We review several properties of string theory and introduce the Brandenberger-Vafa mechanism. This mechanism uses the intrinsic properties of strings to explain why only three of the nine spatial dimensions (ten in the context of M-theory) predicted by string theory are observable. We develop two methods used to verify the BV mechanism and later employ these methods to corroborate previous findings.

In this thesis we examine the viability of a recent proposal, known as Supersymmetric Large Extra Dimensions (SLED), for solving both the cosmological constant and the hierarchy problems. Central to this proposal is the requirement of two large extra dimensions of size r_c ~ 10 micrometres together with a low value for the higher-dimensional scale of gravity, M_* ~ 10 TeV. In order not to run into immediate conflict with experiment, it is presumed that all fields of the Standard Model are confined to a four-dimensional domain wall (brane). A realization of the SLED idea is achieved by relying on the 6D supergravity of Nishino and Sezgin (NS), which is known to have 4D-flat compactifications. When work on this thesis first began, there were many open questions which are now answered either partially or completely. In particular, we expand on the known solutions of NS supergravity, which now include: warped compactifications having either 4D de Sitter or 4D anti-de Sitter symmetry, static solutions with broken 4D Lorentz invariance, and time-dependent "scaling'' solutions. We elucidate the connection between brane properties and the asymptotic form of bulk fields as they approach the brane. Marginal stability of the 4D-flat solutions is demonstrated for a broad range of boundary conditions. Given that the warped solutions of NS supergravity which we consider are singular at the brane locations, we present an explicit regularization procedure for dealing with these singularities. Finally, we derive general formulae for the one-loop quantum corrections for both massless and massive field in arbitrary dimensions, with an eye towards applying these results to NS supergravity / Cette thèse examine la viabilité d'une approche récente, dite des Dimensions Supplémentaires Larges Supersymétriques (Supersymmetric Large Extra Dimensions, or SLED), qui propose une solution au problème de la constante cosmologique et à celui de la hiérarchie. Un aspect central de cette approche est l'existence de deux dimensions supplémentaires de grande taille r_c ~ 10 micromètres, et la faible valeur de l'échelle de gravité, M_* ~ 10 TeV. Afin d'éviter un conflit immédiat avec l'expérience, tous les champs du Modèle Standard sont supposés être confinés dans les quatre dimensions observées (i.e. sur une brane). Une implémentation de cette idée de SLED est realisée par le biais de la supergravité 6D de Nishino et Sezgin (NS), dont on sait qu'elle a des compactifications 4D-plates. Un certain nombre de questions, laissées ouvertes lorsque cette thèse à débutée, sont à présent partiellement ou complètement résolues. En particulier, nous étendons les solutions connues de la supergravité NS; elle incluent à present: compactifications déformées ayant la symétrie de Sitter ou anti-de Sitter 4D, solutions statiques avec invariance de Lorentz 4D brisée, et solutions d'échelle ("scaling'') dépendentes du temps. La relation entre les propriétés des branes et la forme asymptotique des champs de bulk lorsqu'ils approchent la brane est mise en lumière et expliquée. La stabilité marginale des solutions 4D-plate est démontrée pour une large classe de conditions de bord. Etant donné que les solutions déformées de la supergravité NS que l'on considère sont singulières à l'emplacement de la brane, une procédure explicite de rég

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A well known feature of thermal field theories is the breakdown of the standard perturbative expansion. This breakdown is due to the appearance of the Bose-Einstein distribution which is singular in the low-momentum limit. In this thesis it is argued that an effective-Lagrangian approach can be used to restore perturbative calculability. To illustrate this point, the induced thermal mass of a scalar theory is computed to both one and two-loop order. It is shown that the results can be largely determined without the explicit evaluation of Feynman graphs. This technique is then used to calculate the finite-temperature effective potential in a scalar model with spontaneous symmetry breaking. One finds that the resummed expression for the effective potential is not valid in the region of parameter space where evidence of a first-order phase transition is observed. Therefore, contrary to some of the literature, one cannot conclude that this model exhibits a first-order phase transition.

