Studies on tt+bb production at the CMS experiment

In this thesis, the interplay of two elementary particles of the SM, the two heaviest quarks top (t) and bottom (b), is studied in detail. Processes in which a top quark-antiquark pair occurs in associated production with two bottom quarks (tt+bb) play an important role in particle physics. The two quarks show a high mass difference and their associated production is therefore particularly difficult to model, accompanied by large uncertainties [6]. In addition, events involving this process constitute a large irreducible background in measurements of tt+H production in H → bb decays. These measurements are an essential test of the SM and an important constraint of physics beyond the SM. Consequently, the tt+bb process needs to be thoroughly understood.

Events comprising tt+bb processes are analyzed at two different Monte Carlo event simulation levels in this thesis.
The first part compares different existing event simulations used at the CMS experiment for the tt+bb process in the single-lepton channel and investigates possible differences in the modeling. The analysis is technically realized in a way that allows a direct comparison of the simulations with those of the ATLAS experiment. The comparison of simulations of both experiments is performed building on this analysis. This enables the design of a common strategy between ATLAS and CMS.

The second part focuses on strategies for assigning b jets to their origin. Since top quarks decay into b quarks almost exclusively, the final state of an event involving the tt+bb process usually consists of four or more b jets. However, it is unknown in the event reconstruction which b jets result from top quark decays and which originate from additional gluon radiation and subsequent splittings into pairs of b quarks in the event.