Search for the Higgs boson decay to a Z boson and a photon in pp collisions at √s = 13 TeV and 13.6 TeV with the ATLAS detector

A search for the Higgs boson decay to a Z boson and a photon in the ℓℓγ (ℓ = e, μ) final state is performed using pp collisions at √s = 13.6 TeV recorded with the ATLAS detector at the Large Hadron Collider during 2022-2024, corresponding to an integrated luminosity of 165 fb⁻¹. The signal yield, normalised to the Standard Model prediction, is measured to be μ = 0.9^+0.7_−0.6, compared to an expected value of μ = 1.0 ± 0.7. This corresponds to an observed (expected) signal significance of 1.4 (1.5) standard deviations for the background-only hypothesis. This result is combined with that of a similar search performed with 140 fb⁻¹ of √s = 13 TeV pp collisions to provide the most stringent expected sensitivity to date to this rare decay, namely an observed (expected) signal strength of μ = 1.3^+0.6_−0.5 (μ = 1.0^+0.6_−0.5), corresponding to an observed (expected) significance of 2.5 (1.9) standard deviations. The measurement is consistent with the Standard Model expectation.