This thesis presents the design qualification and construction of the Beam Scintillator Counter (BSC) for the CMS Collaboration at CERN in 2007 - 2008. The BSC detector is designed to aid in the commissioning of the Compact Muon Solenoid (CMS) during the first 2 years of operation and provide technical triggering for beam halo and minimum-bias events. Using plastic scintillator tiles mounted at both ends of CMS, it will detect minimum ionizing particles through the low-to-mid luminosity phases of the Large Hadron Collider (LHC) commissioning. During these early phases, the BSC will provide probably the most interesting and widely used data of any of the CMS sub-detectors and will be employed in the track based alignment procedure of the central tracker and commissioning of the Forward Hadron Calorimeter.

Many interesting physical processes produce non-interacting particles that could only be measured using the missing transverse momentum. The increase of the proton beam intensity in the Large Hadron Collider (LHC) provides sensitivity to rare physics processes while inevitably increasing the number of simultaneous proton collisions in each event. The missing transverse momentum (MET) is a variable of great interest, defined as the negative sum of the transverse momentum of all visible particles. The precision of the MET determination deteriorates as the complexity of the recorded data escalates. Given the current complexity of data analysis, a new algorithm is developed to effectively determine the MET. Several well-understood physics processes were used to test the effectiveness of the newly designed algorithm. The performance of the new algorithm is also compared to that of the standard algorithm used in the ATLAS experiment. / Graduate

The LHCb experiment is dedicated to making precision measurements involving beauty and charm hadrons at the CERN Large Hadron Collider. The LHCb RICH detectors provide charged particle identification required to distinguish final states in many decays important to the LHCb physics programme. Time alignment of the RICH photon detectors is necessary in order to ensure a high photon collection efficiency. Using both a pulsed laser and proton-proton collision data the photon detectors are aligned to within 1 ns. The LHCb detector is uniquely positioned to measure production cross-sections at energies and rapidities inaccessible to other experiments. With 1.81 nb−1 of proton-proton collisions collected by the LHCb experiment in 2010 at center-of-mass energy √s = 7 TeV the production crosssection of D±s and D± mesons decaying to the φ{K+K−}π ± final state have been determined in bins of transverse momentum and rapidity. These measurements use a data-driven recursive optimisation technique to improve signal significance. The cross-section ratio is measured to be σ(D± ) σ(D± s ) = 2.32±0.27(stat)±0.26(syst), consistent with the ratio of charm-quark hadronisation fractions to D± and D±s mesons. Time-dependent interference between mixing of B0s -B0s mesons and decay to the final state J/ψφ gives rise to a CP violating phase φs. This phase is constrained to be small within the Standard Model, a significant deviation from which would be a signal of new physics. φs has been measured with 0.37 fb−1 of protonproton collision data recorded during 2011 by the LHCb experiment. Isolation of the signal distribution is achieved using the S-plot technique, and the analysis accounts for inclusive B0s →J/ψK+K− s-wave contributions. The measured value of φs = 0.16±0.18(stat)±0.06(syst) rad is the most precise measurement to date, and is consistent with Standard Model predictions.

This thesis presents the design qualification and construction of the Beam Scintillator Counter (BSC) for the CMS Collaboration at CERN in 2007 - 2008. The BSC detector is designed to aid in the commissioning of the Compact Muon Solenoid (CMS) during the first 2 years of operation and provide technical triggering for beam halo and minimum-bias events. Using plastic scintillator tiles mounted at both ends of CMS, it will detect minimum ionizing particles through the low-to-mid luminosity phases of the Large Hadron Collider (LHC) commissioning. During these early phases, the BSC will provide probably the most interesting and widely used data of any of the CMS sub-detectors and will be employed in the track based alignment procedure of the central tracker and commissioning of the Forward Hadron Calorimeter.

