Search for Rare Decays of the Observed Higgs Boson and Additional Higgs Bosons with the ATLAS Detector

Searches for rare decays of the Standard Model Higgs boson and additional Higgs bosons are performed by the ATLAS experiment at the LHC. They use the unprecedented amount of collision data collected during the LHC $pp$ collision run at $\sqrt{s}=13$ TeV in 2015–2018, corresponding to an integrated luminosity of $139\text{ fb}^{-1}$ 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 $\mu=1.2\pm 0.6$. 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 not only to the increase in the dataset but also to the improvement in the analysis techniques.