Probing Massive Invisible Dark Photons in Higgs Decays via Gluon--Gluon Fusion at ATLAS Run 3

The ATLAS Collaboration has searched for Higgs-boson decays into a photon and missing transverse momentum using $135{\mathrm{f}\mathrm{b}}^{-1}$ of Run~3 proton–proton collision data at $\sqrt{s}=13.6\mathrm{T}\mathrm{e}\mathrm{V}$. For the benchmark decay $H\to \gamma \text{gD}$ with a massless invisible dark photon, ATLAS reports observed (expected) 95% confidence-level limits of $1.4\mathrm{\%}$ ($1.2\mathrm{\%}$) on the branching fraction. We reinterpret this search for a massive detector-invisible dark photon with $0\le m_{\text{gD}}\le 70\text{gev}$, focusing on Higgs production through gluon–gluon fusion. \audit{The relative signal acceptance, normalized to the massless benchmark, remains nearly unchanged up to $m_{\text{gD}}\simeq 40\text{gev}$, but decreases to $0.888$, $0.736$, and $0.524$ at $50$, $60$, and $70\text{gev}$, respectively, as the photon-$p_T$ and transverse-mass spectra move toward the analysis thresholds.} Using the published massless gluon–gluon-fusion yield for normalization, we obtain simplified expected (observed) 95% confidence-level limits on $\text{BR}(H\to \gamma \text{gD})$ from $1.64\mathrm{\%}$ ($1.76\mathrm{\%}$) at $m_{\text{gD}}=30\text{gev}$ to $3.50\mathrm{\%}$ ($3.75\mathrm{\%}$) at $70\text{gev}$. These results extend the ATLAS interpretation to massive invisible dark photons and quantify the resulting loss of sensitivity at large dark-photon mass.