Search for top quark-antiquark pair resonances with the ATLAS experiment

Diese Dissertation beschreibt die Suche nach Resonanzen in der Erzeugung von Top-Quark-Antiquark-Paaren in Proton-Proton-Kollisionen bei einer Schwerpunktsenergie von 8 TeV. Die ausgewerteten Daten entsprechen einer integrierten Luminosität von 20.3 fb-1 und wurden mit dem ATLAS-Detektor am Large Hadron Collider aufgezeichnet. Die Analyse basiert auf Ereignissen mit genau einem geladenen Lepton, fehlendem Transversalimpuls und Teilchenjets im Endzustand. Mithilfe eines künstlichen neuronalen Netzes wird bei gleicher Signaleffizienz eine präzisere Identifikation von hochenergetischen, hadronisch zerfallenden Top-Quarks erzielt als mit bisher verwendeten Verfahren. Darauf aufbauend wird eine Ereignisselektion für die Erkennung von s-Kanal-Resonanzen mit Spin-1 optimiert. Eine verbesserte Abschätzung des erwarteten Untergrunds wird eingeführt, die den Einfluss der systematischen Unsicherheiten verringert. Die untersuchten Massenspektren der rekonstruierten Top-Quark-Paare weisen keine signifikante Abweichung von den Vorhersagen des Standardmodells auf. Im Vergleich zu schnittbasierten Suchstrategien lassen sich für Massen über 2 TeV etwa 30% bessere bayessche obere Ausschlussgrenzen auf den Wirkungsquerschnitt einer schmalen Resonanz eines leptophobischen Z'-Bosons in einem Technicolour-Modell sowie einer breiten Kaluza-Klein-Anregung des Gluons in einem Randall-Sundrum-Modell erzielen. Ausgehend von den beobachteten Ausschlussgrenzen auf den Wirkungsquerschnitt werden eine Z'-Resonanz bis zu einer Masse von 2.5 TeV und ein Randall-Sundrum Kaluza-Klein-Gluon bis zu einer Masse von 2.9 TeV ausgeschlossen. Dies stellt eine Verbesserung gegenüber bisherigen Suchen in vergleichbaren Datensätzen dar.A search for new heavy particles that decay into top quark-antiquark pairs is presented using an integrated luminosity of 20.3 fb-1 of proton-proton collision data recorded at a centre-of-mass energy of 8 TeV with the ATLAS experiment at the Large Hadron Collider. The analysis considers events with exactly one charged lepton, missing transverse momentum and jets in the final state. An artificial neural network is utilized to identify hadronically decaying top quarks with high Lorentz boost more precisely than established methods at the same signal efficiency. Based on this, a novel method optimized for the detection of s-channel resonances with spin 1 is created. An enhanced estimation of the background expectations is introduced, which reduces the impact of the systematic uncertainties on the analysis. No significant deviation from the background predicted by the Standard Model of particle physics is observed in the invariant mass spectrum of the top-quark pair candidates. Bayesian upper cross-section limits on a narrow resonance from a leptophobic Z' boson in the framework of a technicolour model and a broad Kaluza-Klein excitation of the gluon in a Randall-Sundrum model are both found to be about 30% better at resonance masses of at least 2 TeV relative to a cut-based search strategy. Based on the observed cross-section limits, a topcolour-assisted technicolour Z' boson up to a mass of 2.5 TeV and a Randall-Sundrum Kaluza-Klein gluon up to a mass of 2.9 TeV are excluded. These are the most stringent exclusion limits on the resonance mass as compared to other searches based on similar data sets

ATLAS Run 1 searches for direct pair production of third-generation squarks at the Large Hadron Collider

This paper reviews and extends searches for the direct pair production of the scalar supersymmetric partners of the top and bottom quarks in proton-proton collisions collected by the ATLAS collaboration during the LHC Run 1. Most of the analyses use 20 fb$^{-1}$ of collisions at a centre-of-mass energy of $\sqrt{s}$ = 8 TeV, although in some case an additional 4.7 fb$^{-1}$ of collision data at $\sqrt{s}$ = 7 TeV are used. New analyses are introduced to improve the sensitivity to specific regions of the model parameter space. Since no evidence of third-generation squarks is found, exclusion limits are derived by combining several analyses and are presented in both a simplified model framework, assuming simple decay chains, as well as within the context of more elaborate phenomenological supersymmetric models

Search for top quark-antiquark pair resonances with the ATLAS experiment

AbstractA search for new heavy particles that decay into top quark-antiquark pairs ispresented using an integrated luminosity of 20.3 fb−1 of proton-proton collision datarecorded at a centre-of-mass energy of √s = 8 TeV with the ATLAS experiment atthe Large Hadron Collider. The analysis considers events with exactly one chargedlepton, missing transverse momentum and jets in the final state. An artificialneural network is utilized to identify hadronically decaying top quarks with highLorentz boost more precisely than established methods at the same signal efficiency.Based on this, a novel method optimized for the detection of s-channel resonanceswith spin 1 is created. An enhanced estimation of the background expectations isintroduced, which reduces the impact of the systematic uncertainties on the analysis.No significant deviation from the background predicted by the Standard Modelof particle physics is observed in the invariant mass spectrum of the top-quarkpair candidates. Bayesian upper cross-section limits on a narrow resonance froma leptophobic Z′ boson in the framework of a technicolour model and a broadKaluza-Klein excitation of the gluon in a Randall-Sundrum model are both foundto be about 30% better at resonance masses of at least 2 TeV relative to a cut-basedsearch strategy. Based on the observed cross-section limits, a topcolour-assistedtechnicolour Z′TC2 up to a mass of 2.5 TeV and a Randall-Sundrum Kaluza-Kleingluon up to a mass of 2.9 TeV are excluded. These are the most stringent exclusionlimits on the resonance mass as compared to other searches based on similar datasets

Monte-Carlo-Simulation von Top-Paaren in nächstführender Ordnung am ATLAS-Experiment

Due to the probabilistic nature of quantum mechanics, the structure of individual events in particle physics is not calculable ab initio. Therefore, Monte Carlo simulations mark an important tool for understanding experiments in elementary particle physics. Most current Monte Carlo generators employ leading-order matrix elements; these cannot meet the level of accuracy that can be reached by the ATLAS experiment. Although calculations in next-to-leading order have been known for some years, their implementation for generating physically meaningful events is not trivial. The first successful implementation, which defines the standard in this area, is MC@NLO. An alternative approach is the Powheg method, which is examined in more detail in this thesis on the basis of top pair production. The results of both methods are compared and differences are evaluated. It is shown that Powheg represents a viable alternative to MC@NLO that avoids some of its shortcomings. In particular, the method is independent of the parton shower formalism, which allows combining it with the Pythia generator. In this combination, variations of space- and time-like emissions in parton showers are investigated and compared with the results in leading order. The analysis suggests that variations remain substantially valid with matrix elements in next-to-leading order. Nevertheless, significant differences between the two matrix elements are discovered tha t require further study