Abstract
We consider the effect on early Universe cosmology of the dark photon associated with the gauging of $U(1)_{T3R}$, a symmetry group under which only right-handed Standard Model fermions transform non-trivially. We find that cosmological constraints on this scenario are qualitatively much more severe than on other well-studied cases of a new $U(1)$ gauge group, because the dark photon couples to chiral fermions. In particular, the dark photon of $U(1)_{T3R}$ is always produced and equilibrates in the early Universe, no matter how small the gauge coupling, unless the symmetry-breaking scale is extremely large. This occurs because, no matter how the weak the coupling, the Goldstone mode (equivalently, the longitudinal polarization) does not decouple. As a result, even the limit of an extremely light and weakly-coupled dark photon of $U(1)_{T3R}$ is effectively ruled out by cosmological constraints, unless the symmetry-breaking scale is extremely large. We also discuss the possibility of ameliorating Hubble tension in this model.
Highlights
In recent times, precise measurements of the cosmic microwave background (CMB) by the Planck experiment have placed tight constraints on the number of effective relativistic degrees of freedom in the early Universe, encoded in the quantity ΔNeff [1]
We have considered the effect of the dark photon of Uð1ÞT3R on cosmology in the early Universe
Unlike other recently studied cases, such as B − L and Li − Lj, if the dark photon is the gauge boson of Uð1ÞT3R, cosmological constraints are much tighter
Summary
Precise measurements of the cosmic microwave background (CMB) by the Planck experiment have placed tight constraints on the number of effective relativistic degrees of freedom in the early Universe, encoded in the quantity ΔNeff [1]. Uð1ÞT3R has been investigated for the purpose of building a well-motivated model of sub-GeV dark matter [14] This model explains the hierarchies among the light fermion masses and contains a light gauge boson and a light scalar particle. If the dark photon is the gauge boson of Uð1ÞT3R, this second option is foreclosed; the dark photon is always produced in the early Universe, no matter how weak the coupling unless the symmetry-breaking scale is ≳106 GeV This result might at first seem counterintuitive. The longitudinal polarization instead becomes the massless Goldstone mode of the spontaneously broken global symmetry, which need not decouple These considerations apply for any choice of the Uð1Þ gauge group. If only second-generation fermions are charged under Uð1ÞT3R, collider and other astrophysical constraints are largely unaffected by these considerations, whereas constraints arising from early Universe cosmology become much more severe
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