Thermal Emission of Dark Photons from Earth's Core

Abstract

Dark photons in the sub-eV regime may be produced by the Earth's hot core, representing a much less extreme environment than stellar cores. We consider this possibility and estimate constraints on the kinetic mixing parameter $\varepsilon$ that governs dark photon coupling to charged particles, using Earth core cooling arguments and via dark matter direct detection experiments. Our estimates do not find new constraints on the relevant parameter space from these considerations. Depending on the underlying parameters governing the thermal production of dark photons in the Earth, either core cooling or a future experiment, like Oscura with 30 kg-yr of exposure, can constrain $\varepsilon$ about one order of magnitude above the current astrophysical bounds, over the dark photon mass range $\sim 10^{-4}-0.1$~eV.