Combination of searches for Higgs boson decays into a photon and a massless dark photon using $pp$ collisions at $\sqrt{s} = 13$ TeV with the ATLAS detector

A combination of searches for Higgs boson decaying into a visible photon and a massless dark photon ($H\to\gamma\gamma_{\text{d}}$) is presented using 139 fb$^{-1}$ of proton--proton collision data at a centre-of-mass energy of $\sqrt{s} = 13$ TeV recorded by the ATLAS detector at the Large Hadron Collider. The observed (expected) 95\% confidence level upper limit on the Standard Model Higgs boson decay branching ratio is determined to be $\mathcal{B}(H\to\gamma\gamma_{\text{d}}) < $ 1.3% (1.5)%. The search is also sensitive to higher-mass Higgs bosons decaying into the same final state. The observed (expected) 95% CL limit on the cross section times branching ratio ranges from 16 fb (26 fb) for $m_H = 400$ GeV to 1.0 fb (1.5 fb) for $m_H = 3$ TeV. Results are also interpreted in the context of a minimal simplified model.