Microwave Background Constraints on Mixing of Photons with Hidden Photons

Various extensions of the Standard Model predict the existence of hidden photons kinetically mixing with the ordinary photon. This mixing leads to oscillations between photons and hidden photons, analogous to the observed oscillations between different neutrino flavors. In this context, we derive new bounds on the photon-hidden photon mixing parameters using the high precision cosmic microwave background spectral data collected by the Far Infrared Absolute Spectrophotometer instrument on board of the Cosmic Background Explorer. Requiring the distortions of the CMB induced by the photon-hidden photon mixing to be smaller than experimental upper limits, this leads to a bound on the mixing angle χ0 ∼< 10 −7 − 10 for hidden photon masses between 10 eV and 10 eV. This low-mass and low-mixing region of the hidden photon parameter space was previously unconstrained.