Search for the Higgs boson decay to a $Z$ boson and a photon in $pp$ collisions at $\sqrt{s}=13$ TeV and $13.6$ TeV with the ATLAS detector

Abstract

A search for the Higgs boson decay to a $Z$ boson and a photon in the $\ell \ell {\hat{I}}^3$ ($\ell =e,\hat{I}¼$) final state is performed using $pp$ collisions at $\sqrt{s}=13.6$ 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 $\hat{I}¼=0.9_{-0.6}^{+0.7}$, compared to an expected value of $\hat{I}¼=1.0\pm 0.7$. 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 $\sqrt{s}=13$ TeV $pp$ collisions to provide the most stringent expected sensitivity to date to this rare decay, namely an observed (expected) signal strength of $\hat{I}¼=1.3_{-0.5}^{+0.6}$ ($\hat{I}¼=1.0_{-0.5}^{+0.6}$), corresponding to an observed (expected) significance of 2.5 (1.9) standard deviations. The measurement is consistent with the Standard Model expectation.

43 pages in total, author list starting page 26, 4 figures, 4 tables. All figures including auxiliary figures are available at https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/HIGP-2024-19