Processes with Weak Gauge Boson Pairs at Hadron Colliders – Precise Predictions and Future Prospects

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Dokumentart: PhDThesis
Date: 2016-12-14
Language: English
Faculty: 7 Mathematisch-Naturwissenschaftliche Fakultät
Department: Physik
Advisor: Jäger, Barbara (Prof. Dr.)
Day of Oral Examination: 2017-02-08
DDC Classifikation: 530 - Physics
Keywords: Elementarteilchenphysik
Other Keywords:
High Energy Physics - Phenomenology
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In the last years, scattering processes comprising pairs of the massive weak gauge bosons gain more and more attention. Those reactions provide particularly promising means to investigate the very mechanism responsible for electroweak symmetry breaking in the Standard Model of particle physics and to search for new physics entering via the weak sector of the theory. Precisely predicting the differential distributions of the final-state particles in realistic conditions is an essential prerequisite to potentially reveal tiny deviations induced by physics beyond the Standard Model. In this thesis we present a calculation of the next-to-leading order (NLO) electroweak corrections to W-boson pair production at CERNs Large Hadron Collider (LHC), as well as a detailed analysis of vector-boson scattering (VBS) processes at a future high-energy proton–proton collider. In particular, our calculation of the NLO electroweak corrections to the hadronic process pp → W + W − → 4 leptons takes the leptonic W-boson decays as well as all off-shell effects fully into account and, thus, is the first prediction providing NLO accuracy everywhere in phase space. Employing realistic event selection criteria, we study the influence of the corrections in situations that are typical for the experimental analyses in the high-energy region and for Higgs-boson precision studies in the channel H → WW ∗ , to which direct W-boson pair production represents an important irreducible background. We observe non-trivial distortions of the differential distributions that, if not properly included in upcoming analyses, could easily be misidentified as first signs of new physics. Furthermore, we compare our predictions to previous results obtained by employing the so-called double-pole approximation. At small and intermediate scales the two approaches show the expected agreement at the level of fractions of a percent, while in the TeV range the differences may easily reach several tens of percent. Due to the comparably small production cross sections and the generally large QCD backgrounds, studying VBS reactions at hadron colliders is an intricate task, and even with the target luminosity of several 100 fb −1 presumably collected at the end of LHC Run II, dedicated differential analyses will hardly be realizable. In our analysis we therefore investigate the opportunities of a potential follow-up project of the LHC which is proposed to operate at a center-of-mass energy of 100 TeV and assumed to deliver a total integrated luminosity of 30 ab −1 . For several decay modes we perform a detailed signal-to-background analysis, revealing the excellent possibilities for future measure- ments of VBS processes at yet unprecedented energy scales that such a machine facilitates. With process-specific event-selection criteria we manage to significantly reduce the background contribution, while due to the deep energy reach definitely sufficient events of the VBS signal remain for a detailed examination at the differential level.

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