# Searching for new physics in decays of the Higgs boson with the ATLAS detector

Reynolds, Elliot Thomas James ORCID: 0000-0002-1506-5750 (2019). Searching for new physics in decays of the Higgs boson with the ATLAS detector. University of Birmingham. Ph.D.

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## Abstract

Three searches sensitive to physics beyond the Standard Model in Higgs boson decays are presented, using $$pp$$ collision data collected at $$\sqrt{s}=13~\text{TeV}$$ by the ATLAS experiment during Run 2 of the LHC.

First, a direct search is performed for Higgs boson decays to charm quarks. An upper limit is set at $$110\times$$ the SM expectation on $$\sigma(ZH)\times\text{BR}(H\to cc)$$. The expected sensitivity is estimated for the ATLAS detector at the HL-LHC, assuming $$3000~\text{fb}^{-1}$$ of $$pp$$ collision data at $$\sqrt{s}=14~\text{TeV}$$. An upper limit of $$6.3$$ times the Standard Model expectation for $$\sigma(ZH)\times\text{BR}(H\to cc)$$ is expected, in the absence of systematic uncertainties.

Second, a search is performed for Higgs boson decays to pairs of beyond the Standard Model resonances, in the four-muon final state. No events are observed, in agreement with the background-only expectation of $$0.4\pm 0.1$$ events. Mass-dependent upper limits are set on two benchmark models, and model independent upper limits are set by defining a fiducial acceptance.

Third, a search is performed for Higgs boson decays to a $$Z$$ boson and a light resonance in two lepton plus jet events. In the absence of a signal, upper limits are set on $$\sigma(H)\times\text{BR}(H\to Za^0) / \sigma_\text{SM}(H)$$, with values starting from 44.8%.

Type of Work: Thesis (Doctorates > Ph.D.)
Award Type: Doctorates > Ph.D.
Supervisor(s):
Supervisor(s)EmailORCID
Nikolopoulos, KonstantinosUNSPECIFIEDorcid.org/0000-0002-3048-489X
Thompson, PaulUNSPECIFIEDorcid.org/0000-0002-6239-7715
College/Faculty: Colleges (2008 onwards) > College of Engineering & Physical Sciences
School or Department: School of Physics and Astronomy
Funders: European Research Council, Other, Science and Technology Facilities Council
Other Funders: University of Birmingham
Subjects: Q Science > QC Physics
URI: http://etheses.bham.ac.uk/id/eprint/9807

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