Dark Photon as Fractional Cosmic Neutrino Masquerader

TL;DR

This paper explores a gauged version of a Higgs portal model where dark photons could mimic fractional cosmic neutrinos, analyzing their contribution to cosmic radiation and constraints from collider data.

Contribution

It extends Weinberg's model by gauging the global U(1), introducing dark photons and Higgs, and constrains their parameters using cosmological and collider data.

Findings

Dark photons can account for a significant fraction of effective neutrino species.

Parameter space restrictions are derived from freeze-out conditions and collider constraints.

Dark Higgs mass is constrained to be less than a few GeV.

Abstract

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.