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Ce travail présente la mesure de la section efficace différentielle du boson Z en impulsion transverse (ptz), dans le canal de désintégration electron-positron, avec le détecteur ATLAS au LHC. La mesure utilise 4.64 inverse fb de données de collisions proton-proton, prises en 2011 avec une énergie du centre de masse de 7 TeV. Le résultat est combiné avec une mesure indépendante faite dans le canal muon-antimuon. La mesure est faite jusqu'à ptz = 800 GeV, et a une incertitude typique de 0.5 % pour ptz < 60 GeV, atteignant jusqu'à 5 % vers la fin du spectre. La mesure est comparée avec modèles théoriques et prédictions des générateurs Monte Carlo.

<p>The CERN Large Hadron Collider (LHC) offers a unique opportunity to test the Standard Model of particle physics. The Standard Model predicts the existence of a Higgs boson and provides accurate estimates for the strength of the interactions of the Higgs boson with other particles. After the discovery of the Higgs boson, the measurement of its properties, such as its couplings to other particles, is of paramount importance. </p> <p>The projects described in this thesis explore different aspects of one of such measurements – the search for the Higgs boson decay into a pair of muons (H→<em>μμ</em>), conducted by the CMS experiment at the LHC. This decay plays an important role in elementary particle physics, as it provides a direct way to measure the coupling of the Higgs boson to the muon. The first evidence of the H→<em>μμ</em> decay was reported in 2020 as a result of an elaborate statistical analysis of the dataset collected by the CMS experiment during Run 2 of the LHC (2016–2018). The observed (expected) upper limit on the signal strength modifier for this decay at 95% confidence level was found to be 1.93 (0.81), constituting the most precise measurement to date. </p> <p>The details of this analysis, along with studies to establish possible directions for the development of the next iteration of  the H→<em>μμ</em> analysis using Run 3 data, are discussed in this thesis. In addition, a novel machine learning-based algorithm for the muon high level trigger is presented, which ultimately improves the data-taking efficiency of the CMS experiment, and hence, helps to increase the sensitivity of future H→<em>μμ</em> searches. Finally, projections of the H→<em>μμ</em> search sensitivity to the data-taking conditions at the High-Luminosity Large Hadron Collider are presented, estimating the achievable precision for future measurements of the Higgs boson properties.</p>

This thesis is concerned with experimental investigations of properties of the top quark and processes involving this particle. In the first part of the thesis, the first determination of the electric charge of the top quark is presented. The measurement was made using top quark pair events produced in proton-antiproton collisions recorded by the D0 detector at the Fermilab Tevatron. It is based on the reconstruction of the charge of the top quarks decay products from the dominant decay to a W boson and a b-quark. The method uses a jet charge algorithm, calibrated with data, to discriminate between b- and antib-quark jets. A constrained kinematic fit is also performed to resolve the ambiguities of the pairing of the top quark decay products and to extract the top quark electric charge. The result is in good agreement with the Standard Model top quark electric charge of 2e/3 and an upper limit of 0.8 at 90\% confidence level on the fraction of exotic quarks with charge 4e/3 in the data sample is obtained. The second part of the thesis concerns the estimation of the top quark pair background to searches for new physics, such as supersymmetry, with the ATLAS  experiment at the CERN Large Hadron Collider. These searches will require a robust estimation of standard model backgrounds in order to make any claims of discovery or to exclude models of new physics. For searches with a final state signature characterized by two isolated charged leptons, multiple jets and large missing transverse energy the largest source of background is expected to be top quark pairs with leptonic decay of the two W bosons from the top quarks in the event. A data-driven method to estimate this contribution based on full kinematic reconstruction of the top quark pair events is studied using simulated proton-proton collisions. It is shown that the method is capable of estimating the top quark pair background to within 12% using data corresponding to approximately 1fb-1. The systematic uncertainty is of the order of 20% and, depending on the model, the contamination of signal events can potentially be large. / QC 20100730

