Constraints on millicharged particles from nuclear γ -decays

We consider nuclear gamma decays and γ -emitting reactions that can be an efficient source of hypothetical millicharged particles ( χ ). In particular, we revisit the production of millicharged particles in nuclear reactor environment, pointing out that γ cascades from U 239 is an overlooked yet a powerful source of χ χ ¯ pairs. This leads to an increased flux compared to previous studies. We then apply new estimates of the flux to derive novel limits on the value of millicharge, ϵ = Q χ / e , from the electron recoil searched for in a variety of experiments placed in proximity to the reactor cores. The derived limits on ϵ are the strongest in the interval of masses ∼ 0.7 – 2 MeV . We also derive the millicharged particles flux from the Sun and point out potential sensitivity of the low-threshold dark matter search experiments.