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.

We examine the sensitivity of the ATLAS detector to extra-dimensional scalars in scenarios having the extra-dimensional Planck scale in the TeV range and n = 2 large extra dimensions. Such scalars appear as partners of the graviton in higher-dimensional supersymmetric theories. Using first the scalar's lowest-dimensional effective couplings to quarks and gluons, we compute the rate of production of a hard jet together with missing energy. We find a nontrivial range of bulk scalar couplings for which ATLAS could observe a signal, and in particular, higher sensitivity to couplings to gluons than to quarks. / Bulk scalar emission increases the missing-energy signal by adding to graviton production, and so complicates the inference of the extra-dimensional Planck scale from the observed rate of jet + EmissT . Because bulk scalar differential cross sections resemble those for gravitons, it is unlikely that these can be experimentally distinguished should a missing energy signal be observed. However, given, for example, the Supersyminetric Large Extra Dimension (SLED) scenario, which can provide a framework for a solution to the cosmological constant problem, the D-dimensional Planck scale is approximately fixed and so is the graviton production rate. An excess of events would then be due to graviton superpartners like bulk scalars. / Next, we identify the lowest-dimension interaction which is possible between Standard Model brane fields and bulk scalars. The lowest-dimension interaction is unique and involves a trilinear coupling between the Standard Model Higgs and a bulk scalar. We compute its influence on Higgs physics at ATLAS and identify how large a coupling can be detected at the LHC. Besides providing a potentially interesting signal in Higgs searches, such couplings provide a major observational constraint on 6D large-extra-dimensional models with scalars in the bulk. / Finally we consider the Universal Extra Dimensions scenario in which all the SM fields propagate. Tree-level KK number conservation dictates that the associated KK excitations cannot be singly produced. We recalculate the cross sections obtained by Macesanu et al. for the direct production of KK excitations of the gluon, g*, and two distinct towers of quarks, q• and q°, in proton-proton collisions at the LHC. According to the SLED scenario, these KK states will quickly decay in a parton plus a graviton. We find that the LHC mass bound for KK quark and gluon final states is 2675 GeV at its nominal luminosity of 100 fb-1.

In this thesis, we discuss two important aspects of cosmological inflation, which is a prominent part of modern cosmology. / Since inflation typically occurs at ultrahigh energy scales, string theory is a popular framework to achieve inflation. We investigate the fine-tuning problem in string theoretic inflation models, based on a brane-antibrane potential. We phenomenologically quantify the degree of fine-tuning by defining an effective volume in the part of parameter space which is consistent with observational constraints. Moreover, we use Monte Carlo methods to illustrate that the degree of fine-tuning can be substantially alleviated. / We also explore a novel mechanism of dynamical tuning of a flat potential in brane-antibrane inflation. Because of competing effects between its interactions with D7-branes and anti-D3-branes, a stack of D3-branes gives rise to a local minimum in the potential. As branes successively tunnel out, the potential barrier is eventually replaced by a flat inflection point, around which the remaining branes easily inflate. We give an example where this time-dependent flattening of the inflaton potential is possible. / Another important aspect of inflation is the primordial power spectrum. In this thesis, we consider a primordial power spectrum with a k^3 component, which can be generated through different mechanisms and is a good representative of the generic spikelike features. We explore the possibility that the features could be observably large. Although there is no strong evidence, large spikes are not ruled out by the data, and we give constraints on the k^3 component. / Dans cette thèse, nous discutons deux aspects importants de l'inflation cosmologique, qui est une partie importante de la cosmologie moderne. / Dans la mesure où l'inflation se produit généralement à des échelles ultra-haute d'énergie, la théorie des cordes est un cadre populaire pour atteindre l'inflation. Nous étudions le problème de mise au point dans les modèles d'inflation provenant de la théorie des cordes, basé sur un potentiel de Brane-antibrane. Phénoménologiquement, nous quantifions le degré de mise au point en définissant un volume effectif dans la partie de l'espace des paramètres qui est compatible avec les contraintes observationnelles. En outre, nous utilisons des méthodes de Monte Carlo pour illustrer le fait que le degré de mise au point peut être considérablement allégé. / Nous explorons aussi un nouveau mécanisme de réglage dynamique d'un potentiel plat dans l'inflation Brane-antibrane. En raison des effets de compétition entre ses interactions avec les D7-branes et les anti-D3-branes, une pile de D3-branes donne lieu à un minimum local du potentiel. Comme les branes franchissent la barrière de potentiel l'une après l'autre, cette dernière est finalement remplacée par un point d'inflexion plat, autour duquel les branes restantes s'étendent avec facilité. Nous donnons un exemple où cet aplatissement du potentiel de l'inflaton qui dépend du temps est possible. / Un autre aspect important de l'inflation est le spectre de puissance primordial. Dans cette thèse, on considère un spectre de puissance primordial avec un terme en k^3 pouvant être généré par différents mécanismes et qui est un bon représentant des caractéristiques génériques des pics. Nous explorons la possibilité que ces caractéristiques pourraient être observationnellement larges. Bien qu'il n'existe aucune preuve solide, des grands pics ne sont pas exclus par les données, et nous donnent des contraintes sur la composante en k^3.

