Ja. Formaggio et al., Search for the lepton family number violating process (nu)over-bar(mu)e(-)->mu(-)(nu)over-bar(e) - art. no. 071803, PHYS REV L, 8707(7), 2001, pp. 1803
The NuTeV experiment at Fermilab has used a sign-selected neutrino beam to
perform a search for the lepton number violating process <(<nu>)over bar>(m
u)e(-) --> mu-<(<nu>)over bar>(e), and to measure the cross section of the
standard model inverse muon decay process nu (mu)e --> mu-nu (e). NuTeV mea
sures the inverse muon decay asymptotic cross-section slope sigma /E to be
(13.8 +/-1.2 +/-1.4) x 10(-42) cm(2)/GeV. The experiment also observes no e
vidence for lepton number violation and places one of the most restrictive
limits on the cross-section ratio sigma(<(<nu>)over bar>(mu)e(-)--> mu-<(<n
u>)over bar>(e))/sigma(nu (mu)e(-)--> mu (-)nu (e)) less than or equal to1.
7% at 90% C.L. for V-A couplings and less than or equal to0.6% for scalar c
ouplings.