Hidden Photon and Axion Dark Matter from Symmetry Breaking
  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 $q\sim 1$ 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.
