Probing active-sterile neutrino transition magnetic moments at LEP and CEPC

We consider the sterile neutrino, which is also known as heavy neutral lepton, interacting with the Standard Model (SM) active neutrino and photon via a transition magnetic moment, the so-called dipole portal, which can be induced from the more general dipole couplings which respect the full gauge symmetries of the SM. Depending on the interactions with SU(2)L and U(1)Y field strength tensors Wa mu nu and B mu nu, we consider four typical scenarios and probe the constraints on the couplings with photon d gamma at LEP using the analyses to search monophoton signature and the measurement of Z decay. We find that in the considered scenarios assuming the coupling with Z boson dZ not equal 0, the measurement of Z decaying into photon plus invisible particles can provide stricter constraints than the monophoton searches at the LEP1. The complementary constraints to existing experiments can be provided by the LEP. We also investigate the sensitivity on the dipole portal coupling d gamma from the monophoton searches at future electron colliders, such as CEPC, and find that CEPC can explore the previously unconstrained parameter space by current experiments.