Neutrino mediated muon--electron conversion in nuclei revisited

The non-photonic neutrino exchange mechanism of the lepton flavor violating muon-electron conversion in nuclei is revisited. First we determine the nucleon coupling constants for the neutrino exchange mechanism in a relativistic quark model taking into account quark confinement and chiral symmetry requirements. This includes a new, previously overlooked tree-level contribution from neutrino exchange between two quarks in the same nucleon. Then for the case of an additional sterile neutrino we reconsider the coherent mode of this process. The presence of a mixed sterile-active neutrino state heavier than the quark confinement scale Lambda_c (~1 GeV) may significantly improve the prospects for observation of this process in future experiments as compared to the conventional scenario with only light neutrinos. Turning the arguments around we derive new experimental constraints on |U_(mu h)| and |U_(e h)| mixing matrix elements from the non-observation of muon--electron conversion.