Limits on the magnetic moment of sterile neutrino and two-photon neutrino decay

It is shown that the non-zero transition magnetic moment (μtran) between the sterile neutrino (νs) and the muon neutrino (νμ) could be effectively searched for via the Primakoff effect, in the process of νμZ→νsZ conversion in the external Coulomb field of a nucleus Z, with the subsequent νs→νμ+γ decay. From the recent results of the NOMAD neutrino detector at CERN a model-independent constraint of μtran<(10−6−10−9)μB is obtained depending on the value of νs mass. For the ms∼O(1) GeV region these bounds are comparable with the present experimental ones on νμ and νe diagonal magnetic moments and are more sensitive than those on ντ magnetic moment. Further improvement of the bounds might be interesting for understanding of the origin of the KARMEN anomaly. From the same analysis the constraint on ντ(νs)→νμ+γ+γ decay lifetime τ>2×1013 s/mν7 MeV) is obtained. The limit is valid for neutrino masses up to mν∼O(1) GeV.