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Recently, Weinberg proposed a Higgs portal model with a spontaneously brokenglobal $U(1)$ symmetry in which Goldstone bosons may be masquerading asfractional 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 darkphotons can constitute about 0.912 (0.167) to the effective number of lightneutrino species if they decouple from the thermal bath before the pions becomenon-relativistic and after (before) the QCD transition. Restriction on theparameter space of the portal coupling and the dark Higgs mass is obtained fromthe freeze-out condition of the dark photons. Combining with the collider dataconstraints on the invisible width of the standard model Higgs requires thedark Higgs mass to be less than a few GeV.

Dark Photon as Fractional Cosmic Neutrino Masquerader