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