Cette these a pour sujet la recherche de nouvelle physique et l'etude de la production Drell-Yan dans le canal di-electron a l'aide du detecteur CMS au LHC. Certaines theories au dela du Modele Standard (extra dimensions, theories de grande unification) predisent l'existence de particules massives pouvant se desintegrer en une paire d'electrons. La selection des evenements recherches est presentee et etudiee. La strategie d'analyse est introduite et testee. Enfin, l'analyse des premieres donnees a 7 TeV est decrite et les resultats sont commentes.

The Minimal Scale Invariant extension of the Standard Model (MSISM) is a model of low-energy particle physics which is identical to the Standard Model except for the inclusion of an additional complex singlet scalar and tree-level scale invariance. Scale invariance is a classical symmetry which is explicitly broken by quantum corrections whose interplay with the quartic couplings can be used to trigger electroweak symmetry breaking. The scale invariant Standard Model suffers from a number of problems, however the inclusion of a complex singlet scalar results in a perturbative and phenomenologically viable theory. We present a thorough and systematic investigation of the MSISM for a number of representative scenarios along two of its three classified types of flat direction. In these scenarios we determine the permitted quartic coupling parameter space, using both theoretical and experimental constraints, and apply these limits to make predictions of the scalar mass spectrum and the energy scale at which scale invariance is broken. We calculate the one-loop effective potential and the one-loop beta functions of the pertinent couplings of the MSISM specifically for this purpose. We also discuss the phenomenological implications of these scenarios, in particular, whether they realise explicit or spontaneous CP violation, contain neutrino masses or provide dark matter candidates. Of particular importance is the discovery of a new minimal scale invariant model which provides maximal spontaneous CP violation, can naturally incorporate neutrino masses, produces a massive stable scalar dark matter candidate and can remain perturbative up to the Planck scale. It can be argued that the last property, along with the classical scale invariance, can potentially solve the gauge hierarchy problem for this model.

Everything that humans have ever touched, created or built something from consists of a type of matter that only makes up 15 percent of the total matter in the universe. The remaining 85 percent is attributed to dark matter, a so far not discovered and non-luminous type of matter. In this thesis a potential dark matter particle candidate has been studied by investigating an extension of the SU(2) symmetry into a dark gauge sector, where the new sector is connected to the standard model through a vector-like fermion portal. In order to understand how such an extension is made, the Lagrangian density of the standard model and its different gauge sectors were derived. The cross sections of the process of pair production of dark matter particles and tau leptons in the final state due to proton-proton collisions at the LHC was simulated with the software \texttt{MadGraph}. The cross sections were used to draw significance contours for the exclusion and discovery regions for parts of the parameter space of the new model, for current and projected luminosities of the LHC. The projected luminosity scans also consider how lowering the uncertainty in the number of background events through hypothetical improvements to detectors would impact the exclusion and discovery contours. The significance contours were combined with relic density constraints, derived from comparisons between measurements of the Planck telescope and calculations from the software \texttt{MicrOMEGAs}. The resulting graphs show that there are non-forbidden regions of the parameter space that are significant for exclusion and discovery for luminosity of current searches. Increasing the luminosity while keeping the uncertainty in the number of background events the same yielded only minor increases to the exclusion and discovery contours. Combining the projected luminosities with improvements to the background uncertainty instead produced exclusion and discovery regions that were significantly larger than those for the current luminosity.

In the current LHC era, a vast number of models for BSM physics are being tested. For predictions accurate enough to match experimental errors, theoretical calculations have to go beyond LO estimates. However, calculating one-loop corrections in BSM models involves many new particles with specific model dependent properties. Therefore, they are done largely by hand, or in partially–automated ways. I present a fully automated tool for the calculation of generic massive one-loop Feynman diagrams with four external particles, implemented as a module within the fully automated MadGolem framework. With this one can compute the NLO–QCD corrections to generic BSM heavy resonance production processes, for example in the context of supersymmetric theories.