Parton distribution functions (PDFs) provide a description of the quark and gluon content of the proton. They are important input into theoretical calculations of hadronic observables, and are obtained by fitting to a wide range of experimental data. The NNPDF approach to fitting PDFs provides a robust and reliable determination of their central values and uncertainties. The PDFs are modelled using neural networks, while the uncertainties are generated through the use of Monte Carlo replica datasets. In this thesis I provide an in depth description of development of the latest NNPDF determination: NNPDF3.0. A number of novel adaptations to the genetic algorithm and network structure are outlined and the results of tests as to their effectiveness are shown. Centrally, the use of closure tests, where artificial data is generated according to a known theory and used to perform a fit, has been instrumental in both the development and validation of the NNPDF3.0 approach. The results of these tests, which demonstrate the ability of our methodology to reproduce a known underlying law, are investigated in detail. Finally, results from the NNPDF3.0 PDF sets are presented. The parton distributions obtained are compared with results from other PDF collaborations, and PDFs fit to limited datasets are also discussed. Physical observables relevant for future collider runs are presented and compared to other determinations.

Unparticle physics is the physics of a hidden sector which is conformal in the infrared and coupled to the Standard Model. The concept of unparticle physics was introduced by Howard Georgi in 2007 and has since then received a lot of attention, including many studies of its phenomenology in different situations. After a review of the necessary background material, the implications of the self-interactions of the unparticle sector for LHC physics is studied. More specifically, analyses of four-body final states consisting of photons and leptons are performed. The results are upper bounds on the total cross sections as well as distributions of transverse momentum.

