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Bhabha scattering, the process of e⁺e⁻ → e⁺e⁻, has been studied at center-of-mass energies from 50 to 58 GeV with the AMY detector at the KEK e⁺e⁻ storage ring TRISTAN. The study is based on a data sample of 79.7 <i>pb⁻¹</i> integrated luminosity. The differential cross section of Bhabha scattering has been measured. The measured cross section is found to agree fairly well with the Standard Model of the electroweak theory. The measured cross section is also compared with various four-fermion contact interaction models, and confidence level lower limits on the composite scale, A, are determined. In addition, the limits on VV model are converted to SM-break-down scales, which indicate the validity of the SM down to the distance of order ~ 10⁻¹⁷ cm and the electron charge radius of ~ 10⁻¹⁶ cm. Attempts are made in searching for an additional boson Z'. No clear signal of the existence of a Z' boson is found up to energy of ~160 GeV/c². The effect of transverse beam polarization on Bhabha scattering is also studied. The ϕ dependence of Bhabha events are fitted to the QED prediction and found to agree with the theory. However, no quantitative conclusion on polarization effect can be drawn based on current data sample, which does not provide enough statistics. More data is being accumulated and further study should be carried out. / Ph. D.

The electron energy loss spectrometer at Virginia Tech has been modified with new momentum deflectors and various other hardware improvements. In addition to some hardware changes all the software to run the spectrometer and analyze the data has been rewritten. It was suggested by Tsuei (1990) that a nested Fermi surface could be used to explain both the fact that these materials have high superconducting transition temperatures and have linear relaxation rates. It was suggested in that same paper that EELS might be able to confirm the presence of Fermi surface nesting. We have attempted to use transmission electron energy loss spectroscopy in a novel way to search for evidence of Fermi surface nesting in the high T_c superconductor Bi₂Sr₂Ca₁Cu₂O₈. High quality single crystal samples were obtained and thinned for use in the EELS. Data was taken at low energy (<1.0eV) and momentum was scanned along three different momentum transfer directions for |q| =0.0 to 3.0Å⁻¹. The data was scaled to be Im(-1/ε). A calculation of Im(-1/ε) was performed using a tight binding model and it was found that the features associated with the Fermi surface (from this TB calculation) were too small to be seen in our spectra. We interpret this problem to be associated with the presence of thermal diffuse scattering. / Ph. D.

Le mécanisme de génération de masses des neutrinos, la nature de la matière noire et l’origine de l’asymétrie baryonique de l’Univers sont les trois questions les plus pressantes dans la physique moderne des astroparticules, qui exigent l’introduction d’une nouvelle physique au-delà du Modèle Standard. Dans cette thèse, nous nous concentrons sur ces trois questions en fournissant une solution possible en termes d'une extension minimale du Modèle Standard, constituée par l’ajout d'un ensemble de fermions stériles au contenu des champs de la théorie. Les fermions stériles sont des champs qui sont singlets de jauge et qui peuvent interagir avec les neutrinos actifs à travers des termes de mélange. Nous nous concentrons sur le mécanisme dit de l’Inverse Seesaw (ISS), qui est caractérisé par une faible échelle de la nouvelle physique (de l’ordre TeV ou inférieure) et qui peut être testé dans les installations expérimentales actuelles et futures.
Nous présentons l'analyse qui permet d’identifier les réalisations minimales de ce mécanisme et l'étude phénoménologique pour prendre en compte la masses des neutrinos légers et pour imposer toutes les contraintes expérimentales pertinentes au modèle, ainsi que les signatures expérimentales attendues. Nous montrons la viabilité de l’hypothèse que les neutrinos stériles constituent la matière noire, et les caractéristiques de cette solution dans le mécanisme minimale de l’ISS. La possibilité d’expliquer avec succès l'asymétrie baryonique à travers un processus de leptogenèse dans une réalisation testable du mécanisme est aussi adressée.
Il est important de chercher des manifestations des fermions stériles dans les expériences de laboratoire. Nous abordons ce point en faisant des prévisions sur les rapports des branchement attendus pour les désintégrations des bosons vectoriels qui violent le saveur leptonique, qui peuvent être véhiculés par les fermions stériles. Nous étudions aussi l'impact des fermions stériles sur les fits globaux des données de précision électrofaible. / The neutrino mass generation mechanism, the nature of dark matter and the origin of the baryon asymmetry of the Universe are three compelling questions that cannot be accounted for in the Standard Model of particle physics. In this thesis we focus on all these issues by providing a possible solution in terms of a minimal extension of the Standard Model, consisting in the addition of a set of sterile fermions to the field content of the theory. Sterile fermions are gauge singlet fields, that can interact via mixing with the active neutrinos. We focus on the Inverse Seesaw mechanism, which is characterised by a low (TeV or lower) new physics scale and that can be tested in current and future experimental facilities. We present the model building analysis that points towards the minimal realisations of the mechanism, and the phenomenological study in order to accommodate light neutrino masses and to impose all the relevant experimental constraints in the model, as well as the expected experimental signatures. We show the viability of the sterile neutrino hypothesis as dark matter component, together with the characteristic features of this scenario in the minimal Inverse Seesaw mechanism. The possibility of successfully accounting for the baryon asymmetry in a testable realisation of the leptogenesis mechanism is also addressed.On the other side it is important to look for manifestations of sterile fermions in laboratory experiments. We address this point by making predictions for the expected rates of rare lepton number violating decays of vector bosons, that can be mediated by sterile fermions, as well as by studying the impact of sterile fermions on global fit of electroweak precision data.

