Probing millicharge at BESIII via monophoton searches

We propose to search for millicharged particles at the BESIII detector which is operated at the Beijing Electron Positron Collider. We compute the monophoton signal events at the BESIII detector due to millicharged particle production, as well as due to standard model irreducible/reducible backgrounds. By utilizing all the data accumulated at the BESIII detector since 2011, we derive new leading upper limits on millicharge, $ϵ\ensuremath{\lesssim}(0.86--2.5)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$, for the mass range, $0.1\text{ }\text{ }\mathrm{GeV}\ensuremath{\lesssim}m\ensuremath{\lesssim}1\text{ }\text{ }\mathrm{GeV}$. Furthermore, projections with more data to be collected at the BESIII detector are made. Our analysis significantly reduces the parameter region of millicharged dark matter to account for the anomalous 21 cm signal near redshift $z\ensuremath{\simeq}17$ recently observed by the EDGES experiment.