Thesis presented in fulfilment of the requirements for the degree of Master of Science in Physics at the University of the Witwatersrand School of Physics, University of the Witwatersrand, 2015. / A search for the pseudoscalar A boson, which is predicted by in many models with an extended Higgs sector, gives a gateway to searches for physics beyond the Standard Model (SM). This thesis presents the results of a search for gluon-fusion produced A in the decay to Zh, with a final state of two electrons or muons and two τ leptons, in 20.3 fb−1of proton-proton collision data at √s = 8 TeV. Each tau lepton is allowed to dacay either leptonically, τlep, or hadronically,τhad, giving rise to three final states, τlepτlep, τlepτhad and τhadτhad. Focus is placed on the methodology and results of the fully hadronic channel. No evidence for the existence of an A boson is found in the scanned range of 220 ≤ mA ≤ 1000 GeV and 95% CL upper limits are placed on the gluon-fusion cross section times branching ratio, σ × BR(A → Zh) × BR(h → ℓℓττ). The results are combined with a complementing A → Zh search, where h → b¯b, and interpreted in view of two-Higgs-Doublet-Models (2HDMs), where exclusion limits are placed on large sections of phasepace.

A search for Beyond the Standard Model physics is performed with the ATLAS detector in the opposite sign dimuon channel using the 20 fb 1 of data collected in 2012 at √s = 8 TeV. No excess is found above the Standard Model expectation. Using a Bayesian statistical analysis, model dependent 95% Credibility Level Bayesian exclusion limits are extracted for two models of gravitationally-related beyond the Standard Model phenomena. For the ADD and RS quantum black hole models, limits of 3.32 and 1.95 TeV are set on the extradimensional Planck Scale, and for ADD Large Extra dimensions, limits ranging from 2.8-4.4 TeV are set on the string scale for the GRW, HLZ and Hewett formalisms. In addition, a study is performed to estimate the effect of increasing noise cuts in the ATLAS Level-1 Calorimeter Trigger on the physics efficiency of W ! e and t¯t and on the Level-1 missing transverse energy trigger rate. Results suggest that higher noise cuts could reduce the Level-1 missing transverse energy trigger rate with a minimal loss of physics efficiency.

In 2012 a resonance with a mass of 125 GeV resembling the elusive Higgs boson was discovered simultaneously by the ATLAS and CMS experiments using data collected from the Large Hadron Collider (LHC) at CERN. Its observation finally confirms the mechanism for Spontaneous Electroweak Symmetry Breaking (EWSB) necessary for describing the mass structure of the electroweak (EW) gauge bosons. In 2013, Peter Higgs and Francois Englert were awarded the Nobel Prize in physics for their work in developing this theory of EWSB now referred to as the Higgs mechanism. The explanation for EWSB is often referred to as the last piece of the puzzle required to build a consistent theory of particle physics known as the Standard Model. But does that mean that there are no new surprises to be found? Many EW processes have yet to be measured and are just starting to become accessible with the data collected at the LHC. Indeed, this unexplored region of EW physics may provide clues to as yet unknown new physics processes at higher energy scales. Using the 2012 LHC data recorded by the ATLAS experiment, we seek to make the first observation of one such EW process, the massive tri-boson final state: WWW. It represents one of the first searches to probe the Standard Model WWWW coupling directly at a collider. This search looks specifically at the channel where each W boson decays to a charged lepton and a neutrino, offering the best sensitivity for making such a measurement. In addition to testing the Standard Model directly, we also use an effective field theory approach to test for the existence of anomalous quartic gauge couplings which could offer evidence for new physics at higher energies than those produced by the LHC.

A search for the Standard Model Higgs Boson decaying to two muons in proton-proton collisions with the Compact Muon Solenoid experiment is performed. Building on top of the success of previous CMS analyses (CMS Run I campaign), results are presented using 35.9 fb−1 of data collected over the course of 2016 (CMS Run II campaign) at a center-of-mass energy of √s = 13 TeV. During the Long Shutdown 1 of the Large Hadron Collider, the CMS detector underwent substantial hardware changes. The second topic discusses the process of calibration of the CMS Hadron Forward Calorimeter in preparation for collisions after LS1. The final chapter discusses the process of building simulations of calorimeter systems. Walking through all the steps from geometry specification to readout definition the results for two standalone calorimeters are presented that have been proposed as potential replacements for respective CMS components.

Détection et identification de la réaction quark + anti-quark -> e+ + e- à l'aide du détecteur CMS (Compact Muon Solenoid) auprès du Grand Collisionneur de Hadrons du CERN, le LHC. Cette réaction permet de tester avec précision le Modèle Standard et de rechercher d'éventuelles nouvelles particules (Z') prédites par les théories de grande unification (GUT) et par les modèles à dimensions spatiales supplémentaires.

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