Neutrino dipole portal at electron colliders

We propose to search for a heavy neutral lepton (HNL), that is also know 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 $m_N$.It is found that BESIII, Belle II and STCF can probe the upper limits for $d$ down to 1.3 $\times 10^{-5}\ {\rm GeV}^{-1}$, 8 $\times 10^{-6}\ {\rm GeV}^{-1}$, and 1.3 $\times 10^{-6}\ {\rm GeV}^{-1}$ with $m_N$ around GeV scale, respectively, and have sensitivity to the previously unexplored parameter space for electron- ($d_e$) and tau-neutrino ($d_\tau$) dipole portal with $m_N$ from dozens to thounsands MeV. On $d_\mu$ for HNL mixing with the {muon}-neutrino, Belle II and STCF can also provide leading constraints.