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Title: In search for a non-standard sector
Author: Englert, David
ISNI:       0000 0004 9355 4204
Awarding Body: Queen Mary University of London
Current Institution: Queen Mary, University of London
Date of Award: 2020
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In this thesis two works are presented, both focusing on the scalar sector: the combination of searches for Higgs boson pair production with the ATLAS detector; and the exploration of the parameter space of the Type-II Two Higgs Doublet Model (2HDM), facing constraints based on theoretical consistency and current experimental searches. The gluon-initiated Higgs boson pair production is measured in its decays to the bbbb, bb and bb channels recorded with the ATLAS detector during the 2015 and 2016 run of the LHC, corresponding to a centre-of-mass energy p s = 13 TeV, and an integrated luminosity of up to 36.1 fb−1. The channels are statistically combined, and confidence intervals are constructed at 95% confidence level using the CLs test statistic. Under the assumption that there is no signal, the expected median upper limit of the production cross section of the non-resonant signal in the gluon-fusion production mode, normalised relative to the value predicted by the Standard Model (SM) ( SM ggF p s= 13TeV = 33.41 fb) is 10.4. The measured data is found to have deficits relative to the no signal hypothesis in all three channels, resulting in a normalised observed upper limit of 6.7. The ratio of the Higgs trilinear scalar coupling normalised to its SM value, hhh = hhh/ hhh|SM is observed (expected) to be constrained to the interval of −5.0 < hhh < 12.1 (−5.8 < hhh < 12.0). The dataset is also analysed assuming a narrow-width, spin-0 signal model, interpreted in the scalar singlet extension model and the habemus Minimal Supersymmetric Standard Model (hMSSM). In a separate work a preliminary study is carried out on the phenomenology of the Type-II 2HDM, with an exploration of its six-dimensional parameter space using Markov Chain Monte Carlo. Non-excluded regions of the parameter space are determined using a combination of measurements based on: Higgs signal strengths, searches for the additional scalars, and the electroweak precision observables, S and T. Theoretical constraints based on arguments of vacuum stability, unitarity of scattering matrix and perturbativity are also considered.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available