Millicharged particles at electron colliders

We propose to search for millicharged particles in electron colliders operated with the center-of-mass energies at $\mathcal{O}(1--10)\text{ }\mathrm{GeV}$, which include Belle II, BESIII, BABAR, and also the proposed experiment Super Tau Charm Factory (STCF). We use the monophoton final state at electron colliders to probe the parameter space of millicharged particles, that is spanned by millicharge $\ensuremath{\epsilon}$ and mass $m$. We find that electron colliders have sensitivity to the previously unexplored parameter space for millicharged particles with MeV--GeV mass: $\ensuremath{\epsilon}\ensuremath{\lesssim}\mathcal{O}({10}^{\ensuremath{-}1})$ for $0.5\text{ }\text{ }\mathrm{GeV}\ensuremath{\lesssim}m\ensuremath{\lesssim}3.5\text{ }\text{ }\mathrm{GeV}$ in BABAR, $\ensuremath{\epsilon}\ensuremath{\lesssim}\mathcal{O}({10}^{\ensuremath{-}3})$ for $0.1\text{ }\text{ }\mathrm{GeV}\ensuremath{\lesssim}m\ensuremath{\lesssim}1.5\text{ }\text{ }\mathrm{GeV}$ in BESIII, $\ensuremath{\epsilon}\ensuremath{\lesssim}{10}^{\ensuremath{-}3}\ensuremath{-}{10}^{\ensuremath{-}2}$ for $0.1\text{ }\text{ }\mathrm{GeV}\ensuremath{\lesssim}m\ensuremath{\lesssim}4\text{ }\text{ }\mathrm{GeV}$ in Belle II, and $\ensuremath{\epsilon}\ensuremath{\lesssim}\mathcal{O}({10}^{\ensuremath{-}4})$ for $1\text{ }\text{ }\mathrm{MeV}\ensuremath{\lesssim}m\ensuremath{\lesssim}1\text{ }\text{ }\mathrm{GeV}$ in STCF.