Neutrino dipole portal at electron colliders

We propose to search for a heavy neutral lepton (HNL), that is also known as sterile neutrino, in electron colliders running with the center-of-mass energies at few GeV, including BESIII, Belle II, and the proposed Super Tau Charm Factory (STCF). We consider the HNL interacting with Standard Model neutrino and photon via a transition magnetic moment, the so-called dipole portal. We use the monophoton signature at electron colliders to probe the constraints on the active-sterile neutrino transition magnetic moments d as the function of the HNL's mass mN. It is found that BESIII, Belle II and STCF can probe the upper limits for d down to 1.3×10−5GeV−1, 8×10−6GeV−1, and 1.3×10−6GeV−1 with mN around GeV scale, respectively, and have sensitivity to the previously unexplored parameter space for electron- (de) and tau-neutrino (dτ) dipole portal with mN from dozens to thousands MeV. On dμ for HNL mixing with the muon-neutrino, Belle II and STCF can also provide leading constraints.