In this thesis the jet energy scale uncertainty of small and large radius jets at the ATLAS detector is evaluated in-situ using gamma-jet events. The well calibrated photon in the gamma-jet events is used to probe the energy scale of the jets. The studies of the jet energy scale of small radius jets are performed using 4.7 fb^-1 of data collected at sqrt{s} = 7 TeV in 2011. The gamma-jet methods which were developed are then adapted and applied to large radius jets, using 20.3 fb^-1 of data collected at sqrt{s} = 8 TeV in 2012. The new jet energy scale uncertainties are found to be ~1 % for |eta| < 0.8, rising to 2-3 % for |eta| > 0.8. These uncertainties are significantly lower than the 3-6 % precision which has previously been achieved at ATLAS using track jets as a reference object. Due to the increase in precision, uncertainties due to pile-up and the topology of the jet also had to be evaluated. The total energy scale uncertainties for large radius jets are reduced by ~1-2 % (0.5-1 %) for |eta| < 0.8 (> 0.8). This reduction will be beneficial to analyses using large radius jets and it is specifically shown to benefit the t-tbar resonance search in the semi-leptonic channel. The t-tbar search looks for events with two top quarks in the final state, where one decays leptonically and the other hadronically. The hadronically decaying top quark is reconstructed using a large radius jet, and the jet energy scale uncertainty is a dominant source of uncertainty in the analysis. In addition to the studies of the jet energy scale of large radius jets, the first derivation of a calibration, and jet energy scale uncertainties derived with gamma-jet events, are shown for Variable R jets. The Variable R jet algorithm is a new type of jet algorithm with a radius that is inversely proportional to the size of the jet, making it useful for the study of high momentum top quarks. It is shown that similar methods can be used to calibrate and assess the uncertainties of Variable R jets as are used for standard, fixed radius jets at the ATLAS detector, although some adaptations will be necessary. The studies provide a basis for the calibration of Variable R jets in the future.

Many theories of physics beyond the Standard Model predict the existence of TeV-scale resonances that decay to top quark pairs. This thesis presents a search for such resonances produced in proton-proton collisions at a centre-of-mass energy of 7 TeV at the CERN Large Hadron Collider with 4.7 fb−1 of data collected by the ATLAS detector in 2011. The search is performed in the channel where both top quarks decay hadronically and emerge in highly boosted states, and the collimated decay products of each boosted top quark are reconstructed as a single large jet (a “top quark jet”). A tagging technique based on the distinctive masses and substructures of these top quark jets is used to distinguish them from light quark and gluon jets. The data are found to be consistent with Standard Model predictions, and 95% credibility level upper limits are set on the cross section times branching ratio for leptophobic Z' bosons in a Topcolour model and Kaluza-Klein gluons as predicted by the bulk Randall-Sundrum model. These limits exclude Kaluza-Klein gluons with masses between 1.02 TeV and 1.62 TeV. The substructure of light quark and gluon jets, which form the dominant background to hadronically decaying boosted top quarks, is studied in an auxiliary measurement using 35 pb−1 of data collected by ATLAS during the 2010 run period. The observed substructure of these jets are found to be in good agreement with theoretical predictions.

Many theories of physics beyond the Standard Model predict the existence of TeV-scale resonances that decay to top quark pairs. This thesis presents a search for such resonances produced in proton-proton collisions at a centre-of-mass energy of 7 TeV at the CERN Large Hadron Collider with 4.7 fb−1 of data collected by the ATLAS detector in 2011. The search is performed in the channel where both top quarks decay hadronically and emerge in highly boosted states, and the collimated decay products of each boosted top quark are reconstructed as a single large jet (a “top quark jet”). A tagging technique based on the distinctive masses and substructures of these top quark jets is used to distinguish them from light quark and gluon jets. The data are found to be consistent with Standard Model predictions, and 95% credibility level upper limits are set on the cross section times branching ratio for leptophobic Z' bosons in a Topcolour model and Kaluza-Klein gluons as predicted by the bulk Randall-Sundrum model. These limits exclude Kaluza-Klein gluons with masses between 1.02 TeV and 1.62 TeV. The substructure of light quark and gluon jets, which form the dominant background to hadronically decaying boosted top quarks, is studied in an auxiliary measurement using 35 pb−1 of data collected by ATLAS during the 2010 run period. The observed substructure of these jets are found to be in good agreement with theoretical predictions.

