School of Physics - Theses
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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.
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ItemLearning invariant representations with applications to high-energy physics( 2020)In searches for new physics in high-energy physics, experimental analyses are primarily concerned with physical processes which are rare or hitherto unobserved. To claim a statistically significant discovery or exclusion of new physics when studying such decays, it is necessary to maintain an appropriate signal to noise ratio. This makes systems capable of efficient discrimination of signal from datasets overwhelmingly dominated by background events an important component of modern experimental analyses. However, na\"ive application of these methods is liable to raise poorly understood systematic effects which may ultimately degrade the significance of the final measurement. To understand the origin of these systematic effects, we note that there are certain protected variables in experimental analyses which should remain unbiased by the analysis procedure. Variables that the input parameters of models of new physics are strongly dependent upon and variables used to model background contributions to the total measured event yield fall into this category. Systems responsible for separating signal from background events achieve this by sampling events with signal-like characteristics from all candidate events. If this procedure introduces sampling bias into the distribution of protected variables, this introduces systematic effects into the analysis which are difficult to characterize. Thus it is desirable for these systems to distinguish between signal and background events without using information about certain protected variables. Beyond high-energy physics, building systems that make decisions independent of certain protected or sensitive information is an important theme in the real-world application of machine learning and statistics. We address this task as an optimization problem of finding a representation of the observed data that is invariant to the given protected quantities. This representation should satisfy two competing criteria. Firstly, it should contain all relevant information about the data so that it may be used as a proxy for arbitrary downstream tasks, such as inference of unobserved quantities or prediction of target variables. Secondly, it should not be informative of the given protected quantities, so that downstream tasks are not influenced by these variables. If the protected quantities to be censored from the intermediate representation contain information that can improve the performance of the downstream task, it is likely that removing this information will adversely affect this task. The challenge lies in balancing both objectives without significantly compromising either requirement. The contribution of this thesis is a new set of methods for addressing this problem. This thesis is divided into two parts, which are largely independent of one another. The first part of this thesis is about constraining the optimization procedure by which the representation is learnt to reduce the informativeness of the representation of the given protected quantities, such that the representation is invariant to changes in these quantities. The second part of this thesis approaches the problem from a latent variable model perspective, in which additional unobserved (latent) variables are introduced which explain the interaction between different attributes of the observed data. These latent variables can be interpreted as a more fundamental, compact lower-dimensional representation of the original high-dimensional unstructured data. By constraining the structure of this latent space, we demonstrate we can isolate the influence of the protected variables into a latent subspace. This allows downstream tasks to only access a relevant subset of the learned representation without being influenced by protected attributes of the original data. The feasibility of our proposed methods is demonstrated through application to a challenging experimental analysis in precision flavor physics at the Belle II experiment - the study of the $b \rightarrow s \gamma$ transition, a sensitive probe of potential new physics.
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ItemMeasurement of Direct CP Asymmetry and Branching Fraction in B0→D0𝜋0 and B+→D0𝜋+ at the Belle Experiment( 2019)This thesis describes the measurement of direct CP asymmetry and branching fraction for the hadronic B decays B0 -> D0 pi0 an B+ -> D0 pi+. The study uses the full dataset of 711 fb^(-1) collected at the Y(4S) resonance by the Belle experiment at the KEKB accelerator in Tsukuba, Japan. Event reconstruction, background suppression and modelling are first studied using Monte Carlo simulations, before yield and direct CP asymmetry are extracted in a three-dimensional unbinned extended maximum likelihood fit. B+ -> D0 pi+ is measured first as the control mode to validate the methodology, before same techniques are used on B0 -> D0 pi0 . The measured branching fractions and direct CP asymmetries are: Br(B0 -> D0 pi0) = (2.69 +/- 0.06 +/- 0.09) x 10^(-4), A_CP(B0 -> D0 pi0) = (0.10 +/- 2.05 +/- 1.29) x 10^(-2), Br(B+ -> D0 pi+) = (4.53 +/- 0.02 +/- 0.14) x 10^(-3), A_CP(B+ -> D0 pi+) = (0.19 +/- 0.36 +/- 0.60) x 10^(-2), for B0 -> D0 pi0 and B+ -> D0 pi+ respectively, where the first uncertainty is statistical and the second is systematic. The represents the world’s first measurement of direct CP asymmetry for B0 -> D0 pi0. This measurement of branching fraction of B0 -> D0 pi0 and B+ -> D0 pi+, and direct CP asymmetry of B+ -> D0 pi+ are the most precise to date, and consistent with the current world average values.
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ItemSearching for signals of Dark Matter produced with top quark pairs using the ATLAS detector( 2019)Understanding the nature of Dark Matter is a key goal in modern physics. The observed gravita-tional interactions of galaxies and galactic clusters, along with theories of structure formation in the early universe, indicate the existence of Dark Matter. Evidence of the specific nature of Dark Matter remains elusive however. Particle collider experiments search for evidence of Dark Matter production within energetic proton collisions. One strategy employed in this field is to make minimal assumptions about new particles and couplings to Standard Model particles, in order to explore the range of possibilities without being overly constrained by narrow assumptions. This thesis focuses on the assumption that Dark Matter couples strongly to the heavier quarks, which motivates searching for processes where it is produced in association with pairs of top quarks. An analysis is presented on the 2015 and 2016 “Run 2" dataset taken with the ATLAS detector, consisting of 36.1 fb -1 of proton-proton collisions at the Large Hadron Collider. This analysis studies the hypothesis of Dark Matter production in conjunction with hadronically decaying top quarks. No excess above the estimated Standard Model backgrounds is observed, and constraints on the allowed cross-sections are presented. When making minimal assumptions about the nature of Dark Matter, scalar mediator masses below 20 GeV are excluded. These results are then translated to more specific and complete Two Higgs Doublet models that feature for example in Supersymmetry that also predict the same final states, and constraints on the parameter space of these models are presented.