Cette thèse présente des recherches sur la nouvelle physique produite par le processus de Fusion de Bosons Vecteur (VBF) dans les états finaux avec une grand impulsion transverse manquante (Etmiss) en utilisant 36.1 fb⁻¹ de données de collisions proton-proton avec une énergie dans le centre de masse de 13 TeV, recueillies par l'expérience ATLAS au Large Hadron Collider (LHC) au CERN en 2015 et 2016. En particulier, elle se concentre sur la recherche de la désintégration invisible du boson de Higgs produit via le mode VBF. Comme le modèle standard de la physique des particules (MS) prédit une désintégration invisible de Higgs uniquement à travers le mode H->ZZ*->4v avec un rapport d’embranchement BR ~ 0,1%, si une désintégration en particules invisibles du boson de Higgs était observée avec un BR supérieur, ce serait un signe de nouvelle physique. Plusieurs modèles au-delà du modèle standard (BSM) prédisent des désintégrations du boson de Higgs en particules de matière noire (DM, non détectées) ou en particules massives neutres à vie longue. Parmi les recherches H->particules invisibles, la plus sensible est celle où le Higgs est produit via le mode VBF. Son état final est caractérisé par deux jets énergétiques, avec les caractéristiques typiques du mode VBF (c'est-à-dire une grande séparation angulaire et une grande masse invariante des deux jets) et une grande impulsion transverse manquante (Etmiss>180 GeV). Pour sélectionner un échantillon d'événements candidats de signal, une région de signal (SR) est définie pour maximiser la fraction d'événements de signal attendus par rapport à la prédiction du MS (bruit de fond). Les processus MS qui peuvent peupler la SR proviennent principalement des processus Z->vv+jets et W->lv+jets, où le lepton est perdu ou non reconstruit. Leur contribution est estimée avec une approche semi-data driven : des régions dédiées enrichies en événements W->lv/Z->ll sont utilisées pour normaliser les données des estimations de Monte Carlo (MC) en utilisant une technique de fit simultané (méthode du facteur de transfert) et pour les extrapoler à la SR. L'estimation de fond prédit est comparée aux données SR observées. Comme aucun excès n'est trouvé, une limite supérieure sur le BR (H-> invisible) est calculée. L'analyse est ensuite réinterprétée dans le cadre de modèles inspirés du modèle Minimal Dark Matter. Le cas d'un nouveau triplet fermionique électrofaible, avec une hypercharge nulle et avec interactions respectant le nombre B-L, ajouté au MS fournit un bon candidat Dark Matter (WIMP pure). Si on considère l'abondance thermique, la masse du composant neutre est d’environ 3 TeV. Cependant des masses plus faibles sont également envisageables dans le cas de mécanismes de production non thermiques ou lorsque le triplet ne constitue qu'une fraction de l'abondance de DM. Il peut être produit à des collisionneurs proton-proton tels que le LHC et il peut être sondé de différentes manières. Une fois produites, les composantes chargées du triplet se désintègrent dans le composant neutre le plus léger, χ0 , avec en plus des pions très mous, en raison de la petite différence de masse entre les composants neutres et chargés. Ces pions de très faible impulsion ne peuvent pas être reconstruits et sont donc perdus. Le χ0 est reconstruit comme de l’Etmiss dans le détecteur. Par conséquent, lorsqu'il est produit via VBF, il donne lieu à une signature avec deux jets VBF et de l’Etmiss, le même état final que celui qui a été étudié pour l'analyse de VBF H->invisible. Des points de masse différentes (de 90 GeV à 200 GeV) ont été engendrés avec les programmes Monte Carlo Madgraph+Pythia, dans le cadre du logiciel officiel ATLAS, et les limites supérieures sont définies sur la section efficace fiducielle de production. Des extrapolations à des luminosités plus élevées (Run3 et HL-LHC) en utilisant une approche simplifiée sont également présentées. / This thesis presents searches for new physics produced via Vector Boson Fusion (VBF) in final states with large Missing Transverse Momentum (Etmiss) using 36.1 fb⁻¹ of data from proton-proton collisions at center-of-mass-energy of 13 TeV, collected by the ATLAS experiment at the Large Hadron Collider at CERN during 2015 and 2016. In particular, it focuses on the search for the invisible decay of the Higgs boson produced via the vector boson fusion (VBF) process. As the SM predicts an Higgs invisible decay only through H->ZZ*->4v with Branching Ratio BR~0.1%, if an invisibly decaying Higgs boson would be observed with a higher BR, this would be a sign of new physics. Several Beyond the Standard Model (BSM) models predict invisibly decaying Higgs boson where the Higgs can decay into dark matter particles or neutral long-lived massive particles. Among the H->invisible searches the most sensitive one is the one where the Higgs is produced via the VBF process. Its final state is characterized by two energetic jets, with the typical features of the VBF mode (i.e. large angular separation and large invariant mass) and large missing transverse momentum (Etmiss>180 GeV). To select a sample of signal candidate events, a Signal Region (SR) is designed to maximize the fraction of expected signal events with respect to the SM prediction (backgrounds). The SM processes which can populate the SR comes mainly from Z->vv+jets and W->lv+jets processes, where the lepton is lost or not reconstructed. Their contribution is estimated with a semi data driven approach: dedicated regions enriched in W->lv/Z->ll events are used to normalize to data the Monte Carlo (MC) estimates using a simultaneous fitting technique (transfer factor) and to extrapolate them to the SR. The predicted background estimate is compared to the observed SR data. Since no excess is found, an upper limit on the BR(H->inv) is set. The analysis is then reinterpreted in the context of models inspired by the Minimal Dark Matter model. The case of a new electroweak fermionic triplet, with null hypercharge and with interactions respecting the B-L number, added on top of the SM provides a good Dark Matter candidate. As such, it is an example of pure Weakly Interacting Massive Particle (WIMP), meaning that it is a DM particle with SU(2)_L SM interactions which is not mixing with other states (pure).If the thermal abundance is assumed, the mass of the neutral component is around 3 TeV, however smaller masses are also allowed in case of non-thermal production mechanisms or if the triplet constitutes only a fraction of the DM abundance. It can be produced at proton-proton colliders such as the LHC and it can be probed in different ways. Once produced, the charged components of the triplet decays into the lightest neutral component chi0 plus very soft charged pions. chi0 is reconstructed as Etmiss in the detector while the pions, because of the small mass splitting between the neutral and charged components, are so soft that are lost and are not reconstructed. Therefore, when produced via VBF, it gives rise to a signature with two VBF jets and Etmiss, the same final state that has been investigated for the VBF Higgs invisible analysis. Different mass point (from 90 GeV to 200 GeV) have been generated with the Madgraph+Pythia, Monte Carlo programs within the official ATLAS software, and upper limits are set on the fiducial cross section. Extrapolations to higher luminosities using a simplified approach are also presented.

