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):