We review the Regge theory and the minijet model for pp and pp collisions. We show that, in the conventional minijet approach, the asymptotic behavior of the total cross section calculated using currently accepted gluon distribution function and perturbative QCD rises too rapidly when compared with the data and fails to satisfy Froissart bound. To tame the rise, we have developed a new formalism for the interaction between QCD gluon and the classical color field of the colliding nucleons, and show how the gluon propagator is modified. The corresponding gauge invariance condition of the propagator is derived and shown to be satisfied. The modified gluon propagator leads to a suppression of the minijet cross section due to the gg [rightwards arrow] gg sub-process in the small-x region. We show that the pp and pp total cross section from [square root]s = 5 GeV to 30 TeV can be described as a sum of a hard component contributed by the classical field modified minijet model and a soft component due to the exchange of the Pomeron and the I=0 exchange-degenerate [omega] and f Regge trajectories. The soft-component model is motivated by the notion of duality. The predicted total cross section has a ln s asymptotic behavior. / text

Pendant mes trois ans de doctorat j'ai eu le plaisir de travailler sur trois projets très variés ayant un but commun: mieux contraindre certains modèles de nouvelle physique entre cosmologie et le LHC. Le fait que les densités reliques de matière noire et de baryons sont similaires semble indiquer qu'il y a un lien entre les deux. Nous essayons d'expliquer les valeurs observées en reliant un modèle de leptogenèse au miracle des WIMPs, qui produit naturellement la bonne densité relique. Si l'asymétrie baryonique est produit dans des désintégrations hors équilibre à l'échelle électro-faible et si la matière noire est constituée de WIMPs, les deux densités reliques sont contrôlées par des processus électro-faibles hors équilibre. Je construis un modèle de leptogenèse à l'échelle du TeV en utilisant une extension du type seesaw inverse du modèle standard avec des singlets additionnels. Pour produire suffisamment d'asymétrie baryonique il faut une violation CP ~ O(1) qui est difficile à obtenir dans mon cadre. Les axions, tout comme les WIMPs sont de bons candidats de matière noire bien motivés. Il serait très utile de pouvoir les distinguer. Sikivie argumente que si des axions sont dans un condensat de Bose-Einstein, alors ils forment des halos galactiques différents des halos de WIMPs. D'après Sikivie ce sont les interactions gravitationnelles qui thermalisent les axions et qui les condensent. La formation d'un condensat nécessite la génération d'entropie qui ne peut pas être fourni par les interactions gravitationnelles au premier ordre. J'étudie la génération d'entropie par les interactions gravitationnelles en estimant une longueur de dissipation dans le fluide d'axions qui vient de la présence d'une pression anisotrope. Je ne peux pas confirmer la thermalisation rapide d'axions causé par leurs interactions gravitationnelles. Des nouveaux bosons de jauges comme le Z0 apparaissent dans un grand nombre d'extensions du modèle standard. On les recherche le plus souvent comme une résonance dans le spectre de masse invariante de leurs produits de désintégration. Le Z0 doit être produit sur couche de masse dans ces recherches résonantes. Mais la présence d'un Z0 peut aussi influencer d'autres observables cinématiques sans être produit directement, ce qu'on peut utiliser dans des recherches non-résonantes. Je compare ces deux types de recherches au LHC et trouve que pour des petits couplages les recherches résonantes sont plus adaptées mais pour de plus grandes masses et couplages les recherches non-résonantes sont plus performantes.

The LHC and the ATLAS detector offer an unprecedented opportunity to test theories beyond the Standard Model of particle physics. Some of these theories predict heavy particles that would decay predominantly into a top and an antitop quark. This thesis presents a technique to experimentally measure the invariant mass (Mtt) of top-antitop pairs, event-by-event, allowing for a complete reconstruction of the Mtt spectrum. Two different models of heavy narrow resonances were considered: a particle X with a negligible mass width, and a Z' particle. Under these models, no resonances in the Mtt spectrum were found in 35.3 pb-1 of 7 TeV centre of mass proton-proton collision data. An upper limit on the production cross section times the branching ratio of the heavy particle decaying into a top-antitop (Sup) as a function of its mass (Mx or Mz) was established at a 95% confidence level. Including systematic errors, the observed (expected) Sup at 95% varies from 3.2 pb (27.9 +11.3 -19.9 pb) to 77.7 pb (9.8 +10.7 -5.6 pb) for Mx ranging from 760 GeV to 1000 GeV, and from 2.9 pb (55.9 +56.9 -47.0 pb) to 43.4 pb (7.8 +8.9 -4.7 pb) for Mz ranging from 700 GeV to 1000 GeV. / Graduate

