CP violation in D0 → KSKS

The direct CP asymmetry a(CP)(dir)(D-0 -> KSKS) involves exchange diagrams which are induced at tree level in the Standard Model. Since the corresponding topological amplitude E-KK can be large, D-0 -> KSKS is a promising discovery channel for charm CP violation. We estimate the penguin annihilation amplitude with a perturbative calculation and extract the exchange amplitude E-KK from a global fit to D branching ratios. Our results are further used to predict the size of mixing-induced CP violation. We obtain vertical bar a(CP)(dir)(D-0 -> KSKS)vertical bar <= 1.1% ( 95% C.L.). The same bound applies to the nonuniversal part of the phase between the D - (D) over bar mixing and decay amplitudes. If future data exceed our predictions, this will point to new physics or an enhancement of the penguin annihilation amplitude by QCD dynamics. We briefly discuss the implications of these possibilities for other CP asymmetries.