Searching for New Physics in Decays of the Higgs Boson with the ATLAS Detector

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%.