Le sujet de cette thèse est la recherche du boson de Higgs du Modèle Standard par sa désintégration en quatre leptons avec l'expérience ATLAS au CERN. La théorie qui prédit le boson du Higgs est présentée et les contraintes de la théorie et des recherches directes et indirectes sont citées. L'expérience ATLAS et ses composants sont décrits et le Système de Contrôle du Détecteur pourle fonctionnement et la surveillance des blocs d'alimentation des chambres à tubes de dérive est détaillé. La reconstruction et l'identification des électrons et des muons sont résumées. Des études de performance sur les taux de faux muons, sur l'effet d'empilement sur l'isolation des muons, et sur l'efficacité de l'isolation et du paramètre d'impact des muons sont présentées. L'analyse de la désintégration de Higgs en quatre leptons est détaillée en mettant l'accent sur l'estimation du bruit de fond, les méthodes employées et les régions de contrôle utilisées. Les résultats de la recherche avec les données de 2011 à √s = 7TeV sont présentées, qui ont conduit à l'indication de l'observation du boson de Higgs. L'optimisation réalisée pour la recherche d'un boson de Higgs de basse masse est décrite et l'effet sur les données de 2011 sont présentés. L'analyse est effectuée avec les données de 2011 à √s = 8TeV collectées jusqu'au juillet et les résultats sont présentés, incluant la combinaison avec les données de 2011. Ces derniers résultats ont conduit à l'observation d'une nouvelle particule compatible avec le boson de Higgs du Modèle Standard.

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.

The study of the muon decay process μ+ → e+νeν ̄μ is a powerful constraint on the behaviour of the weak interaction, without contamination of the other, stronger, fundamental interactions. The spectrum measured from the momentum and angles of the decay positrons is parametrized using a set of four decay parameters. The purpose of the TWIST experiment is to measure these decay parameters to an unprecedented precision; an order of magnitude improvement in the uncertainties over measurements completed before the TWIST experiment. Measurements of the muon decay parameters constrain the values of a series of 19 weak coupling constants. In the standard model, V-A weak interaction, 18 of these constants are 0, while the remaining constant describes interactions between left handed particles, gV_LL= 1.The decay parameter ρ quantifies the behaviour of the spectrum with respect to momentum. According to the standard model the value of this parameter is 3/4. TWIST measured a value of ρ = 0.74991±0.00009(stat)±0.00028(sys). The measurement is limited by its systematic uncertainty, so a large focus of the experiment was on the determination and control of these uncertainties. The systematic uncertainties are derived from uncertainties in the detector construction and uncertainties in the biases generated by differences between the data and a matching Monte Carlo. Muon decay also limits the possibility of family symmetry breaking interactions. TWIST can be used to search for the possibility of muons decaying into a positron and a single unidentified neutral particle μ+ → e+X0 that does not otherwise interact with normal matter. The large momentum and angle acceptance of the TWIST spectrometer allows for searches of two body decays for masses of the X0 boson mX0 ∈ [0,80] MeV/c, with a variety of behaviours with respect to the angle of the positron track. Upper limits on massive and mass-less X0 decays are set with a 90% confidence level separately at parts per million for massive decays and parts in 10000 for mass-less decays.

It is widely believed that the Standard Model (SM) can not be a fundamental theory of the basic interactions. Originated from this fact, many new physics models have been proposed. Among them, the two Higgs doublet model (2HDM), the SM enlarged by adding one extra scalar doublet, is considered as the simplest extension of the SM. In this work, within the framework of the model III version of the 2HDM, the exclusive decay the branching ratio is calculated and discussed in various physical regions determined by model parameters. It is observed that it is possible to reach present experimental upper limits in model Finally, the avor changing top quark decay,

Using the general, model indepenent form of the effective Hamiltonian, the general expressions of the longitudinal, normal and transversal polarization asymmetries for l+ and l- for the effective B -&gt / K(K*)l+l- decays has been calculated. Existence of regions of Wilson coefficients for which the branching ratio coincides with the Standard Model result, while the lepton polarizations differ from the standard model prediction is expected. Hence, studying lepton polarizations in these regions of Wilson coefficients may be helpful in establishing new physics beyond standard model.

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.

