CP violation in τ ^±→ π ^±K _svand D ^±→π± K _s: The importance of K _S - K _L interference

The B-factories have measured CP asymmetries in the τ → πKS and D → KSπ modes. The KS state is identified by its decay to two pions at a time that is close to the KS lifetime. Within the Standard Model and many of its extensions, the asymmetries in these modes come from CP violation in K0 - K 0 mixing. We emphasize that the interference between the amplitudes of intermediate KS and KL is as important as the pure KS amplitude. Consequently, the measured asymmetries depend on the times over which the relevant decay rates are integrated and on features of the experiment.