Superheavy Dark Matter in Light of Dark Radiation
  Superheavy dark matter can satisfy the observed dark matter abundance if the
stability condition is fulfilled. Here, we propose a new Abelian gauge symmetry
${\rm U(1)}_H$ for the stability of superheavy dark matter as the
electromagnetic gauge symmetry to the electron. The new gauge boson associated
with ${\rm U(1)}_H$ contributes to the effective number of relativistic degrees
of freedom in the universe as dark radiation, which has been recently measured
by several experiments, e.g., PLANCK. We calculate the contribution to dark
radiation from the decay of a scalar particle via the superheavy dark matter in
the loop. Interestingly enough, this scenario will be probed by a future LHC
run in the invisible decay signatures of the Higgs boson.
