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Humboldt-Universität zu Berlin - Mathematisch-Naturwissen­schaft­liche Fakultät - Experimentelle Elementarteilchenphysik

Carsten Kendziorra (2011)

Improvement of the Single Top Quark Reconstruction Based on B-Tagging at ATLAS

Diplom Thesis, Humboldt-Universität zu Berlin.

The detection of jets with beauty quarks (b-tagging) for the analysis of electroweak produced single top quarks (single-top) at the ATLAS detector at the particle accelerator LHC was studied. The weak coupling of the top quark to the W boson can be measured precisely, hence single-top is of special interest. Since the top quark decays to almost 100% into a W boson and a beauty quark, the detection of the b-jet is crucial for the signal extraction. First, basic performance studies of four b-tagging algorithms (b-tagger) were done. The IP3D+SV1 tagger produced for the same efficiency the highest purity and rejection of jets from lighter quarks, followed by the SV0 tagger. The TrackCounting2D and JetProb tagger had the lowest performance. Selected working points of this study have been used for the event selection of the single-top analysis for the t-channel with semileptonic decay. This analysis uses a kinematic fit for the enhancement of the signal. A cut based analysis was additionally used for comparison. The kinematic fit provided for all of the b-tagging working points the highest signal-to-background ratio and signal efficiency. It was shown, that b-tag weight cuts can be relaxed, compared to the standard recommended working points with 50% b-tag efficiency, what improves the signal efficiency. The differences in the performance of the taggers agree with the first study. The IP3D+SV1 tagger provides the most powerful analysis, but is not usable until now. Currently the SV0 tagger with a working point of 59% b-tag efficiency is recommended for the single-top analysis. This provides a signal to background ratio of 0.21 and a signal efficiency of 10.5% for the t-channel with an electron in the final state.