Data from atmospheric and solar neutrinos indicate that there are at least
three neutrino types involved in oscillation phenomena. Even if the corresp
onding neutrino mass scales are very different, the inevitable reference to
mixing between more than two neutrino types has profound consequences for
the planning of the accelerator experiments suggested by these results. We
discuss the measurement of mixing angles and CP phases in the context of th
e neutrino beam emanating from a neutrino factory: the straight sections of
a muon storage ring. We emphasize the importance of charge identification.
The appearance of wrong-sign muons in a long baseline experiment may provi
de a powerful test of neutrino oscillations in the mass-difference range in
dicated by atmospheric-neutrino observations. (C) 1999 Elsevier Science B.V
.