We show that the generator of field mixing transformations in Quantum Field
Theory induces a non trivial structure in the vacuum which turns out to be a
coherent state, both for bosons and for fermions, although with a different
condensate structure. The Fock space for mixed fields is unitarily inequivalent
to the Fock space of the massive (free) fields in the infinite volume limit. As
a practical application we study neutrino mixing and oscillations. A new
oscillation formula is found where the oscillation amplitude is depressed, with
respect to the usual one, by a factor which is momentum and mass dependent. In
the relativistic limit, the usual formula is recovered. We finally discuss in
some detail phenomenological features of the modified oscillation formula.Comment: 13 pages, LaTex + 1 figure. Talk given at the XIXth International
Conference on Particle Physics and Astrophysics in the Standard Model and
Beyond, Bystra, Poland, September 19-26, 199
