The construction of a scalar extension to the Standard Model and the search for a heavy scalar at Ps = 13 TeV with the ATLAS detector
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Date
2020
Authors
Von Buddenbrock, Stefan
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Abstract
Searches for physics beyond the Standard Model (SM) have always been a focus
of the particle physics community, more so since the discovery of a Higgs-like
boson (h) in 2012. These are typically done using either the top-down or bottom-up
methodology. In this thesis, a model is constructed in order to successfully explain
a variety of anomalous results from Run 1 and 2 of the Large Hadron Collider
(LHC), using a hybrid methodology that iterates between theory and experiment.
Initially in 2015, a new heavy scalar boson H was postulated to explain anomalies
in the Run 1 LHC data. The result of this early study implied that a heavy scalar
boson with a mass around 270 GeV, produced through gluon fusion, could explain
these anomalies with a significance of 3s, with a cross section comparable with that
of a heavy Higgs boson as well as a dominant h-associated decay mode. Theoretical
developments of the model then hinted towards the existence of an additional
scalar boson S, which acted as this associated decay product. The S boson, with a
mass of around 150 GeV, was likely to decay to final states comprising of multiple
leptons. The culmination of these studies found that the H ! Sh decay mode was
dominant and could be searched for in final states with multiple leptons and btagged
jets at the LHC. In 2017 and 2018, using the input from these initial studies,
anomalies in the LHC data were successfully explained by the model, as predicted.
It was found that the new physics model improved on the SM description of the
data at the level of at least 8s with just a single degree of freedom. Should these
discrepancies not be resolved, this can be considered as indirect evidence for new
physics processes at the LHC, since the current SM tools are unable to provide a
reasonable explanation for the anomalies. The potential of correlating this result
with a mild excess at 245 GeV in the search for H decaying to four leptons in the
ATLAS detector is considered, and future potential developments of the model are
discussed
Description
A thesis submitted to the Faculty of Science, University of the Witwatersrand,
Johannesburg, in fulfilment of the requirements for the degree of Doctor of Philosophy,
School of Physics, 2020