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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 ωP2/m2 (plasma frequency ωP) 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.