In this thesis, the semileptonic decays of heavy mesons are treated using fully relativistic kinematics. By means of an effective vertex, the effects of Fermi momentum are included both at the inclusive and at the exclusive levels, and the spins of both parent and daughter particles are taken into account. The differential decay rates with respect to the lepton energy and momentum transfer are compared with data from ARGUS and CLEO. This approach is superior to both the ACCMM and ISGW models in that it enables us to precisely calculate CKM matrix elements and decay constants in addition to the inclusive and exclusive rates. From the inclusive data, we both conclude that $ rm {v sb{ub} over V sb{cb}} = .07 pm .05$ and estimate the magnitudes of the matrix elements themselves to be $ rm vert V sb{ub} vert = .038 pm .004$ and $ rm vert V sb{cb} vert = .003 pm .002$. Of particular interest is the fact this model predicts that the exclusive decays $B to Dl sbsp{ mu}{-}$ and $B to D sp*l sbsp{ mu}{-}$ together only account for about 60% of the total rate and thus agrees with data that suggests that as much as 40% of the total B semileptonic decay occurs through other channels. The B and D decay constants are, meanwhile, estimated to be $f sb{B}$ = f$ sb{D} approx 0.4$ GeV. After taking the heavy quark limit we also obtain $ rho sp2 = .9 pm .4$.

Data taken with the ARGUS detector at DESY, Hamburg, specifically BB pairs produced from $e sp+e sp-$ collisions at the energy of the $ Upsilon$(4S) resonance, are used to investigate the decay channel $B sp- to rho sp0 ell sp- bar nu.$ Observation of a signal would be conclusive evidence that the CKM matrix element $V sb{ub}$ is non-zero, a necessary condition for the validity of the Kobayashi-Maskawa explanation for CP violation. The recoil mass technique is employed to try to isolate signal events. Monte Carlo data are used to model the signal background, which is dominated by reasonably well understood $b to c$ decays. Using the model of Wirbel, Stech, and Bauer, a model-dependent upper limit of 1.6 $ times$ 10$ sp{-2}$ is placed on the value of $ vert V sb{ub} vert$ at 90% confidence. The results, however, suggest that further study of the $b to c$ background is warranted. In particular, B meson transitions to states with higher mass than the $D sp*$ may contribute significantly.

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We study the production of hadrons by a polarized photon beam on a longitudinally polarized proton target, including the contributions due to the structure of the photon. / We predict cross sections sizeable enough for good tests and show that, in certain kinematic domains, single hadron production is a good probe of the size of the polarized gluon distribution $ Delta$g. / We find that a special combination of cross sections for production of two large $p sb{T}$ hadrons provides a particularly clean probe of the size of $ Delta$g.

The inclusive pizero and direct photon productions by 300 GeV/c $ pi sp-$ and $ pi sp+$ beams on a lithium target, were measured using the E705 spectrometer at Fermilab. The cross sections were determined by analyzing a fraction (20%) of the data recorded by the experiment during the 1987-1988 running period. The photons were measured by a high resolution electromagnetic calorimeter which consisted of scintillation and lead glass blocks. A fast trigger was designed and implemented to select events with high transverse energy depositions in the calorimeter. / The invariant cross sections are presented as a function of the transverse momentum and the Feynman-x in the range between 4 to 7 GeV/c and $-$0.25 and 0.35 respectively. The results are compared to the measurements made by other experiments and to theoretical predictions within the framework of Quantum ChromoDynamics.

This thesis presents the results of a search for the decay B 0 → J/Ψγ at the BABAR experiment. This is expected to be a very rare decay, with a branching fraction of the order of 10 -8, too low to be measured at BABAR; so the aim is instead to determine a 90% CL upper limit for beta(B 0 → J/Ψγ). No prior limit has been set in this channel. The analysis uses the complete data set from Runs 1, 2, and 3, comprising a total integrated onpeak (offpeak) luminosity of 113.1 (12.0) fb-1; and both SP4 and SP5 Monte Carlo samples for optimization and background studies. We obtain a 90% CL upper limit on the branching fraction for B0 → J/Ψγ of 1.2 x 10-6.