Publication Date

2020

Document Type

Dissertation/Thesis

First Advisor

Adelman, Jahred A.

Degree Name

Ph.D. (Doctor of Philosophy)

Legacy Department

Department of Physics

Abstract

This dissertation presents a search for resonant and non-resonant di-Higgs production in the γγb ̄b final state using data from the ATLAS detector at the Large Hadron Collider (LHC). The search is performed on 36.1 fb−1 of data from proton-proton collisions at a center-of-mass energy of √s= 13 TeV collected in 2015 and 2016.

No significant excesses are observed in this search. The non-resonant analysis sets limits on the HH→γγb ̄b cross-section times branching ratio, with an upper observed (expected) limit of 0.73 (0.93) pb. The observed (expected) limits on the Higgs boson trilinear coupling at 95% Confidence Level (CL) are set at -8.2 < κ_λ < 13.3 (-8.5 < κ_λ < 13.7). A model-independent resonant search is also presented, setting limits on a generic scalar resonance under the narrow-width approximation. These limits cover mass hypotheses ranging from 260 GeV to 1000 GeV, and the observed (expected) limits set are 0.85 (0.92) pb at the lowest mass hypothesis to 0.13 (0.15) pb at the highest mass hypothesis.

Work toward the future of this analysis is presented. Improvements in photon identification are studied, investigating optimization through two approaches. First, through adding the moments of topological clusters as inputs, which show additional discriminating power. Second, through using a multivariate approach to define photon identification, studying a Boosted Decision Tree (BDT) and a Neural Network (NN). Through these additional inputs and the employment of a BDT, an improvement of as much as 27% background rejection for the same signal efficiency as the current tight working point is shown.

Additionally, improvements to the analysis through studying the Vector Boson Fusion (VBF) production mode are shown. This provides handles on new couplings, and a dedicated signal region targeting this mode can improve overall Asimov significance. To define this signal region, a multiclass BDT is used, with classes to model the VBF HH production mode, along with gluon-gluon fusion HH production, as well as the dominant γγ-continuum background, and ttH mono-Higgs background. By adding this signal region, a 9.7% improvement in Asimov significance is achieved using the full 140 fb−1 of Run 2 data.

Extent

232 pages

Language

eng

Publisher

Northern Illinois University

Rights Statement

In Copyright

Rights Statement 2

NIU theses are protected by copyright. They may be viewed from Huskie Commons for any purpose, but reproduction or distribution in any format is prohibited without the written permission of the authors.

Media Type

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