A question about Weisskopf-Wigner approximation

[MichaelDMC]

What is the physical interpretation of the Weisskopf-Wigner approximation, when it is applied in the neutral kaon system?

I would say that the approximation means that a decay state has a small probability to suffer a transition (or be "transformed") into another decay state through weak interaction. But I'm not sure about this.

I would please like to know some references that can help me to understand better this approximation in the kaon system. I'm following the book "CP Puzzle " by Kabir

Thanks in advance

 

[vanhees71]

A very nice elementary treatment of precisely this issue can be found in

Nachtmann, O.: Elementary Particle Physics - Concepts and Phenomenology, Springer-Verlag, 1990