The LHC and the ATLAS detector offer an unprecedented opportunity to test theories beyond the Standard Model of particle physics. Some of these theories predict heavy particles that would decay predominantly into a top and an antitop quark. This thesis presents a technique to experimentally measure the invariant mass (Mtt) of top-antitop pairs, event-by-event, allowing for a complete reconstruction of the Mtt spectrum. Two different models of heavy narrow resonances were considered: a particle X with a negligible mass width, and a Z' particle. Under these models, no resonances in the Mtt spectrum were found in 35.3 pb-1 of 7 TeV centre of mass proton-proton collision data. An upper limit on the production cross section times the branching ratio of the heavy particle decaying into a top-antitop (Sup) as a function of its mass (Mx or Mz) was established at a 95% confidence level. Including systematic errors, the observed (expected) Sup at 95% varies from 3.2 pb (27.9 +11.3 -19.9 pb) to 77.7 pb (9.8 +10.7 -5.6 pb) for Mx ranging from 760 GeV to 1000 GeV, and from 2.9 pb (55.9 +56.9 -47.0 pb) to 43.4 pb (7.8 +8.9 -4.7 pb) for Mz ranging from 700 GeV to 1000 GeV. / Graduate

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

This paper presents an outline of the generation of mass for the massive Standard Model particles (fermions, $W^\pm$, $Z^0$) through electroweak symmetry breaking via the Higgs mechanism, and how the Higgs boson emerges from this framework. A Monte Carlo study was done on the decay $H\rightarrow\tau\tau$, with one leptonically and one hadronically decaying tau, with two different production channels for the $H$, gluon-gluon fusion (gg) and vector boson fusion (VBF), at $\sqrt s = 7\tev$ with a Higgs mass $m_H = 120\gev$. The kinematics of these two production channels were compared and it was found that the transverse momentum of muons produced in VBF were higher on average than those produced in gg. This differance was greater in muons originating from the leptonically decaying tau in the Higgs decay, than those produced by other processes in the underlying event. In the latter, however, the difference was still noticable. Jets were slightly more abundant in VBF than in gg, and were in VBF more distributed along the beam axis. The separation in pseudorapidity between the two jets with highest transverse momentum was found to be greater in VBF than in gg. An attempt to reconstruct the Higgs mass using Monte Carlo data run through a simulation of the ATLAS detector was done. The estimator used was the transverse mass of the system consisting of the visible part of the hadronically decaying tau, the lepton from the leptonically decaying tau and the total missing transvese energy. In gg the mean of the transvese mass distribution was found to be $89.26\gev$ with a root mean square uncertainty (RMS) of $23.86\gev$. In VBF the mean was found to be $85.57\gev$ with RMS $27.08\gev$.

Cette étude expérimentale concerne la génération de faisceaux d'électrons aux propriétés inédites. Ces faisceaux d'électrons sont issus de l'interaction d'une impulsion laser ultra-intense et brève avec un jet de gaz. Jusqu'à récemment, les faisceaux d'électrons ainsi produits avaient une divergence importante et un spectre très large. Un tournant important de cette discipline a été franchi lorsqu'un faisceau d'électrons faiblement divergent (10 mrad) et, par dessus tout, dont le spectre est piqué à haute énergie (170 MeV) a été observé en utilisant un nouveau spectromètre monotir à électrons. Une étude paramétriquede l'interaction a permis de suivre l'évolution des propriétés du faisceau d'électrons. Des expériences ont ensuite été menées pour approfondir la caractérisation de ce faisceau. L'observation du spectre de rayonnement de transition généré par les électrons à une interface montre que le faisceau d'électrons interagit avec l'impulsion laser pendant l'accélération. Des oscillations radiales du faisceau d'électrons autour de l'axe laser, qualifiées d'oscillations bêtatroniques, ont aussi été visualisées sur les spectres électroniques. Un tel spectre quasi-monoénergétique est essentiel pour de nombreuses applications. Afin de justifier l'intérêt d'un tel faisceau, plusieurs expériences d'applications sont présentées: la radiographie γ submillimétrique d'objets denses, un profil de dose du faisceau d'électron comparable aux performances actuelles des sources de photons pour la radiothérapie, un profil temporel très bref utile en radiolyse de l'eau et la génération d'une source de rayons X faiblement divergente.

