Solar constraints on hidden photons re-visited

We re-examine solar emission of hidden photons gamma' (mass m) caused by kinetic gamma gamma' mixing. We calculate the emission rate with thermal field theory methods and with a kinetic equation that includes gamma-gamma' flavor oscillations and gamma 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 omega(2)(P)/m(2) (plasma frequency omega(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 chi < 4 x 10(-12) (eV/m) for the kinetic mixing parameter. This is the most restrictive stellar limit below m similar to 3 eV, 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.