CP violation in τ± → π± KSυ and D± → π± KS: the importance of KS-KL interference

The B-factories have measured CP asymmetries in the tau -> pi K(S)v and D -> K-S pi modes. The K-S state is identified by its decay to two pions at a time that is close to the K-S lifetime. Within the Standard Model and many of its extensions, the asymmetries in these modes come from CP violation in K-0 - (K) over bar (0) mixing. We emphasize that the interference between the amplitudes of intermediate K-S and K-L is as important as the pure K-S amplitude. Consequently, the measured asymmetries depend on the times over which the relevant decay rates are integrated and on features of the experiment.