Two-charged-particle correlations are measured as a function of pseudorapidity and azimuthal angle difference in pp collisions at √s = 13, 2.76 and 5.02 TeV with the ATLAS detector at the Large Hadron Collider. A long-range structure in the two-dimensional function centered at ∆φ = 0 and extending over a large range of ∆η referred to as the “ridge” is seen in the three data sets. A template fitting method is implemented to extract the Fourier harmonics of the flow and gives the dependence of the harmonics on the charged-particle multiplicities. In this method a rescaled correlation function from peripheral events representing the recoil component plus a cosine modulation representing the ridge is used to describe the whole one-dimensional correlation function. Different multiplicity intervals for the peripheral events are used to extract the harmonics. The results presented show that vn,n from correlation functions can be factorized into the products of single particle vn. Significant contributions from v₂, v₃ and v₄ are obtained and their dependences on multiplicity and transverse momentum are studied. It is also shown that there is significant vn even in the lowest multiplicity bins. In addition, the second harmonics v₂ in pp do not have a significant dependence on both the multiplicity and collision energy. Results of pp and pPb at the same energy are compared with each other in both multiplicity and pT distributions. Both chᵗʳᵏ−chᵗʳᵏ and chᵗʳᵏ-muon correlations are measured in pPb collisions at √sNN = 8.16 TeV. Long-range correlations are studied through template fitting procedure. chᵗʳᵏ-v₂ increases with the number of reconstructed charged tracks at low multiplicity and saturates at high multiplicity. Muon-v₂ is considerably smaller than chᵗʳᵏ-v₂ and only has a weak dependence on event multiplicity. Factorization in both cases works pretty well. Two-charged-particle correlation functions are also measured in Xe+Xe events at √sNN = 5.44 TeV. In the most central collisions direct Fourier decomposition is preferred to avoid negative recoil component that might appear in the template fitting method. vn reaches its maximum value in the mid-centrality region and becomes smaller at both low and high centralities. Results are compared with Pb+Pb events at √sNN = 5.02 TeV showing that vn obtained from these two systems have similar values and behaviors.

Longstanding evidence from observational astronomy indicates that non-luminous "dark matter" constitutes the majority of all matter in the universe, yet this mysterious form of matter continues to elude experimental detection. This dissertation presents a search for dark matter at the Large Hadron Collider using 139 fb\(^{-1}\) of proton-proton collision data at a centre-of-mass energy of \(\sqrt{s} = 13\,\)TeV, recorded with the ATLAS detector from 2015 to 2018. The search targets a final state topology in which dark matter is produced from the proton-proton collisions in association with a pair of W bosons, one of which decays to a pair of quarks and the other to a lepton-neutrino pair. The dark matter is expected to pass invisibly through the detector, resulting in an imbalance of momentum in the plane transverse to the beam line. The search is optimized to test the Dark Higgs model, which predicts a signature of dark matter production in association with the emission of a hypothesized new particle referred to as the Dark Higgs boson. The Dark Higgs boson is predicted to decay to a W boson pair via a small mixing with the Standard Model Higgs boson discovered in 2012. Collisions that exhibit the targeted final state topology are selected for the search, and an approximate mass of the hypothetical Dark Higgs boson is reconstructed from the particles in each collision. A search is performed by looking for a deviation between distributions of the reconstructed Dark Higgs boson masses and Standard Model predictions for the selected collisions. The data is found to be consistent with the Standard Model prediction, and the results are used to constrain the parameters of the Dark Higgs model. This search complements and extends the reach of existing searches for the Dark Higgs model by the ATLAS and CMS collaborations. / Graduate