<p dir="ltr">This thesis presents two studies in search for violation of lepton flavor universality as predicted by the Standard Model. The first searches for signs of the violation by studying beyond the Standard Model (BSM) physics models involving same flavor and opposite sign dilepton pair and bottom quarks as final states. This study was done using the dataset collected during years of 2016, 2017 and 2018, with center of mass energy $\sqrt{s} = 13$ TeV and integrated luminosity of 138 $fb^-1$. In the study, scale factors were derived in order to correct deviations between simulation and real life data, specifically for high transverse momentum muons and top\&anti-top quark background. Furthermore, lower limits of energy scale were calculated leading to exclusion of the BSM models with energy scale values lower than that of the calculated value.</p><p dir="ltr">The second study also searches for of the lepton flavor universality violation, but in the specific decay of a tauon into three muons ($\tau \rightarrow 3\mu$). In the study, graph based neural network model (GNN) designed to classify $\tau \rightarrow 3\mu$ events at the CMS detector was converted to high level synthesis (HLS) code, so that the GNN could be coded into a custom hardware such as field programmable gate arrays (FPGA) for deployment. Moreover, techniques such as pruning and quantization were applied in an attempt to make the GNN more light weight, due to strict requirements of FPGA.</p>

No description available.

The discovery of the Higgs boson by the LHC provided the last piece of the puzzle neces- sary for the Standard Model (SM) to be a successful theory of electroweak scale physics. However there exist various phenomenological reasons which serve as pointer towards the existence of physics beyond the Standard Model. For example the explanation for the smallness of the neutrino mass, baryon asymmetry of the universe, the presence of dark matter and dark energy etc. are not within purview of the Standard Model. Con- ceptual issues like the gauge hierarchy problem, weakness of gravity provide some of the theoretical motivation to pursue theories beyond the SM. We consider scenarios with warped extra-dimensions (Randall-Sundrum (RS) Model ) as our preferred candidate to answer some of the questions raised above. RS model gives an elegant geometric solution to address the hierarchy between the two fundamental scales of nature i.e. Planck scale and electroweak scale. In addition to this, the geometry of RS serves as a useful setup wherein the fermion mass hierarchy problem can also be solved. The goal of this thesis is to investigate whether RS model can be an acceptable theory of avour while at the same time serving as a solution to the hierarchy problem. In Chapter[1] we begin with a brief introduction of the SM, highlighting issues which pro- vides the necessary motivation for constructing new physics models. Various candidates of Beyond Standard Model (BSM) physics are introduced and a few preliminaries es- sential to understand frameworks with additional spatial-dimensions ( at and warped) is provided. In Chapter[2] we specialize to the case of warped extra-dimensions and motivate the need to have the SM elds in the bulk. Mathematical details related to the analysis of various spin elds (0; 12; 1 and 2) in a warped background necessary to understand relevant phenomenology is provided. The lack of knowledge of Dirac or Majorana nature of the neutrino leads to a wide variety of possibilities as far as neutrino mass generation is concerned. In Chapter[3] we focus on the leptonic sector where three cases of neutrino mass generation are consid- ered: a)Planck Scale lepton number violation (LLHH case) b) Dirac neutrinos c) Bulk Majorana mass terms. We then study the implications of each case on the charged lepton mass tting. The case with Planck scale lepton number violation in normal RS scenario requires large and negative values for the bulk mass parameters for the charged singlets cE. Dirac neutrinos and the case with Bulk Majorana mass terms give good t to data. For completeness, the ts for the hadronic sector is provided in the appendix. In Chapter[4] avour violation for each of three cases introduced in Chapter[3] is studied. For the case with Planck scale lepton number violation, the non-perturbative Yukawa coupling between the SM singlets and the KK states render the higher order diagrams incalculable. Lepton avour violation (LFV) is particularly large for the Dirac case and the bulk Majorana case for low Kaluza-Klein(KK) mass scales. We then invoke the ansatz of Minimal Flavour violation to suppress LFV with low lying KK scales and examples of avour group is provided for both cases. In Chapter[5] we present an example with a type II two Higgs doublet model applied to the LLHH case. The setup o ers a solution where LLHH scenario can be consistently realized in RS model, where the masses and mixing angles in the leptonic sector can bet with O(1) choice of bulk parameters. Assumption of global lepton number conservation (like in Dirac neutrinos) could lead to problems in theories of quantum gravity where it does not hold. This leads us to the question whether Dirac neutrinos can be naturally realized in nature. In Chapter[6] we consider the special case of bulk Majorana mass encountered in Chapter[3] where the bulk Dirac mass terms for the right handed neutrino is set to zero. We nd that this leads to a case where the e ective zero mode neutrino mass is of Dirac type with negligible e ects from the tower of Majorana states. In Chapter[7] we consider RS at the GUT scale which no longer serves as a solution to the hierarchy problem. SUSY is introduced in the bulk and the low energy SUSY serves as a solution to the hierarchy problem. Such models serve as a useful alternative to SUSY models with family symmetries (e.g. Froggatt-Nielsen Model). However the solutions to the Yukawa hierarchy problem are constrained due to anomaly cancellation conditions. In Chapter[8] supersymmetry breaking due to radion mediation in addition to brane localized sources is considered and detailed analysis of the running of soft masses and the low energy avour observables is considered for both cases separately. In Chapter[9] we conclude and present future directions.