Search for rare decays of the Higgs boson with the ATLAS detector

The Standard Model predicts several rare Higgs boson decay channels, some of which are currently within experimental reach. Among these is the decay to a lepton pair and a photon, $H\rightarrow\ell\ell\gamma$ ($\ell=e,\mu$). The observation of this decay could open the possibility of studying the CP and coupling properties of the Higgs boson in a complementary way to other analyses. Two distinct regimes with different lepton pair invariant masses are explored: low lepton pair mass ($m_{\ell\ell}<30~\text{GeV}$), where the decay predominantly occurs through the virtual photon, $\gamma^*$, and intermediate lepton pair mass (in a window around $m_{\ell\ell}=91.2~\text{GeV}$), where the decay predominantly occurs through the $Z$ boson. Results are based on $139~\text{fb}^{-1}$ of $pp$ collision data at $13~\text{TeV}$ center-of-mass energy collected with the ATLAS detector at the Large Hadron Collider. An upper limit at 95% confidence level on production cross-section times the branching ratio for $pp\rightarrow H\rightarrow Z\gamma$ is set at 3.6 times the Standard Model prediction (with 2.6 times expected in the presence of the Standard Model Higgs boson). An excess over the background-only hypothesis is observed in the $m_{\ell\ell}<30~\text{GeV}$ regime at $3.2\sigma$ significance ($2.1\sigma$ expected), establishing evidence for the $H\rightarrow\ell\ell\gamma$ decay. The Higgs boson production cross-section times the $H\rightarrow\ell\ell\gamma$ branching ratio for $m_{\ell\ell}<30~\text{GeV}$ is measured to be $8.7^{+2.8}_{-2.7}~\text{fb}$.