exclusive charmed semileptonic decays (the ``gap problem'') and for improving the precision of future measurements of $\mathcal{R}(D)$ and $\mathcal{R}(D^{*})$, where new physics may be observed. The tag-side $B$ meson is fully reconstructed in a hadronic decay mode with the latest \ac{BDT} tagging algorithm. The measured branching fractions are $\mathcal{B}(B^{+}\rightarrow D^{-}\pi^{+}\ell^{+}\nu) = (0.396 \pm 0.014 \pm 0.020)\% $, $\mathcal{B}(B^{+}\rightarrow D^{*-}\pi^{+}\ell^{+}\nu) = (0.509 \pm 0.019 \pm 0.030)\%$, $ \mathcal{B}(B^{0}\rightarrow \bar{D}^{0}\pi^{-}\ell^{+}\nu) = (0.364 \pm 0.020 \pm 0.020)\%$, $\mathcal{B}(B^{0}\rightarrow \bar{D}^{*0}\pi^{-}\ell^{+}\nu) = (0.589 \pm 0.030 \pm 0.040)$. Each of which are in agreement with current world averages, apart from $\mathcal{B}(B^{+}\rightarrow D^{*-}\pi^{+}\ell^{+}\nu)$, which falls below the world average by 1.8$\sigma$. Each of these measurements offer a higher precision than previous results.
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ItemExploring dark matter interactions( 2021)Understanding Dark Matter (DM) is one of the foremost goals of modern day particle physics. This thesis is focused on interactions between DM and visible matter. We examine the phenomenology that arises when existing frameworks are extended to make them theoretically consistent, and explore novel means of detection. The first chapter summarises current DM literature and experimental searches, as well as the motivation for pursuing a gauge invariant description of the interactions between the dark and visible sectors. The second chapter considers a gauge invariant portal between the dark and visible sectors, and how the phenomenology of a self consistent model described in a gauge invariant framework differs from the simplified models previously considered in the literature. We consider features of the direct detection signals characteristic of such a gauge invariant model, as well as constraints on these models arising from electroweak precision data, stability of the scalar potential, and DM relic density production. In chapter 3, we consider models in which the tree level contributions to nuclear recoil direct detection experiments are strongly suppressed. In this case, the leading order contributions arise at loop level. We investigate the size of these contributions for both the gauge invariant model presented in the previous chapter, as well as an inelastic DM model. In the fourth chapter we consider the capture of DM particles in the Sun, and their subsequent annihilation to other dark sector particles. The decay of these dark annihilation products, outside the Sun, leads to a flux of gamma rays that we compare with recent solar gamma ray measurements. We analyse this scenario in a model independent way, demonstrating excellent sensitivity to both spin-dependent and spin-independent scattering. We also determine constraints in the context of a self consistent model in which both the scattering and annihilation processes involve dark photons.
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ItemResonant Leptogenesis and Quark-Lepton Unification with Low-Scale Seesaws( 2020)The seesaw mechanism, where a hierarchy exists between the moduli of different entries of a mass mixing matrix, is a simple and theoretically attractive explanation for the observed large hierarchy between the neutral- and charged-fermion masses of the Standard Model. The simplest neutrino mass seesaw predicts that, upon diagonalisation, the physical mass states will either all be Majorana or all form pseudo-Dirac pairs. Non-minimal variants of this seesaw often generate a hybrid scenario with the physical mass states being a combination of both Majorana and pseudo-Dirac pairs. Such models often predict unique phenomenology and also allow for much lower mass scales of new physics. This thesis explores the implications such non-minimal variants can have beyond the simple generation of neutrino mass, particularly the possible role they may have in explaining the observed matter-antimatter asymmetry as well as implications for particular models of quark-lepton unification. Chapter 1 reviews the current experimental evidence for neutrino mass and discusses some possible tree-level origins. The matter-antimatter asymmetry is introduced and the conditions necessary for the dynamical generation of this observed asymmetry are reviewed. The idea of thermal leptogenesis is outlined as a simple mechanism for generating an asymmetry dynamically at an epoch between the the period of reheating and the electroweak phase transition of the early universe. Finally, the idea that quarks and leptons are related by hidden symmetries are discussed with a particular emphasis on the quark-lepton unifying Pati-Salam gauge group. In Chapter 2 we consider the leptogenesis implications for the Standard Model extended by two gauge-singlet fermions for each generation of charged lepton. We focus on the possibility of resonant scenarios without the need for inter-generational mass degeneracies and therefore do not require a possible flavour symmetry origin. The possible connection between neutrino parameters measureable in low-energy experiments and the generation of a matter-antimatter asymmetry is explored. In Chapter 3 we extend the analysis of the previous chapter and highlight how a flavour symmetry can allow for leptogenesis in a much wider region of parameter space for the extended seesaw used in \Cref{Chapter2}. The benefits of this extended seesaw, compared to the minimal seesaw scenario, when the proposed flavour symmetry is included are discussed and implications for low-energy flavour-violation experiments are explored. In Chapter 4 different possible Pati-Salam models are discussed with an emphasis on the connection between the scale of Pati-Salam breaking and the scale of heavy neutrino masses. Models allowing for the breaking scale to occur close to the electroweak scale are introduced. The dominant experimental probe of Pati-Salam is discussed and the current limits on the scale of breaking are calculated. Simple extensions of this model are proposed which both break an undesired mass degeneracy in the theory and allow for a significant reduction in the experimental limits on Pati-Salam breaking. A thorough analysis of the possible allowed parameter space in which both of these effects occur is explored and any possible connection to the symmetries of the theory is made. Chapter 5 briefly concludes.
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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.