Searches for rare and flavour-violating exclusive decays of the Higgs and Z bosons with the ATLAS experiment

Ward, Robert James ORCID: 0000-0001-5530-9919 (2023). Searches for rare and flavour-violating exclusive decays of the Higgs and Z bosons with the ATLAS experiment. University of Birmingham. Ph.D.

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Abstract

The latest searches for the rare exclusive Higgs and Z boson decays to a vector quarkonium state and a photon in the \(\mu^+\mu^−\gamma\) final state are presented. These searches probe the magnitude and the sign of the Higgs boson coupling to bottom and charm-quarks using 139 fb\(^{−1}\) of ATLAS data collected at \(\sqrt{s}\) = 13 TeV. The observed data are compatible with the expected backgrounds and the 95% confidence level upper limits on the branching fractions of the Higgs boson decays into \(J/\psi \gamma, \psi(2S) \gamma\), and \(\Upsilon(1S, 2S, 3S) \gamma\) are found to be 2.0 × 10\(^{−4}\), 10.5 × 10\(^{−4}\), and (2.5, 4.2, 3.4) × 10\(^{−4}\), respectively, assuming Standard Model production of the Higgs boson. The corresponding 95% CL upper limits on the branching fractions of the Z boson decays are 1.2 × 10\(^{−6}\), 2.4 × 10\(^{−6}\), and (1.1, 1.3, 2.4) × 10\(^{−6}\). The sensitivities of the searches for the flavour-violating exclusive decays of the Higgs boson to D* \(\gamma\) and the Z boson decays to D\(^0 \gamma\) and \(K_s \gamma\) are also presented. These searches probe potential flavour-violating couplings of the Higgs and Z bosons using 135 fb\(^{−1}\) of ATLAS data collected at \(\sqrt{s}\) = 13 TeV. The expected 95% CL upper limits on the branching fractions of each decay are 13.2\(^{+5.4}_{−3.7}\) × 10\(^{−4}\) for H → D* \(\gamma\), 3.0\(^{+1.2}_{−0.8}\) × 10\(^{−6}\) for Z → D\(^0 \gamma\), and 2.7\(^{+1.2}_{−0.7}\) × 10\(^{−6}\) for Z → \(K_s \gamma\). In addition, a study of the feasibility to measure the track reconstruction efficiency of the ATLAS inner detector in dense environments, using boosted J/\(\psi\) and \(\phi\) decays in the \(\mu^+\mu^−\) final state, is presented. These environments are relevant in the exclusive Higgs and Z boson decays.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):