The Standard Model of particle physics constitutes the most accurate and comprehensive known description of the fundamental building blocks of the universe. However, overwhelming evidence suggests that the theory is incomplete and that new physics may be hiding at the TeV-scale. The Large Hadron Collider (LHC) at CERN probes these high-energy scales, opening a potential gateway to access physics beyond the Standard Model (BSM). Long-lived particles (LLPs) arise in many promising BSM theories, but they remain weakly constrained at the LHC. This thesis presents a novel search for displaced and delayed diphoton and dielectron vertices originating from the decay of a neutral LLP. The analysis uses the full LHC Run 2 dataset of pp collisions at a center-of-mass energy of √s = 13 TeV recorded by the ATLAS detector, corresponding to an integrated luminosity of 139 fb-1. The search harnesses the capabilities of the ATLAS Liquid Argon calorimeter to precisely measure the displacement and delay of the final state electromagnetic objects. The results are interpreted in a gauge-mediated supersymmetry breaking model that features the pair-production of LLPs, with each LLP subsequently decaying into either a Higgs boson or a Z boson. Since no significant excess is observed above the background expectation, the results are used to set upper limits on the cross section of LLP pair-production for signals with an LLP mass between 100 and 725 GeV and lifetime between 0.25 ns and 1 μs. A model-independent limit is also set on the production of pairs of photons or electrons with a significantly delayed arrival at the calorimeter.

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No description available.

Cette thèse décrit la recherche de nouvelles particules scalaires octets de couleur dans les données de l'expérience ATLAS auprès du Grand Collisionneur de Hadrons (LHC). Pour une large gamme de masse, la désintégration de ces scalaires en deux partons du MS domine. Cela motive la recherche de ces nouveaux scalaires dans des états finaux multijet, où ils se signaleraient comme des résonances dijet. Comme les nouveaux scalaires sont produits par paires, un état final contenant au moins quatre jets est utilisé comme environnement de recherche. Une méthode est développée pour extraire une possible résonance multijet scalaire du grand fond QCD et est utilisée pour chercher de tels scalaires dans les données de l'expérience ATLAS collectées en 2010 et 2011. Les données sont en accord avec l'estimation du fond et des limites sont posées sur la section efficace de production des scalaires en fonction de leur masse. En interprétant ces limites dans des modèles de supersymétrie, le gluon scalaire du MRSSM et du modèle hybride N=1/N=2 est exclu à 95 % CL entre 100 et 287 GeV. Les limites sont aussi interprétées dans un modèle de symétrie de jauge à la QCD, où le sgluon est remplacé par l'hyperpion qui est exclu dans une gamme de masse légèrement plus restreinte de part sa section efficace plus faible. / This thesis describes the search for new color-octet scalar particles in the ATLAS experiment data at the Large Hadron Collider (LHC). For a wide range of mass, the decay of the scalar to two SM partons dominates. This motivates the search for these new scalars in multijet final states, where they would manifest as dijet resonances. As the new scalars are products in pairs, a final state containing at least four jets is used as a search environment. A method is developed to extract a possible scalar resonance from the multijet QCD background and is used to search for such scalar in the data from the ATLAS experiment collected in 2010 and 2011. The data are in agreement with the estimation of the background and limits are set on the scalar production cross section as a function of the scalar mass. Interpreting these limits in models of supersymmetry, the scalar gluon of the MRSSM and of the hybrid N=1/N=2 model is excluded at the 95 % CL between 100 and 287 GeV. Limits are also interpreted in a model of gauge symmetry à la QCD, where the sgluon is replaced by the hyperpion excluded in a mass range slightly smaller because of its smaller cross section.

