In the absence of any strangeness violating interaction, the stationary
states, 
and 
, of a 
-meson and 
-meson respectively,
are mass eigenstates of the strong and electromagnetic interactions:
Since the strangeness violating interaction 
is much
weaker than the strong and electromagnetic interaction we can apply
perturbation theory (Wigner-Weisskopf approach)
and obtain for the time evolution of the neutral kaon wave function the
following differential equation:
where the 
matrices M and 
are called mass and decay
matrix respectively and given by:
with the index i = 1(2) corresponding to 
.
The matrix elements of 
can in principle be calculated within the
Standard Model but in practice the uncertainties due to non-perturbative
effects are too large to obtain any interesting predictions. Therefore we use
the following parametrisation of with 8 real and positive parameters:
where 
, 
, 
, 
are equal to the masses and lifetimes of and
respectively.
