Measurement of the associated production of a top quark pair and a Higgs boson (t¯tH) with boosted topologies

Borbely, Albert Gyorgy (2023) Measurement of the associated production of a top quark pair and a Higgs boson (t¯tH) with boosted topologies. PhD thesis, University of Glasgow.

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This thesis presents three studies focusing on boosted topologies that utilise machine learning techniques for boosted H → b¯b reconstruction using the ATLAS detector. The measurement of the t¯tH cross-section is a direct way of accessing the Higgs top Yukawa coupling (yt). Firstly, an all-hadronic feasibility study is shown, aimed at assessing boosted topologies in the all-hadronic t¯tH decay channel. It was found to have low statistical significance, with considerable efforts and data driven techniques required to reduce the QCD-multijet background. Secondly, the boosted contribution to the recent t¯tH, H → b¯b measurement using the full Run-2 ATLAS data set, 139f b−1 at √s = 13 TeV, is analysed. There is a considerable contribution from the boosted region to this result, particularly to the differential cross-section measurement of the Simplified Template Cross-Section (STXS) bins [300, 450) and [450, ∞) GeV. The result of the inclusive profile-likelihood fit is μ = 0.35+0.36−0.34, which corresponds to σ = 1.0(2.7) observed(expected) significance compared to the background-only hypothesis. Thirdly work on retraining the boosted H → b¯b reconstruction deep neural network (DNN) is shown for the Run-2 Legacy re-analysis. The bespoke DNN trained for the analysis showed some improvements over the previous round due to the updated analysis algorithms. It also outperformed the general purpose H → b¯b Xbb tagger. The final motivation for use of the bespoke DNN is that it allows the choice of boosted jet collection (RC-jets vs LR-jets). RC-jets re cluster “small” (∆R = 0.4) jets with ∆R = 1.0 while LR-jets directly cluster the calorimeter clusters with ∆R = 1.0, both using the anti-kt algorithm. The RC-jets jets are found to be advantageous. This is due to the ease of propagating systematics for combining with resolved regions and the good modelling observed using samples made with the Atlfast-2 detector simulation.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Subjects: Q Science > QB Astronomy
Q Science > QC Physics
Colleges/Schools: College of Science and Engineering > School of Physics and Astronomy
Supervisor's Name: Doyle, Professor Anthony and Owen, Dr. Mark
Date of Award: 2023
Depositing User: Theses Team
Unique ID: glathesis:2023-83710
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 10 Jul 2023 09:48
Last Modified: 18 Sep 2023 10:16
Thesis DOI: 10.5525/gla.thesis.83710

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