Constraints on millicharged particles from nuclear gamma-decays

TL;DR

This paper investigates nuclear gamma decays as a source of millicharged particles, deriving new constraints on their charge from reactor and solar fluxes, with implications for dark matter searches.

Contribution

It identifies gamma cascades from $^{239}$U as a significant source of millicharged particles and derives the strongest limits on their charge in the 0.7-2 MeV mass range.

Findings

New limits on millicharge $oldsymbol{ ext{ε}}$ in the 0.7-2 MeV mass range

Gamma cascades from $^{239}$U are a powerful source of millicharged particles

Potential sensitivity of dark matter experiments to solar-produced millicharged particles

Abstract

We consider nuclear gamma decays and $\gamma$-emitting reactions that can be an efficient source of hypothetical millicharged particles ($\chi$). In particular, we revisit the production of millicharged particles in nuclear reactor environment, pointing out that $\gamma$ cascades from $^{239}$U is an overlooked yet a powerful source of $\chi\bar\chi$ 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, $\varepsilon = Q_\chi/e$, from the electron recoil searched for in a variety of experiments placed in proximity to the reactor cores. The derived limits on $\varepsilon$ are the strongest in the interval of masses $\sim 0.7-2 $\,MeV. We also derive the MCP flux from the Sun and point out potential sensitivity of the low-threshold dark matter search experiments.