Dark radiation constraints on minicharged particles in models with a hidden photon

We compute the thermalization of a hidden sector consisting of minicharged fermions (MCPs) and massless hidden photons in the early Universe. The precise measurement of the anisotropies of the cosmic microwave background (CMB) by Planck and the relic abundance of light nuclei produced during big bang nucleosynthesis (BBN) constrain the amount of dark radiation of this hidden sector through the effective number of neutrino species, N-eff. This study presents novel and accurate predictions of dark radiation in the strongly and weakly coupled regime for a wide range of model parameters. We give the value of N-eff for MCP masses between 100 keV and 10 GeV and minicharges in the range 10(-11) - 1. Our results can be used to constrain MCPs with the current data and they are also a valuable indicator for future experimental searches, should the hint for dark radiation manifest itself in the next release of Planck's data.