Search for rare decays of the observed Higgs boson and additional Higgs bosons with the ATLAS detector

The searches for rare decays of the observed Higgs boson and additional Higgs bosons are performed at the ATLAS experiment at the LHC. The searches use the unprecedented amount of collision data collected during the LHC $ pp $ collision run at $ \sqrt{s}=13~\mathrm{TeV} $ in the years 2015--2018, that correspond to the integrated luminosity of $ 139~\mathrm{fb}^{-1} $ after the data quality requirements. For the search for the dimuon decay of the Standard Model Higgs boson, the observed (expected) significance is $ 2.0\sigma $ ($ 1.7\sigma $), and the best-fit value of the signal strength parameter is $ \mu=1.2\pm0.6 $. For the search for the $ Z\gamma $ decay of the Standard Model Higgs boson, the observed (expected) significance is $ 2.2\sigma $ ($ 1.2\sigma $), and the best-fit value of the signal strength parameter is $ \mu=2.0^{1.0}_{-0.9} $. For the search for new resonances decaying into photon pairs, no significant excess is observed. The experimental sensitivity of these analyses to the signal processes is increased due to the increase in the size of the dataset and the improvement in the analysis techniques.