Search for New Physics in tt ̅ Final States with Additional Heavy-Flavor Jets with the ATLAS Detector

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Springer, Jul 20, 2016 - Science - 283 pages
This doctoral thesis focuses on the search for new phenomena in top-antitop quark (tt) final states with additional b-quark jets at the LHC. It uses the full Run 1 dataset collected by the ATLAS experiment in proton-proton collisions at √s=8 TeV. The final state of interest consists of an isolated lepton, a neutrino and at least six jets with at least four b-tagged jets, a challenging experimental signature owing to the large background from tt+heavy-flavor production. This final state is characteristic of ttH production, with the Higgs boson decaying into bb, a process that allows direct probing of the top-Higgs Yukawa coupling. This signature is also present in many extensions of the Standard Model that have been proposed as solutions to the hierarchy problem, such as supersymmetry or composite Higgs models, which predict the pair production of bosonic or fermionic top quark partners, or the anomalous production of four-top-quark events. All these physics processes have been searched for using an ambitious search strategy that has been developed on the basis of a combination of state-of-art theoretical predictions and a sophisticated statistical analysis to constrain in-situ the large background uncertainties. As a result, the most restrictive bounds to date on the above physics processes have been obtained.
 

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Contents

1 Introduction and Theoretical Framework
1
2 The ATLAS Experiment at the Large Hadron Collider
31
3 Event Simulation
49
4 Reconstruction of Physics Objects
61
5 Common Aspects in Searches for New Physics in tbart Final States with Additional HeavyFlavor Jets
86
6 Statistical Analysis
127
7 Searches for New Physics in tbart Final States with Additional HeavyFlavor Jets
139
8 Conclusions
197
Appendix ATiming Performance of the Tile Calorimeterwith Muons from Collision Data
200
Appendix Bt t + HF Modeling
219
Appendix CInput Variables to the Neural Networkin the t tH Analysis
250
Appendix DPostfit Distributions for Kinematic Variables
261
Appendix EGlossary
280
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About the author (2016)

Javier Montejo studied physics and computer science at the Universidad de Salamanca and obtained a Masters and PhD at the Universidad Autónoma de Barcelona. He studied various aspects of the hierarchy problem in the tt plus heavy flavor final state. The search for ttH constitutes currently the most precise determination of the top Yukawa coupling. Additional searches for beyond-the-standard-model signatures in the same final state have placed stringent constraints on the particles predicted by some of most favored theories that address the hierarchy problem. His current work is focused on supersymmetric models with emphasis on unusual signatures through which supersymmetric particles might have gone unnoticed.

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