Alors que la nouvelle génération de détecteurs directs de matière sombre est en cours de construction, dans l’espoir de trouver de la matière sombre avec une masse inférieure au GeV, il est important de comprendre comment le rayonnement naturel peut produire un fond d’interactions à faible énergie. Cette thèse s’intéressera à la simulation de la diffusion élastique des rayons γ, une source possible de fonds pour les détecteurs de matière sombre sub-GeV. La simulation utilise le logiciel SuperSim basé sur Geant4 afin de modéliser l’expérience SuperCDMS SNOLAB. Une version modifiée du G4JAEAElasticScatteringModel appelée CDMSJAEAElasticScatteringModel a été mise en oeuvre dans SuperSim afin de simuler les mécanismes de diffusion de photon élastiques Rayleigh, nucléaire Thomson et Delbrück. Le CDMSJAEAElasticScatteringModel ajoute la possibilité pour les particules γ de déposer de l’énergie après avoir été diffusées élastiquement. La validité de ces deux modèles a été vérifiée et des erreurs dans le logicielle ont été rencontrées dans leur traitement des distributions d’angle de diffusion des photons qui déterminent les spectres d’énergie déposée. Les sections efficaces totales sont en accord avec la documentation et d’autres sources. Malgré les erreurs logicielles, la simulation définit une limite inférieure sur le taux de diffusion élastique des rayons γ de ∼ 0,01 et ∼ 0,035 photon diffusé élastiquement par kg par an pour les détecteurs SuperCDMS SNOLAB au germanium et au silicium, respectivement. Ces limites inférieures sont définies à l’aide d’une coupure d’énergie de recul de 1 eV. Cela fait de la diffusion élastique des rayons γ une source importante de bruit de fond pour détecteurs SuperCDMS proposés avec des capacités de discrimination ER/NR à des énergies de recul à l’échelle eV. / While the new generation of direct dark matter detectors are being built in the hopes of finding sub-GeV dark matter, it is important to understand how natural radiation can produce a background of low-energy interactions. This thesis will analyze simulating γ-ray elastic scattering, a possible source of background for sub-GeV dark matter detectors. The simulation uses Geant4-based SuperSim software in order to model the SuperCDMS SNOLAB experiment. A modified version of the G4JAEAElasticScatteringModel called CDMSJAEAElasticScatteringModel was implemented into SuperSim in order to simulate the Rayleigh, nuclear Thomson and Delbrück γ-ray elastic scattering mechanisms. The CDMSJAEAElasticScatteringModel adds the ability for the γ particles to deposit energy after being elastically scattered. The validity of both these models was checked, and errors were encountered in their treatment of photon scattering angle distributions which determine the deposited energy spectra. The total cross sections are consistent with the documentation and other sources. Despite the bug, the simulation does set a lower bound on the γ-ray elastic scattering rate of ∼ 0.01 and ∼ 0.035 elastically scattered photon per kg per year for germanium and silicon SuperCDMS SNOLAB detectors, respectively. These lower bounds are set using a 1 eV recoil energy cutoff. In conclusion, γ-ray elastic scattering a significant source of background for proposed SuperCDMS detectors with ER/NR discrimination capabilities at eV-scale recoil energies.

In 2016, the ATLAS experiment at CERN released data from 100 trillion proton-proton collisions to the general public. In connection to this release the ATLAS Outreach group has developed several tools for visualizing and analyzing the data, one of which is a Histogram analyzer. The focus of this project is to bridge the gap between the general public's knowledge in physics and what is needed to use this Histogram analyzer. The project consists of both the development and an evaluation of a learning resource that explains experimental particle physics for a general public audience. The learning resource is a website making use of analogies and two perspectives on learning: Variation Theory and Cognitive Load Theory. The evaluation of the website was done using a survey with 10 respondents and it focused on whether analogies and the perspectives on learning helped their understanding. In general the respondents found the analogies to be helpful for their learning, and to some degree they found the explanations based on Variation Theory to be helpful. The implementations of Cognitive Load Theory were considered to be helpful by the respondents who noticed them, but the majority did not, implying that improvements of the design are needed. The results indicate that analogies and the two perspectives on learning can be helpful for explaining experimental particle physics, but there might be other learning theories more suitable for this purpose. / ATLAS-experimentet på CERN släppte år 2016 data från 100 biljoner proton-kollisioner fritt till allmänheten. I anslutning till detta har ATLAS Outreach-grupp utvecklat ett flertal verktyg för att visualisera och analysera datan, varav en är en analys med hjälp av histogram. Fokus för detta projekt är att överbrygga klyftan mellan allmänhetens kunskaper i fysik och vad som behövs for att kunna använda Histogram-analysverktyget. Projektet består både av utvecklandet och utvärderingen av ett läromedel som förklarar experimentell partikelfysik med målgruppen allmänheten. Läromedlet är en webbsida som använder sig av analogier och två perspektiv på lärande, Variationsteori och Kognitiv Belastningsteori. Utvärderingen av webbsidan gjordes med en enkät med tio respondenter, med fokus på huruvida analogier och perspektiven på lärande hjälpte deras förståelse. I allmänhet fann respondenterna analogierna hjälpsamma för deras lärande, och de fann Variationsteori hjälpsamt i viss utsträckning. Implementeringarna av Kognitiv Belastningsteori ansågs vara hjälpsamma av de respondenter som lade märke till dem, men majoriteten gjorde inte det, vilket tyder på att förbättringar av implementeringen krävs. Resultaten indikerar att analogier och de två perspektiven på lärande kan vara hjälpsamma för att förklara experimentell partikelfysik, men det kan finnas andra lärandeteorier som uppfyller syftet bättre.