This thesis describes a measurement of the Standard Model Higgs boson coupling to fermions in decays to two τ-leptons, a search for charged Higgs bosons in decays to a τ-lepton and a neutrino, as well as the reconstruction, identification and triggering of hadronically decaying τ-leptons. The data considered are collected by the ATLAS experiment at the Large Hadron Collider. The reconstruction and identification of hadronically decaying τ-leptons in the ATLAS experiment during Run 2 of the Large Hadron Collider are described, and the performance of the τ trigger is measured in events with top–antitop-quark pairs using data from 13 TeV proton–proton collisions, corresponding to an integrated luminosity of 3.2 fb-1, collected in 2015, and 11.5 fb-1, collected in 2016. Hadronically decaying τ-leptons are of importance to many physical processes involving Higgs bosons. The coupling of the Standard Model Higgs boson to fermions is measured in decays to two τ-leptons using 7 TeV data corresponding to an integrated luminosity of 4.5 fb-1, collected in 2011, and 8 TeV data corresponding to an integrated luminosity of 20.3 fb-1, collected in 2012. The signal strength is measured to be μ = 1.4, corresponding to an excess over the background-only model of 4.5σ. Charged Higgs bosons are searched for in decays to a τ-lepton and a neutrino, where the τ-lepton decays hadronically. 13 TeV data corresponding to an integrated luminosity of 14.7 fb-1, collected in 2015 and 2016, are used. No excess over the Standard Model background is observed, and the 95 % confidence-level exclusion limits on σ(pp → [b]tH+) × (H+ → τν) are set to 2.0 pb–8 fb in the range 200–2000 GeV.

This thesis presents searches for evidence for Weakly Interacting Massive Particles (WIMPs) and Extra Dimensions in proton-proton collisions recorded by the ATLAS experiment at the CERN Large Hadron Collider (LHC). The WIMP is one of the main candidates to constitute the particle content of Dark Matter. Extra Dimensions are introduced in several theories in order to explain the apparent weakness of gravity when compared to the other interactions in Nature. Theories with WIMPs as well as Extra Dimensions can manifest themselves at the LHC, with experimental signatures characterized by an energetic hadronic jet associated with large missing momentum. These signatures are known as monojet signatures, and are investigated in this thesis.  The first analysis is performed using L = 20.3 fb-1 of proton-proton collisions at √s = 8 TeV recorded in the ATLAS Run 1. The second analysis is performed using L = 3.2 fb-1 of proton-proton collisions at √s = 13 TeV recorded in the ATLAS Run 2. No significant excess over the expected background is found in either of the analyses. New exclusion limits are set at 95% confidence level on Dark Matter particle production. New limits are also set on graviton production in the so-called ADD scenario with Extra Dimensions.

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Searches are presented for narrow heavy resonances decaying to a top quark and a bottom quark using data collected by the CMS experiment at a center-of-mass energy of 13 TeV. Final states that include a single lepton (electron, muon), multiple jets, and missing transverse momentum are analyzed. No evidence is found for the production of a W' boson, and the production of right-handed W' bosons is excluded at 95% confidence level for masses up to 3.6 TeV depending on the scenario considered. Exclusion limits for W' bosons are also presented as a function of their coupling strength to left- and right-handed fermions. These limits on a W' boson decaying via a top and a bottom quark are the most stringent published to date. Projections for future searches with an integrated luminosity of up to 3 inverse attobarns are also presented, and suggest that W' boson masses above 4 TeV could be excluded.

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.

This thesis contains the work of two recent developments in the Herwig general purpose event genrator. Firstly, the results from an new implementation of the KrkNLO method in the Herwig event generator are presented. This method allows enables the generation of matched next-to-leading order plus parton shower events through the application of simple positive weights to showered leading order events. This simplicity is achieved by the construction Monte Carlo scheme parton distribution functions. This implementation contains the necessary components to simulation Drell-Yan production as well as Higgs production via gluon fusion. This is used to generate the first differential Higgs results using this method. The results from this implementation are shown to be comparable with predictions from the well established approaches of POWHEG and MC@NLO. The predictions from KrkNLO are found to closely resemble the original configuration for POWHEG. Secondly, a benchmark study focussing on the source of perturbative uncertainties in parton showers is presented. The study employs leading order plus parton shower simulations as a starting point in order to establish a baseline set of controllable uncertainties. The aim of which is to build an understanding of the uncertainties associated with a full simulation which includes higher-order corrections and interplay with non- perturbative models. The uncertainty estimates for a number of benchmark processes are presented. The requirement that these estimates be consistent across the two distinct parton show implementations in Herwig provided an important measure to assess the quality of these uncertainty estimates. The profile scale choice is seen to be an important consideration with the power and hfact displaying inconsistencies between the showers. The resummation profile scale is shown to deliver consistent predictions for the central value and uncertainty bands.

In the Large Hadron Collider (LHC), particle physics events are created by colliding high energy proton beams at a number of interaction points around the ring. One of the main performance indicating parameters of the LHC is the luminosity. The luminosity is limited by, amongst other things, the strength of the beam-beam interaction. In this thesis, the effect of the beam-beam interaction on the luminosity performance of the LHC and the proposed High Luminosity Large Hadron Collider (HL-LHC) is investigated. Results from a number of dedicated, long-range beam-beam machine studies are presented and analysed. In these studies, the minimum beam-beam separation for two different beta star— optics are identified. This separation defines the minimum operational crossing angle in the LHC. The data from these studies are then compared to simulation of the dynamic aperture and the results are discussed. In addition to studies of the LHC, an analytical approach is derived in order to describe the hourglass effect, which may become a contributing factor in limiting the luminosity performance of the HL-LHC.

