  It has been claimed that wave packets must be covariant and also that
decohered neutrino oscillations are always revived during measurement. These
conjectures are supported by general arguments which are not specific to the
electroweak theory, and so if they are true for neutrinos they will also be
true for simplified models. In this paper we produce such a simplified model in
which the neutrino wave function, including its entanglement with the source
particle and the environment, can be calculated explicitly in quantum field
theory. It exhibits neutrino oscillation, which is reduced at late times by
decoherence due to interactions of the source with the environment. One simple
lesson from this model is that only the difference between the environmental
interactions before and after neutrino emission can reduce the amplitude of
neutrino oscillations. The model will be used to test the conjectures in a
companion paper.