The coupling of the Standard Model top quark to the Higgs boson is O(1), which leads to large quantum corrections in the perturbative expansion of the Higgs boson mass. Possible solutions to this so-called naturalness problem include supersymmetric models with gluinos and stop squarks whose masses are at the electroweak scale, O(1 TeV). If supersymmetry is realised in nature at this scale, these particles are expected to be accessible with the Large Hadron Collider at CERN. A search for gluino pair production with decays mediated by stop- and sbottom-squark loops in the initial 14.8 ifb of the ATLAS run 2 dataset is presented in terms of a pair of simplified models, which targets extreme regions of phase space using jet substructure techniques. No excess is observed and limits are set which greatly extend the previous exclusion region of this search, up to 1.9 TeV (1.95 TeV) for gluinos decaying through light stop (sbottom) squarks to the lightest neutralinos. A performance study of top tagging algorithms in the 20.3 ifb 2012 dataset is also presented, which includes the first measurements of substructure-based top tagging efficiencies and fake rates published by ATLAS, as well as a detailed comparison of tagger performance in simulation. A benchmarking study which compares commercially available cloud computing platforms for applications in High Energy Physics, and a summary of ATLAS liquid argon calorimeter data quality work focused on monitoring and characterising the sporadic phenomena of Mini Noise-Bursts in the electromagnetic barrel calorimeter are also included. / Graduate / 0798 / matt.leblanc@cern.ch

Lorsqu'un boson Z de haute impulsion se désintègre en paire e+e-, l'électron et le positron laissent des traces très rapprochées l'une de l'autre, ce qui rend la reconstruction de ce processus peu efficace. L'identification de signature leptoniques d'évènements massifs issus de théories telles l'extension du groupe de jauge électrofaible SU(2)_1 x SU(2)_2 x U(1) pour ajouter des bosons massifs W' et Z' se couplant directement aux bosons vectoriels et aux fermions du modèle standard est important dans l'optique de recherche de nouvelle physique puisque les états finaux leptoniques fournissent le canal de détection de plus pur. Ici, un outil d'identification des paires d'électron-positron issues de bosons Z à haute impulsion basé uniquement sur les variables décrivant les jets est développé en se basant sur des simulations d'évènements massifs produits dans le détecteur ATLAS du LHC au CERN. Une étude topologique des évènements candidats est portée dans un ensemble d'échantillons Monte Carlo. Une série de coupures rectangulaires est optimisée sur un échantillon de bosons Z produits avec une haute impulsion. L'efficacité de reconstruction des paires d'électron-positron est maximisée. Cinq sources de bruit de fonds ont été soumises aux critères de sélection pour l'étude de la suppression du bruit de fonds. Finalement l'outil développé est comparé à l'identification de paires d'électron-positron basé sur la reconstruction usuelle d'objets leptoniques au LHC. Cette comparaison met en évidence le gain significatif de l'outil développé, atteignant une efficacité de reconstruction de 93% dans des échantillons Monte Carlo à haute énergie, avec un bruit de fond négligeable. / When a Z boson with high transverse momentum desintegrates into an e+e- pair, the electron and the positron leave nearly superimposed tracks which makes the detection process very difficult. The identification of massive events arising from theories like the extension of the electroweak gauge group SU(2)_1 x SU(2)_2 x U(1) leading to the existence of massive W' and Z' bosons coupling directly to standard model vector bosons and fermions is more readily performed through leptonic channel. Here, a series of rectangular cuts comprising a purely jet substructure based electron positron pair arising from the decay of a Z boson identification in boosted topologies (p_T(Z)>1TeV) is presented. The selection process of the cuts is done through a topology study of candidate events in various Monte Carlo simulation samples. The reconstruction efficiency of the electron-positron pairs is maximised. Five background sources were tested against the selection criteria to study the background rejection efficiency of the methodology. Finally, it is found that the reconstruction tool developped nearly triples electron positron identification, reaching 93% efficiency in high energy Monte Carlo samples, with insignificant background noise.