<p dir="ltr">The Mu2e experiment is searching for Beyond-Standard-Model, Charged Lepton Flavor Violation (CLFV) in the muon capture reaction μ⁻+ Al → e⁻ + Al. To compare the accessible energy scale of this experiment, the Large Hadron Collider (LHC) is capable of observing new physics at the few TeV mass scale. However, by searching for μ-to-e conversion at a branching ratio sensitivity of 10⁻¹⁷, Mu2e will probe for new physics at mass scales up to 10³ ∼ 10⁴TeV, far beyond the reach of any planned accelerator and surpassing the current world’s best limit by 10⁴ times. In addition, there is no competing Standard Model process that produces this decay to a branching ratio level < 10⁻⁵⁴. To report a reliable result, the number of stopped muons will be normalized to 10% precision utilizing two γ-ray transitions and one x-ray atomic transition. The first, directly proportional to the CLFV signal, is the 1808.7 keV γ-ray emitted promptly in the muon capture process. The second, the 2p→1s atomic transition of muonic aluminum, is the 346.8 keV x-ray line. The third, is the 844 keV γ-ray from the β-decay process. These signals need to be measured in the presence of an energy flux background of 3.2 x 10⁸TeV/sec, consisting of muons, electrons, neutrons, x-rays and γ-rays. Here, two com- 11 photon counting detectors are used in the luminosity measurement. One of them, the LaBr₃ detector, is capable of high rate operation up to and above 800 kcps and energy resolution of 0.7%, producing highly accurate statistical measurements. The other, the HPGe detector is capable of energy resolution of 0.1%, with limited rate capability ∼ 70 kcps, yet producing measurements having low systematic error. Once signals are found within the background, corrections must be understood and applied including: geometric factors, detector efficiency, branching ratio of the observed physics pro- cesses, signal loss due to propagation to the detector, interfering lines, event loss due to pile-up, event loss due to algorithm miscalculation, time evolution of the signal, and others. The normalization measurement will be reported out in real time every 5 to 10 minutes, and a comprehensive off-line analysis will be undertaken using merged data sets.</p>

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Semileptonic B decays of the type Bq⁰→Dq⁻μ⁺ν (where Dq⁻→K⁻K⁺pie⁻) are selected and their lifetimes are corrected using a statistical simulation-based correction called the k-factor. Using 1 fb⁻¹ of LHCb data the B⁰ and Bs⁰ mixing frequencies are measured to be Deltamd = ( 0.503 ± 0.011 (stat) ± 0.013 (syst) ) ps⁻¹ and Deltams = ( 17.93 ± 0.22 (stat) ± 0.15 (syst) ) ps⁻¹. We exclude the null hypothesis of no mixing for the B⁰ and Bs⁰ by 5.8 and 13.0 standard deviations respectively. This is the first observation of Bs Bsbar mixing using only semileptonic B decays. The lepton flavour violating decay D⁰→eμ is searched for, using tagged D⁰ decays from D*→D⁰pie, and the measurement is normalised using D⁰→K⁻pie⁺ decays. No evidence is seen of an excess over the expected background and so a limit is placed B(D⁰→eμ) < 1.3×10⁻⁸ at a 90% confidence level using 3 fb⁻¹ of LHCb data. This improves the previous measurement by a factor of 20 and is the world's best measurement. Possible upgrades to the LHCb VELO detector are simulated and aspects of the upgraded detector are optimised to ensure that all tracks within the angular acceptance can be detected with high precision. Finally the simulated performance of the current and upgraded VELO detectors are compared.

A search is done on the entire BaBar data set for the neutrino-less tau decay tau->e gamma. No evidence for the decay is found and a 90% confidence level upper limit is determined to be 3.3 * 10^(-8) including systematic uncertainty. This measurement is a factor of three improvement from the previous best result from BaBar.

