Evidence for the Standard Model Higgs boson decaying to a photon and a low-mass lepton pair with the ATLAS detector at the LHC

A search for the Higgs boson in the final state with a photon and a low-mass lepton pair (muons or electrons) originating from the decay of a virtual photon is presented. A data set of $pp$ collisions at a center-of-mass energy of 13 TeV corresponding to an integrated luminosity of 139 $\text{fb}^{-1}$ collected with the ATLAS detector at the Large Hadron Collider was used. The search strategy is outlined, with a focus on a novel approach for resolving close-by electron candidates. Evidence for Higgs boson decay to a photon and a low-mass lepton pair is found at 3.2$\sigma$ significance over the background-only hypothesis, compared to an expected significance of 2.1$\sigma$. The best-fit value of the signal strength parameter, defined as the ratio of the observed signal yield to the one expected in the Standard Model, is measured at $\mu=1.5\pm0.5$. The Higgs boson production cross-section times the $H\rightarrow\ell\ell\gamma$ branching ratio for $m_{\ell\ell}<30$ GeV is determined to be $8.7^{+2.8}_{−2.7} \ \text{fb}$.