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The measurement of the ratio of the branching fractions of Υ (3S) decays into τ leptons over dimuons (R τ /μ = B(Υ (3S) → τ + τ − )/B(Υ (3S) → μ + μ − )) is a test of lepton universality. A violation of lepton universality would be evidence of new physics (and possibly of a light CP-odd Higgs boson). A sample of Υ (3S) decays (2.408 fb −1 ) collected with the B A B AR detector at the SLAC National Accelerator Laboratory was used to determine that the ratio R τ /μ is R τ /μ = 1.0385 ± 0.034 ± 0.019. Using the remaining blinded data sample (corresponding to an integrated luminosity of 25.6 fb −1 ) the estimated statistical sensitivity will be 1.1 % and the estimated systematic uncertainty of R τ /μ is 1.9 %. Prior to this work, previous measurements of R τ /μ had an estimated total precision of 10 %. / Graduate

Using data collected with the BABAR detector at the SLAC PEP-II electron-positron storage ring operating at a centre-of-mass energy near 10.58 GeV, the branching fractions $B(\tau^- \to \pi^-\pi^-\pi^+\nu_\tau) =(8.83\pm0.01\pm0.13)\%$, $B(\tau^- \to K^-\pi^-\pi^+\nu_\tau) =(0.273\pm0.002 \pm 0.009)\%$, $B(\tau^- \to K^-\pi^-K^+\nu_\tau) =(0.1346 \pm 0.0010 \pm 0.0036)\%$, and $B(\tau^- \to K^-K^-K^+\nu_\tau) =(1.58 \pm 0.13 \pm 0.12)\times10^{-5}$ are measured where the uncertainties are statistical and systematic, respectively. The invariant mass distribution for the $\tau^- \to \pi^-\pi^-\pi^+\nu_\tau$, $\tau^- \to K^-\pi^-\pi^+\nu_\tau$, $\tau^- \to K^-\pi^-K^+\nu_\tau$, and $\tau^- \to K^-K^-K^+\nu_\tau$ decays are unfolded to correct for detector effects. A measurement of $B(\tau^- \to \phi\pi^-\nu_\tau)=(3.42\pm0.55\pm0.25)\times10^{−5}$, a measurement of $B(\tau^- \to \phi^-K^-\nu_\tau) =(3.39\pm0.20\pm0.28)\times10^{−5}$ and an upper limit on $B(\tau^- \to K^-K^-K^+\nu_\tau [ex.\phi]) < =2.5\times10^{−6}$@90\%CL are determined from a binned maximum likelihood fit of the $\tau^- \to K^-\pi^-K^+\nu_\tau$ and $\tau^- \to K^-K^-K^+\nu_\tau$ $K^+K^−$ invariant mass distributions. The branching ratio $B(\tau^- \to K^-\nu_\tau)/B(\tau^- \to\pi^-\nu_\tau)$ is measured to be $(6.531\pm0.056\pm0.093)\times10^{−2}$ from which $|V_{us}|$ is determined to be $0.2255 \pm 0.0023$. The branching ratio $B(\tau^- \to \mu^-\nu_\tau\bar{\nu}_\mu)/B(\tau^- \to e^-\nu_\tau\bar{\nu}_e) =(9.796 \pm 0.016 \pm 0.035)\times10^{−1}$ is measured enabling a precision test of the Standard Model assumption of charged current lepton universality, $g_{\mu}/g_{e}=1.0036 \pm 0.0020$. The branching ratios $B(\tau^- \to K^-\nu_\tau)/B(\tau^- \to e^-\nu_\tau\bar{\nu}_e)=(3.882 \pm 0.032 \pm 0.056)\times10^{−2}$, and $B(\tau^- \to\pi^-\nu_\tau)/B(\tau^- \to e^-\nu_\tau\bar{\nu}_e)=(5.945 \pm 0.014 \pm 0.061)\times10^{−1}$ are measured which provide additional tests of charged current lepton universality, $(g_{\tau}/g_{\mu})_{\pi}= 0.9856 \pm 0.0057$ and $(g_{\tau}/g_{\mu})_{K}= 0.9827 \pm 0.0086$ which can be combined to give $(g_{\tau}/g_{\mu})_{\pi/K}= 0.9850 \pm 0.0054$. Any deviation of these measurements from the expected Standard Model values would be an indication of new physics.