In der vorliegenden Arbeit wird die Physik von Schwer-Fermion-Systemen, die durch Lanthanid- und Aktinid-Übergangsmetallverbindungen realisiert werden, untersucht. Die Basis für eine theoretische Beschreibung bildet das Anderson-Gitter, welches das Wechselspiel freier Leitungselektronen und stark korrelierter Elektronen aus lokalisierten f-Orbitalen charakterisiert. Als Zugang zu diesem Modell wird die von Wegner vorgeschlagene Flussgleichungsmethode verwendet, ein analytisches Verfahren, welches auf der Konstruktion eines effektiven Hamilton-Operators basiert. Ein zentrales Thema dieser Arbeit ist die Beschreibung der elektronischen Struktur von Schwer-Fermion-Systemen. Insbesondere wird die Abhängigkeit statischer Größen vom Einfluss verschiedener Systemparameter betrachtet. Die Dynamik kollektiver Anregungen in Schwer-Fermion-Systemen wird an Hand der elektronischen Zustandsdichten und dynamischen magnetischen Suszeptibilitäten untersucht. / The physical properties of heavy-fermion systems are examined. These systems are mainly formed by rare earth or actinide compounds. Their essential physics can be characterized by the periodic Anderson model which describes the interplay of itinerant metal electrons and localized, but strongly correlated f-electrons. The present calculations are based on the flow equations approach proposed by Wegner. This method uses a continuous unitary transformation to derive an effective Hamiltonian of an easy to treat structure. Within this framework the electronic structure of heavy-fermion systems is calculated and the influence of external parameters is studied. Beside the derivation of static properties the density of states and dynamic magnetic susceptibilities are investigated in order to characterize the nature of collective excitations.

