Abstract

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 nu(h) heavier than the quark confinement scale A(c) similar to 1 GeV. From the present non-observation of muon-electron conversion we derive new experimental constraints on nu(h) - nu(e,mu) mixing. (C) 2002 Elsevier Science B.V. All rights reserved.