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.
