The Top Quark as Window to New Physics

The goal of my research plan is the improved understanding of the top quark which is the heaviest fundamental particle known. Its high mass makes the top quark special in several ways and precise understanding of its properties might open up a window to new physics. Due to its short lifetime the top quark decays before it can fragment, enabling the study of a bare quark. Furthermore the large top mass results in a strong coupling to the Higgs boson. The top quark therefore plays an important role in the search for the Higgs boson, but also in our quest to find other physics beyond the Standard Model of particle physics. In the Standard Model the top quark decays almost always into a W boson and a b quark. For several extensions of the Standard Model, though, either quarks or bosons might have different properties. As part of my PhD thesis I measured the fraction of top quarks decaying into b quarks. Theories like Supersymmetry predict that the top quark could decay into a charged Higgs boson instead of a W boson. I therefore also searched for charged Higgs bosons coming from the top quark decay. Within the Standard Model only a neutral Higgs boson can be produced in association with a top quark pair. This process is the only one where the coupling between the top and the Higgs boson can be measured directly. The plan is to improve the existing searches at the D0 experiment and apply them to a four times larger data set than was used so far. Besides the improvement of including more final state channels of top quark pair events and the use of additional topological information of the events, the measurement of the fraction of top quarks decaying into b quarks should be extended to single top quark production channels. The overall fit with these channels is directly sensitive to the top quark couplings without any theoretical assumptions. In summary, the top quark is unique due to its high mass and short lifetime and it is therefore an excellent candidate to search for new physics. Only a few hundred events containing top quarks have been studied so far. By adding more data from D0 and the new data from ATLAS I will be able to place stringent bounds on new physics models and perhaps even be able to discover new and unexpected phenomena.