Title: Analysis of J/Psi + photon production in proton-proton collisions on ATLAS at LHC Author: Bc. Zuzana Vidláková Department: Institute of particle and nuclear physics Supervisor: prom. fyz. Václav Vrba, CSc. (Academy of Sciences of the Czech Republic) Abstract: The subject of this thesis is physics connected to quarkonia and their studies at the ATLAS detector situated at LHC in CERN, Geneva. In this thesis, following topics are cov- ered: ATLAS detector, history of physics connected to quarkonia, production mechanisms of quarkonia, software used at CERN for data analysis and cross-section measurement of J/Psi + photon continuum. The analysis was done on behalf of the B-physics working group at ATLAS. Keywords: CERN, LHC, ATLAS, B-fyzika, quarkonia

Le thème des analyses présentées dans ce document est la mesure des propriétés du boson de Higgs se désintégrant dans le mode H→ZZ→4l dans l'expérience ATLAS au CERN. Le document commence par un résumé détaillé concernant la procédure d'étalonnage des électrons: l'algorithme de combinaison trace-cluster améliore la résolution en énergie (surtout pour les électrons ayant une faible énergie transverse) en exploitant les informations du cluster et de la trace dans un ajustement par maximum de vraisemblance. L'amélioration en résolution est approximativement de 18-20% pour les désintégrations du J/Ψ en di-électrons, et 3% pour Z→ee. Par la suite, la combinaison E-p est appliquée au canal H→ZZ→4l avec électrons dans l'état final permettant d’obtenir un gain modéré sur la distribution de la masse invariante (4-5%). En deuxième lieu, la masse du boson de Higgs et sa largeur sont estimées, en particulier afin de comprendre les effets apportés par l'utilisation de l'algorithme de combinaison trace-cluster. La masse a été calculée en se servant d'un ajustement à deux dimensions appliqué sur la masse invariante m4l et un score de discrimination du signal contre le bruit de fond ZZ*. Cette discrimination est obtenue en exploitant les corrélations angulaires dont les distributions sont sensibles au spin et à la parité du boson de Higgs. L’étude sur la largeur du boson est ensuite détaillée : les résultats sont basés sur une approche qui vise à contraindre cette largeur en analysant la région de haute masse m4l où le boson de Higgs se comporte comme un propagateur. La section efficace au pic de la résonance (« on-shell ») dépend de la largeur totale du boson de Higgs, ce qui n’est pas le cas pour la production dans la région de haute masse (« off-shell »). Par conséquent, des limites indirectes sur la largeur peuvent être déterminées en combinant les régions « on-shell » et « off-shell ». Une limite à 6.7 fois la largeur Higgs ΓSMH est obtenue via le canal 4l. En combinant la mesure « on-shell » avec tous les canaux de désintégration étudiés (notamment ZZ→4l, ZZ→2l2ν and WW→lνlν), les résultats aboutissent à une limite observée (attendue) sur la largeur totale de 22.7 (33.0) MeV. La dernière partie de ce travail de thèse est consacrée à l'analyse sur la largeur du boson de Higgs en quatre leptons à haute (High-Luminosity LHC) luminosité intégrée (respectivement 300 fb⁻¹ et 3000 fb⁻¹) : il s’agit d’une étude extrapolant à √s =14 TeV les techniques utilisées pour l’analyse à 8 TeV (Run 1). / The theme of the analyses presented in this Thesis is the measurement of the Higgs boson properties in the H→ZZ→4l decay channel with the ATLAS experiment at the LHC. A detailed overview on the electron calibration process is first presented. In this regard, the track-cluster combination algorithm is found to improve the energy resolution of low ET electrons by exploiting both track and cluster information into a maximum likelihood fit. The improvement in resolution is approximately 18-20% for J/Ψ dielectron decays, and of the order of 3% for Z→ee events. In addition, the E-p combination algorithm has also been applied to the H→ZZ→4l channel with electrons in the final state resulting in a non-negligible gain on the invariant mass distribution (4-5%). Secondly, the Higgs mass and its total width are evaluated in the H→ZZ→4l channel. The Higgs mass is measured in the 4l decay channel with particular interest on the beneficial effects brought by the improved electron calibration and the track-cluster combination. The mass on the full 2011 and 2012 datasets is worked out with a 2-dimensional fit on the invariant mass of the 4 lepton final state, m4l, and on a boosted decision tree (BDT)-based output conceived against the main ZZ irreducible background and constructed on variables that are sensitive to the Higgs boson spin-parity state. Regarding the Higgs width, results are based on a relatively recent approach aimed at indirectly constraining the Higgs boson width by exploiting the m4l high-mass region where the Higgs boson acts as a propagator. The Higgs production cross section in the on-shell m4l region, where the Higgs boson is a resonance, depends on the total Higgs width, whereas this is not the case for the high mass m4l (off-shell). Limits on the Higgs width can be therefore set when merging the off-shell results with the on-shell ones. A limit of ∼ 6.7 times ΓSMH is obtained in the four lepton channel. Secondly, by combining with the on-shell measurement and using all the decay channels in the analysis, i.e. ZZ→4l, ZZ→2l2ν and WW→lνlν, the results lead to an observed (expected) 95% C.L. upper limit on the Higgs boson total width of 22.7 (33.0) MeV (4.2 MeV is the Standard Model predicted Higgs width at mH=125 GeV).The last section of the thesis is devoted to the evaluation of the Higgs width at √s=14 TeV in the high luminosity scenario (High Luminosity LHC), 300 fb⁻¹ and 3000 fb⁻¹, by employing the same techniques exploited in the previous Run 1 analysis at √s=8 TeV.

Tese de mestrado. Engenharia Electrotécnica e de Computadores (Área de especialização de Informática e Automação). Faculdade de Engenharia. Universidade do Porto. 2006