Measurement of the Higgs boson decay to a low-mass dilepton system and a photon in $pp$ collisions at $\sqrt{s}=$ 13 and 13.6 TeV with the ATLAS detector

Abstract

A measurement is presented for the Higgs boson decaying into a photon and a pair of electrons or muons with an invariant mass $m_{\ell \ell }<$ 30 GeV, using $pp$ collision data at $\sqrt{s}=$ 13.6 TeV recorded by the ATLAS detector at the Large Hadron Collider during the years 2022-2024, corresponding to an integrated luminosity of 164 fb${}^{-1}$. 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 $\mu =0.64_{-0.39}^{+0.44}$ compared with the expected value of $\mu =1.00_{-0.41}^{+0.47}$. The corresponding observed (expected) signal significance is 1.7 (2.5) standard deviations under the background-only hypothesis. The result is combined with a similar search performed using 139 fb${}^{-1}$ of $pp$ collision data at $\sqrt{s}=$ 13 TeV collected during the years 2015-2018. This combination gives an observed (expected) signal strength of $\mu =1.03_{-0.32}^{+0.35}(\mu =1.00_{-0.32}^{+0.34})$, corresponding to an observed (expected) significance of 3.4 (3.3) standard deviations.

45 pages in total, author list starting page 28, 7 figures, 3 tables, submitted to JHEP. All figures including auxiliary figures are available at https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/HIGP-2024-16/