Dark photons and resonant monophoton signatures in Higgs boson decays at the LHC

Motivated by dark-photon $\bar{\gamma}$ scenarios extensively considered in the literature, we explore experimentally allowed models where the Higgs boson coupling to photon and dark photon $H\gamma\bar{\gamma}$ can be enhanced. Correspondingly, large rates for the $H\to \gamma\bar \gamma$ decay become plausible, giving rise to one monochromatic photon with $E^{\gamma}\simeq m_H/2$ (i.e., more than twice the photon energy in the rare standard-model decay $H\to \gamma Z\to\gamma\bar\nu\nu$), and a similar amount of missing energy. We perform a model-independent study of this exotic resonant monophoton signature at the LHC, featuring a distinctive $E^{\gamma}_T$ peak around 60 GeV, and $\gamma+\slashed{E}_T$ transverse invariant mass ruled by $m_H$. At parton level, we find a $5\,\sigma$ sensitivity of the present LHC data set for a $H\to \gamma\bar{\gamma}$ branching fraction of $0.5\%$. Such large branching fractions can be naturally obtained in dark $U(1)_F$ models explaining the origin and hierarchy of the standard model Yukawa couplings. We urge the LHC experiments to search for this new exotic resonance in the present data set, and in future LHC runs.