Can millicharge be probed at future Super Tau Charm Facility via mono-$π^0$ searches?

Abstract: We propose a new channel to search for millicharged particles at the future Super Tau Charm Facility (STCF) via mono-$\pi^0$ signature. For the first time, we compute the mono-$\pi^0$ signal events at the future STCF due to millicharged particle production, as well as due to standard model irreducible/reducible backgrounds. By utilizing the assumed 20 ab$^{-1}$ luminosity for each running energy with $\sqrt{s}= 2$ GeV, 4 GeV and 7 GeV, we derive the corresponding upper limits on millicharge, respectively. Via mono-$\pi^0$ searches at the future STCF with $\sqrt{s}=2$ GeV, the upper limits on millicharge can be improved than ArgoNeuT when the mass of millicharged particle is less than about 500 MeV, but are not very competitive compared to a latest derivation from a past BEBC experiment and a new SENSEI experiment. Regardless, the mono-$\pi^0$ searches could be an important complement to investigate the invisible particles, such as dark matter.