  The one-channel Wigner-Weisskopf survival amplitude may be dominated by
exponential type decay in pole approximation at times not too short or too
long, but, in the two channel case, for example, the pole residues are not
orthogonal, and the pole approximation evolution does not correspond to a
semigroup (experiments on the decay of the neutral K-meson system support the
semigroup evolution postulated by Lee, Oehme and Yang, and Yang and Wu, to very
high accuracy). The scattering theory of Lax and Phillips, originally developed
for classical wave equations, has been recently extended to the description of
the evolution of resonant states in the framework of quantum theory. The
resulting evolution law of the unstable system is that of a semigroup, and the
resonant state is a well-defined funtion in the Lax-Phillips Hilbert space. In
this paper we apply this theory to relativistically covarant quantum field
theoretical form of the (soluble) Lee model. We show that this theory provides
a rigorous underlying basis for the Lee-Oehme-Yang-Wu construction.