This investigation address the effect that pressure, p, and temperature, T, have on 4f-states of the rare-earth elements in the isostructural YbRh2Si2, EuCo2Ge2, and EuCu2Ge2 compounds. Upon applying pressure, the volume of the unit cell reduces, enforcing either the enhancement of the hybridization of the 4f-localized electrons with the ligand or a change in the valence state of the rare-earth ions. Here, we probe the effect of a pressure-induced lattice contraction on these system by means of electrical resistivity, from room temperature down to 100 mK. At ambient pressure, the electrical resistivity of YbRh2Si2 shows a broad peak at 130 K related to the incoherent scattering on the ground state and the excited crystalline electrical field (CEF) levels. At T_N = 70 mK, YbRh2Si2 undergoes a magnetic phase transition. Upon applying pressure up to p_1 = 4 GPa , T_N increases monotonously while the peak in the electrical resistivity is shifted to lower temperatures. For p &amp;lt; p_1 a different behavior is observed; namely, T_N depends weakly on the applied pressure and a decomposition of the single peak in the electrical resistivity into several shoulders and peaks occurs. Above p_2 = 9 GPa, the electrical resistivity is significantly reduced for T &amp;lt; 50 K and this process is accompanied by a sudden enhancement of T_N. Thus, our results confirm the unexpected behavior of the magnetization as function of pressure reported by Plessel et al. The small value of the magnetic ordering temperature for p &amp;lt; p_2 and the strength of the mechanism that leads to the peaks and shoulders in the electrical resistivity suggest that the f-electrons are still screened by the conduction electrons. Therefore, the observed behavior for pressures lower than p_2 might be a consequence of the competition of two different types of magnetic fluctuations (seemingly AFM and FM). Furthermore, the results suggest that a sudden change of the CEF scheme occurs at pressures higher than p_1, which would have an influence on the ground state. Additionally, a comparison of the pressure dependent features in the electrical resistivity of YbRh2Si2 with similar maxima in other isostructural YbT2X2 (T = transition metal; X = Si or Ge) compounds was performed. For the comparison, a simple relation that considers the Coqblin-Schrieffer model and the hypothesis of Lavagna et al. is proposed. A systematic behavior is observed depending on the transition metal; namely, it seems that the higher the atomic radii of the T-atom the smallest the pressure dependence of the maximum in the electrical resistivity, suggesting a weaker coupling of localized- and conduction-electrons. It is also observed that an increase in the density of conduction electrons reduces the pressure dependence of the characteristic Kondo temperature. The mechanism responsible for the sudden enhancement of T_N in YbRh2Si2 at about p_2 is still unknown. However two plausible scenarios are discussed. The Eu-ions in EuCo2Ge2 and EuCu2Ge2 have a divalent character in the range 100 mK &amp;lt; T &amp;lt; 300 K. Therefore, these systems order magnetically at T_N = 23 K and T_N = 12 K, respectively. The studies performed on EuCo2Ge2 and EuCu2Ge2 as a function of pressure suggest that a change to a non-magnetic trivalent state of the Eu-ions might occur at zero temperature for pressures higher than 3 GPa and 7 GPa, respectively. A common and characteristic feature on EuCo2Ge2 and EuCu2Ge2 is the absence of a clear first order transition from the divalent to the trivalent state of the Eu-ions at finite temperature for p &amp;gt; 3 GPa and for p &amp;gt; 7 GPa, respectively. In other isostructural Eu-based compounds, a discontinuous and abrupt change in the thermodynamic and transport properties associated to the valence transition of the Eu-ions is typically observed at finite temperatures. In contrast, the electrical resistivity of EuCo2Ge2 and EuCu2Ge2 changes smoothly as a function of pressure and temperature. The analysis of the the electrical resistivity of EuCo2Ge2 suggest that a classical critical point might be close to the AFM-ordered phase, being a hallmark of this compound. The overall temperature dependence of the the electrical resistivity of EuCo2Ge2 changes significantly at 3 GPa; therefore, it seems that the system suddenly enters to a T-dependent valence-fluctuating regime. Additionally, the pressure-dependent electrical-resistivity isotherms show a step-like behavior. Thus, it is concluded that discontinuous change of the ground state might occur at 3 GPa. The electrical resistivity of EuCu2Ge2 at high pressure is characterized by a negative logarithmic T-dependence in the pressure range 5 GPa &amp;lt; p &amp;lt; 7 GPa for T &amp;gt; T_N and by a broad peak in the pressure dependent residual resistivity, whose maximum is located at 7.3 GPa. The first behavior resembles the incoherent scattering process typical for an exchange coupling mechanism between the localized electrons and the ligand. This and the peak effect in the local 4f susceptibility observed in NMR measurements are consistent with such a coupling mechanism. Thus, it would be for the first time that a dense Eu-based compound like EuCu2Ge2 show such a behavior. Combining the results of the experiment performed at high pressures on EuCu2Ge2 with the studies performed in the EuCu2(Ge1-xSix)2 series, a crossover from an antiferromagnetically ordered state into a Fermi-liquid state for pressures higher than 7.3 GPa may be inferred from the analysis. Therefore, it may be possible that the sudden depopulation of 4f-level occur mediated by quantum fluctuation of the charge due to a strong Coulomb repulsion between the localized-electrons and the ligand. This phenomenon would explain the broad peak in the residual resistivity. To our knowledge, this would be the first Eu-based compound, isostructural to ThCr2Si2, that show such a transition as function of pressure at very low temperatures.