The Higgs boson is the only elementary particle predicted by the Standard Model (SM) that has not yet been observed experimentally. If it exists, it explains the spontaneous electroweak symmetry breaking and the origin of mass for gauge bosons and fermions. We test the validity of the SM by performing a search for the associated production of a Higgs boson and a W boson in the channel where the Higgs boson decays to a bottom-antibottom quark pair and the W boson decays to a charged lepton and a neutrino (the WH channel). We study a dataset of proton-antiproton collisions at a centre-of-mass energy of 1.96 TeV provided by the Tevatron accelerator, corresponding to an integrated luminosity of 5.7 inverse femtobarns , and recorded using the Collider Detector at Fermilab (CDF). We select events consistent with the signature of exactly one charged lepton (electron or muon), missing transverse energy due to the undetected neutrino (MET) and two collimated streams of particles (jets), at least one of which is required to be identified as originating from a bottom quark. We improve the discrimination of Higgs signal from backgrounds through the use of an artificial neural network. Using a Bayesian statistical inference approach, we set for each hypothetical Higgs boson mass in the range 100 - 150 GeV/c^2 with 5 GeV/c^2 increments a 95% credibility level (CL) upper limit on the ratio between the Higgs production cross section times branching fraction and the SM prediction.Our main original contributions are the addition of a novel charged lepton reconstruction algorithm with looser requirements (ISOTRK) with respect the electron or muon tight criteria (TIGHT), as well as the introduction of a novel trigger-combination method that allows to maximize the event yield while avoiding trigger correlations and that is used for the ISOTRK category. The ISOTRK candidate is a high-transverse-momentum good-quality track isolated from other activity in the tracking system and not required to match a calorimeter cluster, as for a tight electron candidate, or an energy deposit in the muon detector, as for a tight muon candidate. The ISOTRK category recovers real charged leptons that otherwise would be lost in the non-instrumented regions of the detector. This allows the reconstruction of more W boson candidates, which in turn increases the number of reconstructed WH signal candidate events, and therefore improves the sensitivity of the WH search. Since there is no ISOTRK-dedicated trigger at CDF, for the ISOTRK charged lepton category we employ three MET-plus-jets-based triggers using a novel method, which allows the combination of any number of triggers in order to maximize the event yield while avoiding trigger correlations. On an event-by-event basis, only the trigger with the largest efficiency is used. By avoiding a logical "OR" between triggers, the loss in the yield of events accepted by the trigger combination is compensated by a smaller and easier-to-compute corresponding systematic uncertainty. The addition of the ISOTRK charged lepton category to the TIGHT category produces an increase of 33% in the WH signal yield and a decrease of 15.5 % to 19.0 % in the median expected 95 % CL cross-section upper limits across the entire studied Higgs mass interval. The observed (median expected) 95 % CL SM Higgs upper limits on cross section times branching ratio vary between 2.39 x SM (2.73 x SM) for a Higgs mass of 100 GeV/c^2 to 31.1 x SM (31.2 x SM) for a Higgs mass of 150 GeV/c^2, while the value for a 115 GeV/c^2 Higgs boson is that of 5.08 x SM (3.79 x SM).The novel trigger combination method is already in use by several CDF analyses. It is applicable to any analysis that uses triggers based on MET and jets, such as supersymmetry searches at the ATLAS and CMS experiments at the Large Hadron Collider. In its most general form, the method can be used by any analysis that combines any number of different triggers. / Le boson de Higgs est la seule particule elementaire predite par le Modele Standard qui n'a jamais ete observee experimentalement. S'il existe, il explique la brisure spontanee de la symetrie electrofaible, ainsi que la masse des bosons W et Z et de tous les fermions. On verifie la validite du Modele standard en effectuant une recherche sur la production associee d'un boson de Higgs et d'un boson W dans le cas ou le boson de Higgs se desintegre en une paire de quarks bottom-antibottom et le boson W se desintegre en un lepton charge et un neutrino (le mode WH). Nos donnees furent accumulees en etudiant des collisions proton-antiproton a une energie au centre de masse de 1.96 Tev produites par l'accelerateur Tevatron, a une luminosite integree de 5.7 femtobarns inverses et collectees par le detecteur Collider Detector at Fermilab (CDF). On selectionne des evenements avec une signature correspondante a exactement un lepton charge (electron ou muon), de l'energie manquante transversale a cause du neutrino qui s'echappe du detecteur (MET) et deux jets de particules, dont au moins un doit provenir d'un quark bottom. On ameliore la discrimination entre le signal Higgs et le bruit de fond a l'aide d'un reseau de neurones artificiels. En utilisant une inference statistique bayesienne, on calcule pour chaque masse hypothetique du boson de Higgs dans l'intervalle 100-150 GeV/c^2, avec des increments de 5 GeV/c^2, une limite superieure de 95% d'intervalle de credibilite (CL), sur le rapport entre la section efficace multipliee par le rapport d'embranchement et celle predite par le Modele Standard. Notre contribution principale est l'introduction d'un nouvel algorithme de reconstruction d'un lepton charge avec des criteres plus laches (ISOTRK) par rapport aux criteres stricts de reconstruction des candidats d'electrons et de muons (TIGHT). La deuxieme contribution majeure consiste en l'introduction d'une nouvelle methode pour combiner des declencheurs differents permetant de maximiser le nombre d'evenements selectionnes et en meme temps que d'eviter les correlations entre les declencheurs.Un candidat de ISOTRK est une trajectoire qui correspond aux criteres de qualite, qui a une large quantite de mouvement transverse, qui est isolee d'autres activites dans le detecteur de trajectoires et qui ne doit pas se prolonger dans une zone active du calorimetre (detecteur de muons), comme pour un candidat d'electron (muon). Les candidats de ISOTRK recuperent de vrais leptons charges qui seraient autrement perdus dans les zones non instrumentees du detecteur. L'ajout de la categorie ISOTRK a la categorie TIGHT permet de reconstruire plusieurs bosons W reels et par la suite de recuperer plusieurs evenements WH, ce qui ameliore la sensitivite de la recherche de WH.Comme a CDF il n'y a pas de declencheurs dedies au leptons charges ISOTRK, on utilise pour la categorie ISOTRK trois declencheurs bases sur MET et jets avec une nouvelle methode qui permet de maximiser le nombre d'evenements accumules tout en evitant les correlations des declencheurs. Pour chaque evenement, ce n'est que le declencheur avec la plus grande efficacite qui est utilise. Le nombre d'evenements ramasses est legerement inferieur a celui d'un "OR" logique entre les declencheurs, mais cela est compense par une erreur systematique qui est a la fois moins importante et plus facile a evaluer. Dans sa forme la plus generale, la methode peut etre utilisee par toute analyse qui utilise un nombre variable de declencheurs differents. L'utilisation de la categorie ISOTRK en plus de la categorie TIGHT augmente de 33% le nombre d'evenements WH selectionnes et decroit de 15.5% a 19.0% la limite superieure mediane attendue. La limite superieure observee (attendue) varie entre 2.39 x SM(2.73 x SM) pour un boson de Higgs de 100 GeV/c^2 jusqu'a 31.1 x SM(31.2 x SM) pour un boson de Higgs de 150 GeV/c^2. En meme temps, pour un boson de Higgs de 115 GeV/c^2, la valeur est 5.08 x SM(3.79 x SM).

