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, compatible with the expected value of 𝜇 = 1.0 ± 0.7. This corresponds to an observed (expected) signal significance of 1.4 (1.5) standard deviations under 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 best 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.