Search for high-mass resonances in photon plus missing transverse momentum signatures in $pp$ collisions at $\sqrt{s}$ = 13 TeV with the ATLAS detector

A search for high-mass resonances from heavy Higgs “H” bosons in photon plus missing transverse momentum signatures produced in proton-proton collisions at a center-of-mass energy of $\sqrt{s}=13$ TeV at the Large Hadron Collider (LHC) is presented. Both the gluon-gluon Fusion ($ggF$) and the Vector-Boson Fusion ($VBF$) production modes of the heavy Higgs bosons are considered using the total LHC Run-2 integrated luminosity of 139 fb$^{-1}$ recorded by the ATLAS detector. The RECAST strategy is adopted to reinterpret the previous ATLAS mono-photon $\gamma+E_{\mathrm {T}}^{\mathrm{miss}}$ result in the context of the search for dark photons from heavy Higgs bosons decay. The results of this reinterpretation are presented in terms of exclusion limits on the cross-section times the branching ratio for heavy Higgs boson masses ranging from 400 GeV to 3 TeV. For a massless dark photon, the exclusion limits at 95\%~confidence level on the $\sigma\times BR(H\to\gamma\gamma_d )$ obtained using the combined $ggF+VBF$ production of the heavy Higgs boson are found to be in the range from 1.44 (1.68) fb to 24.2 (28.1) fb observed (expected) respectively. The highest limit corresponds to a heavy Higgs boson with a mass of 400 GeV and the lowest limit corresponds to the heaviest considered mass of 3 TeV. Limits for separate $ggF$ and $VBF$ production modes as well as intermediate heavy Higgs boson masses are provided.