New Light on Dark Photons

"Dark Photons", light new vector particles V kinetically mixed with the photon, are a frequently considered extension of the Standard Model. For masses below 10 keV they are emitted from the solar interior. In the limit of small mass m_V the dark photon flux is strongly peaked at low energies and we demonstrate that the constraint on the atomic ionization rate imposed by the results of the XENON10 Dark Matter experiment sets the to-date most stringent limit on the kinetic mixing parameter of this model: \kappa*m_V < 3*10^{-12} eV. The result significantly improves previous experimental bounds and surpasses even the most stringent astrophysical and cosmological limits in a seven-decade-wide interval of m_V.