Solar constraints on hidden photons re-visited

We re-examine solar emission of hidden photons γ' (mass m ) caused by kinetic γ–γ' mixing. We calculate the emission rate with thermal field theory methods and with a kinetic equation that includes γ–γ' ''flavor oscillations'' and γ absorption and emission by the thermal medium. In the resonant case both methods yield identical emission rates which, in the longitudinal channel, are enhanced by a factor ω P 2 / m 2 (plasma frequency ω P ) in agreement with An, Pospelov and Pradler (2013). The Sun must not emit more energy in a ``dark channel'' than allowed by solar neutrino measurements, i.e., not more than 10% of its photon luminosity. Together with the revised emission rate, this conservative requirement implies χ < 4 × 10 −12 (eV/ m ) for the kinetic mixing parameter. This is the most restrictive stellar limit below m ∼ 3eV, whereas for larger masses the transverse channel dominates together with limits from other stars. A recent analysis of XENON10 data marginally improves the solar limit, leaving open the opportunity to detect solar hidden photons with future large-scale dark matter experiments. Detecting low-mass hidden photons with the ALPS-II photon-regeneration experiment also remains possible.