Abstract
A light hidden photon or axion-like particle is a good dark matter candidate and they are often associated with the spontaneous breaking of dark global or gauged U(1) symmetry. We consider the dark Higgs dynamics around the phase transition in detail taking account of the portal coupling between the dark Higgs and the Standard Model Higgs as well as various thermal effects. We show that the (would-be) Nambu-Goldstone bosons are efficiently produced via a parametric resonance with the resonance parameter q ∼ 1 at the hidden symmetry breaking. In the simplest setup, which predicts a second order phase transition, this can explain the dark matter abundance for the axion or hidden photon as light as sub eV. Even lighter mass, as predicted by the QCD axion model, can be consistent with dark matter abundance in the case of first order phase transition, in which case the gravitational wave signals may be detectable by future experiments such as LISA and DECIGO.
Highlights
A light, or explicitly a sub-keV, axion or hidden photon DM cannot be produced through thermal scatterings like the WIMP case, since otherwise it is too hot
We show that the Nambu-Goldstone bosons are efficiently produced via a parametric resonance with the resonance parameter q ∼ 1 at the hidden symmetry breaking
We show that the NG boson can be efficiently produced soon after the symmetry breaking or phase transition (PT) if the dark Higgs is not thermalized at the moment
Summary
Let us consider the spontaneous symmetry breaking of a hidden global continuous symmetry in the early Universe. Later we will gauge or explicitly break this group to give mass to the (would-be) NG boson. In this part, we show that the NG boson can be efficiently produced soon after the symmetry breaking or PT if the dark Higgs is not thermalized at the moment
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