New aspects of muon–electron nuclear conversion

We found a new important tree-level contribution to muon–electron nuclear conversion from neutrino exchange between two quarks in the same nucleon and demonstrated that this process, contrary to common belief, can be observed in the near future experiments if there exists a mixed sterile-active neutrino state νh heavier than the quark confinement scale Λc∼1 GeV. From the present non-observation of muon–electron conversion we derive new experimental constraints on νh−νe,μ mixing.