Dark Photon as Fractional Cosmic Neutrino Masquerader
  Recently, Weinberg proposed a Higgs portal model with a spontaneously broken
global $U(1)$ symmetry in which Goldstone bosons may be masquerading as
fractional cosmic neutrinos. We extend the model by gauging the $U(1)$
symmetry. This gives rise to the so-called dark photon and dark Higgs. The dark
photons can constitute about 0.912 (0.167) to the effective number of light
neutrino species if they decouple from the thermal bath before the pions become
non-relativistic and after (before) the QCD transition. Restriction on the
parameter space of the portal coupling and the dark Higgs mass is obtained from
the freeze-out condition of the dark photons. Combining with the collider data
constraints on the invisible width of the standard model Higgs requires the
dark Higgs mass to be less than a few GeV.