This thesis comprises several manuscripts, each exploring aspects of the dynamics of the early Universe. The foundations of the work presented lies in the realm of cosmology, but draws heavily on string theory as a source of guidance. The thesis commences with a motivation for the research and provides an introduction to the contemporary views of Cosmology: following a historical perspective of cosmology, we motivate the inflationary paradigm of Big Bang Cosmology, and introduce several world-views promoted by string theory. The string-motivated models will address shortcomings of cosmologies based on General Relativity and the Standard Model, and will provide a comprehensive, coherent description of the early Universe that is expected to transition to our observed Universe. Two possibilities presented here include String Gas Cosmology (SGC) and the Brane World scenario. We provide an introduction to these two constructions, and subsequently report on the possibility of simultaneously stabilizing the dilaton and moduli fields in SGC stabilization models, a mechanism to solve the overshoot problem of racetrack inflation, and explore the possibility of long-lived relics in warped reheating. / Cette th\èse explore la physique du jeune univers. Nous utilisons la th\'eorie des cordes pour nous aider \à comprendre l'\'epoque pr\'ec\'edant la nucl\'eosynth\èse. Deux manuscrits sont present\'es: un concernant la cosmologie de gaz de cordes (string gas cosmology, SGC), et un concernant le sc\'enario d'un univers membrane (brane world, BW). Nous pla\cons des contraintes sur la stabilization du moduli et du dilaton dans la construction SGC. Puis, nous \'etudions l\'epoque du r\'echauffement de l'univers dans la construction BW, et nous pla\c{c}ons des contraintes sur les param\ètres fundamentaux de la th\'eorie des cordes.