The Standard Model has granted exquisite power to predict the behaviour of high-energy particle collisions. It is not, however, without conceptual and empirical weaknesses. Several theories have been proposed which aim to resolve these difficulties. This thesis describes searches for two such theories: models of extra spatial dimensions, and supersymmetry. The Large Hadron Collider has extended the frontiers of energy and intensity in particle physics. In 2015, the LHC resumed proton–proton collisions at a centre-of-mass energy of 13 TeV. This increase over previous operation grants an enhancement in sensitivity to many processes beyond those of the Standard Model. Extra-dimensional theories address the hierarchical nature of the Standard Model. The lowered fundamental scale of gravity in these models allows a rich phenomenology at energies which may be accessible to the LHC. Some models predict the formation of microscopic black holes, which are the target of an analysis of collisions recorded by the ATLAS detector in 2015. No significant deviations from Standard Model predictions were observed. The constraints inferred on the parameters of the model are a significant advance on previous results. Lepton flavour is conserved in the Standard Model. This is not the result of a known fundamental symmetry, however. The latter part of this thesis proposes a search examining asymmetries of charge and flavour in the eμ final state. Models of supersymmetry with an R-parity-violating λ'₂₃₁ coupling are taken as motivation. The strategy is developed using collision data recorded by ATLAS in 2015 and 2016, and the most significant biasing effects are addressed.

This thesis presents work carried out using data from the LHCb experiment during the first three years of data taking, 2010 - 2012. A study of the effects of radiation damage on the silicon sensors of the LHCb Vertex Locator is performed, with an emphasis on the implications for the long term performance of the detector. Following three years of operation the sensors have received a maximum delivered neutron equivalent fluence of approximately 1.6E12 per square centimeter, leading to a number of radiation induced effects. In particular the change in charge collection efficiency and signal/noise with fluence is compared to theoretical expectations, and the current trends are extrapolated to the fluences expected at the end of the LHCb detector lifetime. The development of an unexpected effect due to the structure of the routing lines in the sensors is described in detail. Searches for lepton flavour and baryon number violating decays of the tau lepton using the 2011 LHCb dataset are described. Observation of any lepton flavour or baryon number violation would be an unambiguous sign of new physics, whilst setting improved limits helps to constrain a number of Beyond the Standard Model theories. First LHCb limits are set on the branching fractions of the decays tau- to mu- mu+ mu-, tau- to anti-proton mu+ mu- and tau- to proton mu- mu-, with these results also representing the first limits on lepton flavour violating tau decays at a hadron collider. The limit on tau- to mu- mu+ mu- is expected to approach the world's best result from Belle in the coming years whilst the tau- to anti-proton mu+ mu- and tau- to proton mu- mu- results constitute the first limits on the branching fractions of these decays. The future prospects for these measurements with further data are briefly described.

In this doctoral thesis, we study both low- and high-energy observables related to massive neutrinos. Neutrino oscillations have provided indisputable evidence in favour of non-zero neutrino masses and mixings. However, the original formulation of the Standard Model cannot account for these observations, which calls for the introduction of new Physics. Among many possibilities, we focus here on the inverse seesaw, a neutrino mass generation mechanism in which the Standard Model is extended with fermionic gauge singlets. This model offers an attractive alternative to the usual seesaw realisations since it can potentially have natural Yukawa couplings (O(1)) while keeping the new Physics scale at energies within reach of the LHC. Among the many possible effects, this scenario can lead to deviations from lepton flavour universality. We have investigated these signatures and found that the ratios R_K and R_π provide new, additional constraints on the inverse seesaw. We have also considered the embedding of the inverse seesaw in supersymmetric models. This leads to increased rates for various lepton flavour violating processes, due to enhanced contributions from penguin diagrams mediated by the Higgs and Z bosons. Finally, we also found that the new invisible decay channels associated with the sterile neutrinos present in the supersymmetric inverse seesaw could significantly weaken the constraints on the mass and couplings of a light CP-odd Higgs boson.