Theoretical predictions for generic multi-scale observables in Quantum Chromodynamics (QCD) typically suffer from large Sudakov logarithms associated with the emission of soft or collinear radiation, whose presence spoils the perturbative expansion in the coupling strength which underlies most calculations in QCD. A canonical way to improve predictions wherever these logarithms appear is to resum them to all perturbative orders, which can conveniently be achieved using Effective Field Theory (EFT) methods. In an age of increasing automation using computers, this task is still mostly performed manually, observable-by-observable. In this thesis we identify the 2-loop soft function as a crucial ingredient for the resummation of QCD Sudakov logarithms to Next-to-next-to-leading logarithmic (NNLL) accuracy in Soft-Collinear Effective Theory (SCET), for wide classes of observables involving two massless colour-charged energetic particles, such as dijet event shapes at lepton colliders, or colour singlet production at hadron colliders. We develop a method to evaluate these soft functions using numerical methods based on sector decomposition and the choice of a convenient parametrisation for the phase space. This allows the factorisation of all implicit (real emission) and explicit (virtual correction) divergences made manifest by dimensional and analytic regularisation. The regulator pole coefficients can then be evaluated numerically following a subtraction and expansion, and two computational tools are presented to perform these numerical integrations, one based on publicly available tools, the other based on our own code. Some technical improvements over naive straightforward numerical evaluation are demonstrated and implemented. This allows us to compute and verify two of three colour structures of the 2-loop bare soft functions for wide ranges of observables with a factorisation theorem. A number of example results - both new and already known - are shown to demonstrate the reach of this approach, and a few possible extensions are sketched. This thesis therefore represents a crucial step towards automation of resummation for generic observables to NNLL accuracy in SCET.

An electron-positron linear collider is an option for future large particle accelerator projects. Such a collider would focus on precision tests of the Higgs boson properties. This thesis describes three studies related to the optimisation of highly granular calorimeters and one study on the sensitivity of Higgs couplings at CLIC. Photon reconstruction algorithms were developed for highly granular calorimeters of a future linear collider detector. A sophisticated pattern recognition algorithm was implemented, which uses the topological properties of electromagnetic showers to identify photon candidates and separate them from nearby particles. It performs clustering of the energy deposits in the detector, followed by topological characterisation of the clusters, with the results being considered by a multivariate likelihood analysis. This algorithm leads to a significant improvement in the reconstruction of both single photons and multiple photons in high energy jets compared to previous reconstruction software. The reconstruction and classification of tau lepton decay products was studied. Utilising highly granular calorimeters, the high resolution of energy and invariant mass of the tau decay products enabled a high classification rate. A hypothesis test was performed for expected decay final states. A multivariate analysis was trained to classify decay final states with a machine learning method. The performance of tau decay classification is used for the electromagnetic calorimeter optimisation at the ILC or CLIC. A proof-of-principle analysis using the correlation between the polarisations of the tau pair from a boson decay as a signature to differentiate the Higgs boson from the Z boson is presented. Sensitivity of Higgs couplings at CLIC was studied using the double Higgs production process. Algorithms were developed for signal event selection. The event selection relies on the jet reconstruction and the flavour tagging. A multivariate analysis is performed to select signal events. An attempt at extracting Higgs trilinear self-coupling and quartic coupling was conducted.

The Standard Model has granted exquisite power to predict the behaviour of high-energy particle collisions. It is not, however, without conceptual and empirical weaknesses. Several theories have been proposed which aim to resolve these difficulties. This thesis describes searches for two such theories: models of extra spatial dimensions, and supersymmetry. The Large Hadron Collider has extended the frontiers of energy and intensity in particle physics. In 2015, the LHC resumed proton–proton collisions at a centre-of-mass energy of 13 TeV. This increase over previous operation grants an enhancement in sensitivity to many processes beyond those of the Standard Model. Extra-dimensional theories address the hierarchical nature of the Standard Model. The lowered fundamental scale of gravity in these models allows a rich phenomenology at energies which may be accessible to the LHC. Some models predict the formation of microscopic black holes, which are the target of an analysis of collisions recorded by the ATLAS detector in 2015. No significant deviations from Standard Model predictions were observed. The constraints inferred on the parameters of the model are a significant advance on previous results. Lepton flavour is conserved in the Standard Model. This is not the result of a known fundamental symmetry, however. The latter part of this thesis proposes a search examining asymmetries of charge and flavour in the eμ final state. Models of supersymmetry with an R-parity-violating λ'₂₃₁ coupling are taken as motivation. The strategy is developed using collision data recorded by ATLAS in 2015 and 2016, and the most significant biasing effects are addressed.