A model of dark matter excitations is studied in an attempt to explain the anomalously large 511 keV photon line emission observed by the SPI spectrograph on INTEGRAL to be originating from the galactic bulge of the Milky Way. The proposed dark matter WIMP has a near degenerate mass partner a few MeV heavier. Scattering between dark matter particles leads to excitations, with the subsequent decays producing an electron-positron pair. In this way, the kinetic energy of the massive dark matter particles can be efficiently converted into electron-positron pairs moving slow enough to produce the narrow annihilation line observed. With a sufficiently large mass gap, kinematic considerations and the cuspy dark matter density profile constrain excitations to the galactic bulge where the escape velocity, and thus the fraction of dark matter particles above the kinematic cutoff, is large. / Un model d'excitations matière sombre est etudié dans une tentative d'explication de la ligne d'emission anormalement large observé par le spectrographe SPI sur INTEGRAL originaire du bulbe galactique de la Voie Lactée. La matière sombre WIMP proposée possède un partenaire ayant une masse de quelques MeV supplémentaires. La diffusion entre les particules de matière sombre mène aux excitations et à la désintégration ultérieure en une paire électron-positron. De cette façon, l'énergie cinétique des particules de matière sombre peut être convertie en paires électron-positron se déplaçant suffisement lentement pour produire l'étroite ligne d'annihilation observée. Avec un espacement en masse suffisement grand, les considérations cinématique et un profil de densité de la matière sombre cuspy contraignent les excitations au bulbe galactique, où la vitesse d'échappement, et donc la fraction de particules matière sombre au-dessus du seuil cinétique, est grande.

The classical molecular dynamics model is used for a study of the possible nuclear phase transition in heavy ion collisions at intermediate energies. We implemented this model through Monte Carlo techniques. Different kinds of initial configurations are considered, as well as different methods of particles propagation. In order to simulate the canonical ensemble dynamic evolution we investigate two methods of keeping the temperature constant. / We consider a system of 85 nucleons interacting through two-body nucleon-nucleon potential. The calculations are first carried out ignoring Coulomb interaction and then including it. Data on various thermodynamic quantities are obtained and the question of the existence of the phase transition is investigated. / To estimate the effect of a finite particle number on critical parameters we go to a system of 200 nucleons.

Using the ARGUS detector at the $e sp+e sp-$ storage ring DORIS II at DESY, a study of the decay $ bar{B} sp0 rightarrow D sp{*+} ell sp- bar{v}$ has been performed by exploiting a partial $D sp{*+}$ reconstruction technique. The branching ratio was determined to be (4.4 $ pm$ 0.3 $ pm$ 0.3)% for this mode, and for the higher excited $D sbsp{(J)}{*}$ states $Br( bar{B} sp0 rightarrow D sbsp{(J)}{*} sp+ ell sp- bar{v})=(2.5 pm 0.6 pm0.5)$%. Furthermore, the inclusive $D sp{*+}$ branching ratio in B decays was measured by fully reconstructing $D sp{*+}$ candidates. / Using a tagged subset of this sample of $B sp0$ meson decays in the mode $ =B sp0 rightarrow D sp{*+} ell sp- =v, B sbsp{d}{0} longleftrightarrow =B sbsp{d}{0}$ oscillations have been studied. For this purpose two tagging techniques have been applied: the standard method of using fast leptons, and a new technique which makes use of kaons to tag the b flavour content. Combining the values obtained by these two methods, the $B sp0 =B sp0$ mixing parameter $ chi sb{d}$, used to denote the strength of the oscillations, was determined to be $ chi sb{d} = 0.165 pm0.057$. / In addition, using fully reconstructed $D sp{*+}$ candidates, a third study of the $B sp0 =B sp0$ mixing parameter was carried out by investigating $D sp{*+}K sp{ pm}$ correlations. The mixing measurements obtained using kaons to tag the B meson flavour employ this technique for the first time. Future CP violation measurements at B Factories will place critical reliance on this method. / Finally, using the extracted value for the mixing parameter $ chi sb{d}$, the CKM matrix element $V sb{td}$ was determined and the $B sbsp{s}{0} =B sbsp{s}{0}$ mixing parameter $ chi sb{s}$ was obtained.