Using data collected with the BABAR detector at the SLAC PEP-II electron-positron storage ring operating at a centre-of-mass energy near 10.58 GeV, the branching fractions $B(\tau^- \to \pi^-\pi^-\pi^+\nu_\tau) =(8.83\pm0.01\pm0.13)\%$, $B(\tau^- \to K^-\pi^-\pi^+\nu_\tau) =(0.273\pm0.002 \pm 0.009)\%$, $B(\tau^- \to K^-\pi^-K^+\nu_\tau) =(0.1346 \pm 0.0010 \pm 0.0036)\%$, and $B(\tau^- \to K^-K^-K^+\nu_\tau) =(1.58 \pm 0.13 \pm 0.12)\times10^{-5}$ are measured where the uncertainties are statistical and systematic, respectively. The invariant mass distribution for the $\tau^- \to \pi^-\pi^-\pi^+\nu_\tau$, $\tau^- \to K^-\pi^-\pi^+\nu_\tau$, $\tau^- \to K^-\pi^-K^+\nu_\tau$, and $\tau^- \to K^-K^-K^+\nu_\tau$ decays are unfolded to correct for detector effects. A measurement of $B(\tau^- \to \phi\pi^-\nu_\tau)=(3.42\pm0.55\pm0.25)\times10^{−5}$, a measurement of $B(\tau^- \to \phi^-K^-\nu_\tau) =(3.39\pm0.20\pm0.28)\times10^{−5}$ and an upper limit on $B(\tau^- \to K^-K^-K^+\nu_\tau [ex.\phi]) < =2.5\times10^{−6}$@90\%CL are determined from a binned maximum likelihood fit of the $\tau^- \to K^-\pi^-K^+\nu_\tau$ and $\tau^- \to K^-K^-K^+\nu_\tau$ $K^+K^−$ invariant mass distributions. The branching ratio $B(\tau^- \to K^-\nu_\tau)/B(\tau^- \to\pi^-\nu_\tau)$ is measured to be $(6.531\pm0.056\pm0.093)\times10^{−2}$ from which $|V_{us}|$ is determined to be $0.2255 \pm 0.0023$. The branching ratio $B(\tau^- \to \mu^-\nu_\tau\bar{\nu}_\mu)/B(\tau^- \to e^-\nu_\tau\bar{\nu}_e) =(9.796 \pm 0.016 \pm 0.035)\times10^{−1}$ is measured enabling a precision test of the Standard Model assumption of charged current lepton universality, $g_{\mu}/g_{e}=1.0036 \pm 0.0020$. The branching ratios $B(\tau^- \to K^-\nu_\tau)/B(\tau^- \to e^-\nu_\tau\bar{\nu}_e)=(3.882 \pm 0.032 \pm 0.056)\times10^{−2}$, and $B(\tau^- \to\pi^-\nu_\tau)/B(\tau^- \to e^-\nu_\tau\bar{\nu}_e)=(5.945 \pm 0.014 \pm 0.061)\times10^{−1}$ are measured which provide additional tests of charged current lepton universality, $(g_{\tau}/g_{\mu})_{\pi}= 0.9856 \pm 0.0057$ and $(g_{\tau}/g_{\mu})_{K}= 0.9827 \pm 0.0086$ which can be combined to give $(g_{\tau}/g_{\mu})_{\pi/K}= 0.9850 \pm 0.0054$. Any deviation of these measurements from the expected Standard Model values would be an indication of new physics.

Les courants neutres changeant la saveur b -> sl⁺l⁻ ne sont autorisés dans le Modèle Standard (MS) qu'au niveau des boucles.Par conséquent, ils sont sensibles aux éventuels effets de la Nouvelle Physique (NP) au-delà du MS.Dans le MS, les transitions b -> se⁺e⁻ et b -> sμ⁺μ⁻ ont la même probabilité, cette propriété est appelée l'universalité du couplage aux leptons (UL).Tester l'UL dans les désintégrations de hadrons beaux a récemment été un domaine prometteur pour les recherches de NP. Des tensions par rapport aux prédictions du MS, de l'ordre de 2,5 déviations standards, ont été observées dans les désintégrations rares B -> K(*) l⁺l ⁻. À ce jour, les tests de l'UL ont été effectués uniquement sur des désintégrations de mesons beaux.Cette thèse présente un premier test de la UL dans la désintégration rare du baryon Lambdab -> pK l⁺l⁻ , en utilisant les données collectées par l'expérience LHCb au CERN en 2011, 2012 et 2016. La sensibilité statistique attendue est d'environ 15%, tandis que la valeur centrale reste aveuglée.La première mesure du rapport de branchement de la désintégration Lambdab -> pKμ⁺μ⁻ est effectuée avec une précision statistique de 5,3%.En outre, cette thèse présente la première observation de la désintégration Lambdab -> pKe⁺e ⁻ , ainsi que la mesure de son rapport de branchement.Enfin, la désintégration Xib0 -> pKJ/ψ est observée et étudiée pour la première fois. / Flavor-changing neutral-current b ->sl⁺l⁻ transitions are forbidden at tree level in the Standard Model (SM), and can only occur at loop level. Therefore, they are sensitive to possible New Physics (NP) effects beyond the SM. In the SM, the transitions b ->se⁺e⁻ and b -> sμ⁺μ⁻ have the same probability to happen, this property is called lepton universality (LU). Probing LU in b-hadron decays has been recently a promising area for NP searches. Tensions with respect to the SM predictions at the level of about 2.5 standard deviations have been observed in rare B -> K(*)l⁺l ⁻ decays. To date, tests of LU have been performed only in decays of b-mesons.This thesis presents a first test of the LU in rare b-baryon decays Lambdab -> pK l⁺l⁻ using the data collected by the LHCb experiment at CERN, during 2011, 2012 and 2016 data-taking periods. The expected statistical sensitivity is about 15%, while the central value remains blind at the moment. The first measurement of the branching fraction of the decay Lambdab -> pKμ⁺μ⁻ is performed with the statistical precision of 5.3%.In addition, this thesis presents the first observation of the Lambdab -> pKe⁺e ⁻ decay, and the measurement of its branching fraction.Finally, a suppressed decay Xib0 -> pKJ/ψ is observed and studied for the first time.

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.