Top-Quarks under the LHC's Magnifying Glass: From Process Modelling to Parameter Extraction

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With the successful start of collisions in 2010, the LHC has put another milestone towards a thorough exploration of physics at the Terascale. Its first research run took place from 2010-2013 at an initial energy of 7 TeV, rising to 8 TeV from 2012. After an almost two year shutdown, the LHC started delivering physics data in 2015. This marked the beginning of the LHC Run 2 era and opened the path to an even deeper understanding of the Standard Model (SM) physics and hopefully to new discoveries. The multi-particle events that are observed at the LHC hide or strongly modify all possible signals of physics beyond the SM. Thus, in view of a correct interpretation of the signals of new physics which might be extracted from data, it is of great interest to reduce theoretical uncertainties for the SM processes. i.e. to understand them as precisely as possible. This is especially important when QCD (background) processes are involved. In this respect, the need of precision next-to-leading order (NLO) QCD calculations for physical observables is indisputable. Key measurements that are curried out at the LHC by ATLAS and CMS are connected to the top-quark, the heaviest known fundamental particle. Besides the determination of the top-quark mass, key measurements include the total cross section, differential distributions as well as spin correlations and top-quark couplings to gauge bosons and the SM Higgs boson. The top-quark, however, is an extremely short-lived resonance and only its decay products can be detected experimentally. Thus, for comparison with data, theoretical predictions must include top-quark decays. Moreover, due to the large collision energy at the LHC, top anti-top pairs are abundantly produced with large energy and high pT . Therefore, the probability for the initial state gluon radiation increases making the ttj final state measurable with high statistics. Besides fully inclusive QCD tt production and the ttj production process, more exclusive final states can be accessed at the LHC. Even though their cross sections are typically much smaller, they can provide key information on the properties of the top-quark like for example top-quark couplings to gauge bosons. The purpose of this project is to provide precise and realistic theoretical predictions for tt production in association with additional final states. In practice this requires to include QCD corrections to top-quark production and decays including top-quark spin correlations. To be more precise, we shall focus on calculating NLO QCD corrections to fully realistic final states with complete top-quark off-shell effects included. We plan to use our realistic theoretical results to study broad phenomenological aspects of top-quark physics at the LHC.