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Description
This paper presents a search for semi-visible Higgs boson decays in final states with a photon ($\gamma$) and missing transverse momentum ($p_T^{miss}$). The analysis is optimised for the gluon-gluon fusion production channel and exploits a novel trigger deployed in 2023, with reduced thresholds down to 50 GeV on the photon transverse momentum ($p_T^\gamma$) and 70 GeV on the $p_T^{miss}$, in combination with a selection on the transverse mass of the $p_T^{miss}$-$\gamma$ system ($m_T$) above 80 GeV. The results are interpreted in terms of Higgs boson decays into a $\gamma$ and a dark photon $\gamma_d$ ($H\to\gamma\gamma_d$). The analysis is performed using 135 fb$^{-1}$ of Run 3 data collected by ATLAS in 2023 and 2024. A boosted decision tree is used to identify events with misreconstructed primary vertices, which can affect the $p_T^{miss}$ calculation. Data-driven methods are employed to estimate the backgrounds from jets and electrons misidentified as photons, while the backgrounds with genuine photons are taken from Monte Carlo simulations, normalised to data in control regions with muons. A binned maximum-likelihood fit to the mT distribution is performed to evaluate the compatibility of the observed data with the Standard Model expectation. No significant excess is observed over the Standard Model prediction, and an observed (expected) upper limit on the branching ratio of $H\to\gamma\gamma_d$ of 1.4% (1.2%) is obtained at 95% confidence level. Combining this result with previous Run 2 searches improves the observed (expected) upper limit to 0.9% (0.9%).
