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

Abstract

The ATLAS Collaboration has searched for Higgs-boson decays into a photon and missing transverse momentum using $135~\mathrm{fb}^{-1}$ of Run~3 proton--proton collision data at $\sqrt{s}=13.6~\mathrm{TeV}$. For the benchmark decay $H\toγ\gD$ with a massless invisible dark photon, ATLAS reports observed (expected) 95\% confidence-level limits of $1.4\%$ ($1.2\%$) on the branching fraction. We reinterpret this search for a massive detector-invisible dark photon with $0\leq m_{\gD}\leq70~\gev$, focusing on Higgs production through gluon--gluon fusion. \audit{The relative signal acceptance, normalized to the massless benchmark, remains nearly unchanged up to $m_{\gD}\simeq40~\gev$, but decreases to $0.888$, $0.736$, and $0.524$ at $50$, $60$, and $70~\gev$, 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 $\BR(H\toγ\gD)$ from $1.64\%$ ($1.76\%$) at $m_{\gD}=30~\gev$ to $3.50\%$ ($3.75\%$) at $70~\gev$. These results extend the ATLAS interpretation to massive invisible dark photons and quantify the resulting loss of sensitivity at large dark-photon mass.