Coloured final states are ubiquitous at hadron colliders such as the Large Hadron Collider (LHC). Therefore understanding high energy perturbative quantum chromodynamics (QCD) at these experiments is essential not only as a test of the Standard Model, but also because these processes form the dominant background to many searches for new physics. One such `standard candle' is the production of a dilepton pair in association with dijets. Here we present a new description of this final state (through the production of a Z⁰ boson and γ*). This calculation adds to the fixed-order accuracy the dominant logarithms in the limit of large partonic centre-of-mass energy to all orders in the strong coupling αs. This is achieved within the framework of High Energy Jets. This calculation is made possible by extending the high energy treatment to take into account the multiple t-channel exchanges arising from Z⁰ and gamma* -emissions off several quark lines. The correct description of the interference effects from the various t-channel exchanges requires an extension of the subtraction terms in the all-order calculation. We describe this construction and compare the resulting predictions to a number of recent analyses of LHC data. The description of a wide range of observables is good, and, as expected, stands out from other approaches in particular in the regions of large dijet invariant mass and large dijet rapidity spans. In addition we also present the application of the High Energy Jets framework to two new experimental scenarios. Firstly, we show a comparison of High Energy Jets matched to the ARIADNE parton shower to an ATLAS study of gap activity in dijet events. We see that our description agrees well with the data throughout and in many distributions gives the best theoretical description. This shows the extra logarithmic corrections are essential to describe data already in LHC Run I. Secondly, we present a study of Z⁰/γ* plus dijets at 100 TeV. We compare the behaviour of the high energy logarithmic enhancements to the QCD perturbative series at 7 TeV and 100 Tev and see that at any high energy hadronic Future Circular Collider (FCC) the effects described by our resummation become significantly more important.

<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>

Cette thèse résume une recherche de manifestations de Grandes Dimensions Supplémentaires (GDS, og Large Extra Dimensions fg en anglais) en utilisant 4.91 fb-1 de données enregistrées en 2011 par le détecteur Atlas installé auprès du collisionneur LHC au CERN. En 2011, le LHC a produit des collisions proton-proton à une énergie dans le centre de masse de sqrt(s)= 7 TeV. Les GDS peuvent potentiellement expliquer une énigme connue sous le nom du problème de la hiérarchie : la grande différence entre l'échelle électrofaible et l'échelle de Planck dans le Modèle Standard (MS). Dans le cadre du modèle ADD (nommé selon les auteurs N. Arkani-Hamed, S. Dimopoulos and G. Dvali) des GDS, les effets de la gravitation quantique deviennent plus forts que dans le MS; potentiellement suffisamment forts pour être observés au LHC. Il y a deux mécanismes de production de gravitons dans les collisions proton-proton : production directe de gravitons et échange virtuel de gravitons. Dans cette thèse, nous présentons une recherche de dimensions supplémentaires via l'effet de l'échange virtuel de gravitons dans l'état final di-photon. Le spectre de masse invariante des événements di-photon est étudié, et un bon accord entre les données et le bruit de fond prédit par le MS est observé. Nous utilisons deux méthodes pour estimer des limites sur l'échelle de Planck fondamentale du modèle ADD : une expérience de comptage et une analyse de la forme du spectre de masse. L'expérience de comptage donne des limites entre 2.62 et 3.92 TeV à 95% C.L., en fonction du nombre de dimensions supplémentaires et du formalisme théorique utilisé. L'analyse de la forme du spectre de masse donne des limites légèrement plus strictes : la limite inférieure sur l'échelle de Planck fondamentale augmente d'un facteur de 1.04. / This thesis summarizes a search for manifestations of Large Extra Dimensions (LED) using 4.91fb-1 of data collected in 2011 by the Atlas detector at the LHC collider at CERN. In 2011, the LHC has provided proton-proton collisions at a center-of-mass energy of sqrt(s) = 7 TeV. LED can potentially solve the so-called hierarchy problem, i.e. large apparent difference between two fundamental scales of the Standard Model (SM), the electroweak and the Planck scales. In the context of the ADD model (named after the authors N. Arkani-Hamed, S. Dimopoulos and G. Dvali) of LED, the effects of quantum gravity become much stronger than in the SM; possibly large enough to be observed at the LHC. There are two possibilities of graviton production in proton-proton collisions: direct graviton production and virtual graviton exchange. In this thesis, we present a search for the manifestation of extra dimensions via the effect of virtual graviton exchange on the di-photon final state. The di-photon invariant mass spectrum is studied and found to be in good agreement with SM background expectation. We set limits on the fundamental Planck scale of the ADD model using two different methods: a counting experiment and an analysis of the shape of the di-photon mass spectrum. The counting experiment yields limits between 2.62 and 3.92 TeV at 95% CL, depending on the number of extra dimensions and the theoretical formalism used. The shape analysis yields slightly more stringent limits: the lower limits on the fundamental Planck scale improve by a factor of 1.04.