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 lepton pair (muons or electrons) with an invariant mass $m_{ll}<30$ GeV is presented. The analysis uses 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. The search strategy is presented, with a focus on a novel approach for resolving close-by electron candidates. Evidence for the Higgs boson decaying to a photon and a low-mass lepton pair is found with a $3.2 \sigma$ 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 to be $\mu=1.5\pm 0.5$. The Higgs boson production cross-section times the $H\to\ell\ell\gamma$ branching ratio for $m_{ll}<30$ GeV is determined to be $8.7^{+2.8}_{-2.7}\ \mathrm{fb}$.