The Standard Model of particle physics is currently the most complete theory of subatomic particles. The discovery of the Higgs boson with a mass of 125 GeV in 2012 further validated the Standard Model, providing evidence for the theory that vector bosons obtain non-zero masses through the Higgs mechanism. Studies are ongoing to determine the exact nature and properties of the Higgs boson. A Higgs boson of this mass is predicted to decay to a pair of b-b quarks with a branching ratio of 58%, however this decay mode has not yet been observed. This thesis presents a search for the associated production of a Higgs boson with a leptonically decaying W boson, WH → ℓvb-b, using 20.3 fb-1 of Run 1 data collected by ATLAS at the LHC from pp collisions at a centre-of-mass energy of ps = 8 TeV. The observed (expected) significance of a Higgs boson with a mass of 125 GeV for the WH → ℓvb-b process is found to be 2:7σ (1:3σ). The measured cross section in units of the expected Standard Model cross section has a best-fit value of μ = μ/μSM = 2:2+0:67-0:64(stat:)+0:7-0:59(syst:) = 2:2+0:97-0:87. The results are combined with the search for ZH → v-vb-b and ZH → ℓ+ℓ-b-b to provide a best-fit value of μ = μ/μSM = 1:1+0:61-0:56. The start of Run 2 of the LHC in 2015 saw the collision energy being raised to √s = 13 TeV, increasing the probability of particles being produced with a large momentum boost. At these high energies there is also a possibility to discover new particles and interactions. An extension of the Standard Model, the Heavy Vector Triplet (HVT) model, describes new heavy vector bosons W¹ and Z¹, which can decay to pairs of heavy bosons (W, Z or Higgs bosons). If the W0 and Z0 bosons are sufficiently heavy, the hadronic decays of the diboson final states produce boosted jets. In this thesis, methods for identifying hadronically decaying boosted bosons are developed, based on techniques that examine the internal substructure of the jet. Multiple substructure variables are combined into a single discriminant using two machine learning techniques: boosted decision trees and deep neural networks. Simulated events of W¹→WZ → q-qq-q are used to develop these boosted W boson taggers. An improvement in the background rejection power, whilst keeping 50% of the signal, over previous boosted W boson taggers of up to 13%-when using deep neural networks-and 36%-when using boosted decision trees-is obtained. The performance of the new boosted W boson taggers are evaluated in a search for a narrow WW resonances from the decay of a Z¹ with boson-tagged jets in 3.2 fb-1 of pp collisions at √s = 13 TeV collected with the ATLAS detector.

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Die Arbeit beschreibt die Messung des Wirkungsquerschnittes für die Produktion von Jets in Ereignissen mit Z-Bosonen in Proton-Proton Kollisionen bei einer Schwerpunktsenergie von √s = 7 TeV mit einer integrierten Luminosität von ∫Ldt = 36 pb−1 und ∫Ldt = 4.6 fb−1 aufgenommen mit dem ATLAS Experiment am Large Hadron Collider in Genf. Die inklusiven und differenziellen Wirkungsquerschnitte für Z (→ e+ e−) + jets werden für Jets mit einem Transversalimpuls von pT(jet) > 30 GeV und einer Rapidität von |y(jet)| < 4.4 gemessen. Die Datensätze erlauben Messungen in vorher nicht zugänglichen Phasenraumregionen und können genutzt werden, um die Modellierungen von Z/γ∗ + jets in typischen Phasenraumregionen, die vom Zerfall des Higgs Bosons oder Suchen nach neuer Physik erwarten werden, zu testen. Die Ergebnisse werden auf Partonlevel entfaltet und mit Vorhersagen verschiedener Monte-Carlo Generatoren und Vorhersagen der perturbativen QCD in nächst-führender Ordnung verglichen.

Detector mis-modelling or hardware problems may cause an excess of Missing Transverse Energy (MET) to be inferred in physics events recorded in ATLAS, leading to higher than expected backgrounds for new physics. In particular, non-Gaussian tails in the MET distribution are unlikely to be well modelled in Monte Carlo simulations. To account for this, a technique has been established to improve the background predictions derived from Monte Carlo simulations. This is done using a correction derived through comparison of control samples in data and Monte Carlo simulation. Two different control samples are used to create the correction, giving a range of predictions for the shape of the MET tail and aiding the calculation of systematic errors. This technique is then applied to several samples which are potential backgrounds to new physics which give detector signatures which include MET. / Graduate