This thesis presents gauge fixed gluonic observable and neutral Kaon mixing matrix element measurements using nf=2+1 Domain Wall Fermion (DWF) configurations. These were generated with the Iwasaki gauge action by the RBC and UKQCD collaborations. Results from the first measurement of the QCD strong coupling with these ensembles using the triple gluon vertex are shown. We find that while a very accurate measurement of the coupling is possible using this technique, the systematic error from the perturbative matching at current lattice scales is large. We also discuss the utilisation of this method as a probe for possible Technicolor theories. The calculation of the QCD strong coupling constant from the triple gluon vertex required an implementation of a fast code to fix lattice gauge configurations. I provide details on my implementation of a parallel and optimised Fourier-accelerated algorithm for both Landau and Coulomb gauge fixing. I include the first calculation of the highly accurate W0-scale using these ensembles, allowing for percent-level scale setting. I show results from a wide variety of smearing methods and present the first gluonic measurement of different smearing radii. This thesis also details the first nf=2+1 measurement of the BSM neutral Kaon mixing renormalised matrix elements from lattice simulations with almost exact chiral symmetry in the valence sector and the sea.

In this thesis we present solutions of spontaneous compactifications of D=11, N=1 supergravity on parallelizable manifolds S^1, S^3 and S^7. In Freund-Rubin compactifications one usually obtains AdS vacua in 4D, these solutions usually sets the fermionic VEV's to zero. However giving them non zero VEV's allows us to define torsion given by the fermionic bilinears that essentially flattens the geometry giving us a vanishing cosmological constant on M_4. We further give an analysis of the consistent truncation of the bosonic sector of D=11 supergravity on a S^3 manifold and relate this to other known consistent truncation compactifications. We also consider the squashed S^7 where we check for surviving supersymmetries by analyzing the generalised holonomy, this compactification is of interest in phenomenology.

Dans cette thèse, nous nous sommes intéressés aux propriétés électroniques de deux familles de systèmes à fortes corrélations électroniques, les supraconducteurs à haute température critique (cuprates) et les fermions lourds, lorsqu'ils sont soumis à un fort champ magnétique (60 T). Depuis leur découverte en 1986, le diagramme de phase des supraconducteurs à haute température critique reste énigmatique. L'une des questions fondamentales concerne la nature de l'état normal à basse température. Dans la phase surdopée (p>0.16), on retrouve les caractéristiques d'un métal conventionnel, avec notamment une grande surface de Fermi. Dans la phase sous-dopée, les mesures d'ARPES semblent indiquer que la surface de Fermi n'est pas fermée et est seulement constituée d'arcs de Fermi déconnectés et aucune mesure expérimentale n'a permis jusqu'à présent de mettre en évidence une surface de Fermi fermée. En mesurant la résistance de Hall dans deux oxydes de cuivre de type YBCO, nous avons mis en évidence des oscillations quantiques établissant l'existence, à basse température, d'une surface de Fermi fermée et cohérente pour les cuprates sous-dopés. La faible fréquence d'oscillation mesurée indique que la surface de Fermi est constituée de petites poches, en fort contraste avec le grand cylindre observé du côté surdopé. De plus, l'observation d'un effet Hall négatif dans l'état normal à basse température suggère la présence d'électrons dans la surface de Fermi. On discute alors la possibilité qu'une reconstruction de la surface de Fermi entraîne l'apparition de petites poches d'électrons et de trous. Dans un second temps, nous présentons des mesures de magnétotransport et d'effet Nernst dans le fermion lourd URu2Si2 . Il apparaît que la mystérieuse phase ordre caché apparaissant à T <17.5 K soit caractérisée par des porteurs en faible nombre mais très mobiles, pouvant induire l'émergence d'un effet Nernst important. L'application d'un champ magnétique supérieur à 35 T à basse température déstabilise cet ordre et un état métallique plus conventionnel semble être restauré au dessus de 40 T.