Beyond the Weisskopf-Wigner approximation in the K-0-(K)over-bar(0) system

We reanalyze the time evolution of the K-0-(K)over-bar(0) system in the lan guage of certain spectral function whose Fourier transforms give the time-d ependent survival and transition amplitudes. Approximating the spectral fun ction by a one-pole ansatz we obtain insight into the limitation of the val idity of the one-pole approximation. It will be shown that the same validit y restrictions apply to the known formulae of the Weisskopf-Wigner approxim ation as well. The present analysis can also be applied to the effect of so -called vacuum regeneration of K-L, and K-S. As a result of this possibilit y new contributions to the well-known oscillatory terms will enter the time -dependent transition probabilities. It will be shown that the order of mag nitude of this new effect is very small and, in principle, its exact determ ination lies outside the scope of the one-pole ansatz.