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

TL;DR

This paper reports a search for the Higgs boson decay to a Z boson and a photon using ATLAS data at 13.6 TeV, finding results consistent with the Standard Model and providing the most sensitive measurement to date.

Contribution

It presents the most sensitive search for Higgs to Z gamma decay combining new data with previous results, improving the constraints on this rare process.

Findings

Measured signal strength close to Standard Model prediction

Observed significance of 2.5 standard deviations

Results are consistent with the Standard Model

Abstract

A search for the Higgs boson decay to a $Z$ boson and a photon in the $\ell\ell\gamma$ ($\ell = e, \mu$) 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 $\mu=0.9^{+0.7}_{-0.6}$, compared to an expected value of $\mu=1.0\pm0.7$. 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 $\mu = 1.3^{+0.6}_{-0.5}$ ($\mu = 1.0^{+0.6}_{-0.5}$), corresponding to an observed (expected) significance of 2.5 (1.9) standard deviations. The measurement is consistent with the Standard Model expectation.