Dans cette thèse, nous étudions à la fois des observables de basse et de haute énergie liée à la présence de neutrinos massifs. Les oscillations de neutrinos ont apporté des preuves indiscutables en faveur de l'existence de masses non-nulles et de mélanges. Néanmoins, la formulation originale du Modèle Standard ne permet pas d'expliquer ces observations, d'où la nécessité d'introduire de nouveaux modèles. Parmi de nombreuses possibilités, nous nous concentrons ici sur le seesaw inverse, un mécanisme générant des neutrinos massifs par l'ajout de fermions singulets de jauge au Modèle Standard. Ce modèle offre une alternative attractive aux réalisations habituelles du seesaw puisqu'il a des couplages de Yukawa potentiellement naturels (O(1)) tout en conservant l'échelle de la nouvelle physique à des énergies accessibles au LHC. Parmi de nombreux effets, ce scénario peut générer de larges écarts à l'universalité leptonique. Nous avons étudié ces signatures et trouvé que les rapports R_K et R_π constituent de nouvelles contraintes pour le seesaw inverse. Nous nous sommes aussi intéressé à l'intégration de l'inverse seesaw dans différents modèles supersymétriques. Ceci conduit à une augmentation de la section efficace de divers processus violant la saveur leptonique du fait de contributions plus importantes venant des diagrammes pingouins comportant un boson de Higgs ou Z. Finalement, nous avons aussi trouvé que les nouveaux canaux de désintégration ouverts par la présence de neutrinos stériles dans les modèles de seesaw inverse supersymétriques peuvent significativement relaxer les contraintes sur la masse et les couplages d'un boson de Higgs CP-impair. / In this doctoral thesis, we study both low- and high-energy observables related to massive neutrinos. Neutrino oscillations have provided indisputable evidence in favour of non-zero neutrino masses and mixings. However, the original formulation of the Standard Model cannot account for these observations, which calls for the introduction of new Physics. Among many possibilities, we focus here on the inverse seesaw, a neutrino mass generation mechanism in which the Standard Model is extended with fermionic gauge singlets. This model offers an attractive alternative to the usual seesaw realisations since it can potentially have natural Yukawa couplings (O(1)) while keeping the new Physics scale at energies within reach of the LHC. Among the many possible effects, this scenario can lead to deviations from lepton flavour universality. We have investigated these signatures and found that the ratios R_K and R_π provide new, additional constraints on the inverse seesaw. We have also considered the embedding of the inverse seesaw in supersymmetric models. This leads to increased rates for various lepton flavour violating processes, due to enhanced contributions from penguin diagrams mediated by the Higgs and Z bosons. Finally, we also found that the new invisible decay channels associated with the sterile neutrinos present in the supersymmetric inverse seesaw could significantly weaken the constraints on the mass and couplings of a light CP-odd Higgs boson.

<p>In this work, we consider the addition of a single neutral massive vector boson to the Standard Model (SM). This boson, which we refer to as N⁰, induces dipolar transitions between electrons and muons. We obtain bounds on the strength of its coupling and its mass: from the scattering process e+e- to mu+mu-; from its contribution to muonium-antimuonium oscillations; and from its possible contribution to the rare muon decay mu- to e+e-e-. In particular, we examine the two cases where the mediator is both heavy and light compared with the scattering energies for, and place constraints on the relevant parameters based on their contributions to the cross section and the forward-backward asymmetry. For muonium-antimuonium oscillations, we consider only the case where the mediator is heavy compared to all other scales, reducing its effect to an effective contact interaction. Finally, we consider an SU(2) invariant theory from which the N⁰ interaction emerges, and find that flavour diagonal interactions also emerge, giving a tree-level path for the decay mu- to e+e-e-. We find that the heavy N⁰ is not strongly constrained by this contribution, while the light N⁰ is very strongly constrained by it. Very generally, we find that the heavy N⁰ is much less constrained than other lepton flavour violating processes, while the constraints on the light N0 vary in strength between processes.</p> / Master of Science (MSc)