Diagnostic for new physics in B→πK decays

A recent analysis of B→πK decays concludes that present data do not clearly indicate whether (i) the standard model (or ΔI=0 new physics) is sufficient, or (ii) ΔI=1 new physics is needed. We show that these two possibilities can be distinguished by whether a sum rule relating the CP asymmetries of the four B→πK decays is valid. If case (i) is favored, the sum rule holds, and one predicts ACP(π0K0)=−0.15, while in case (ii) fits to new physics involving large values of a color-suppressed tree amplitude entail ACP(π0K0)=−0.03. The current experimental average ACP(π0K0)=−0.01±0.10 must be measured a factor of at least three times more precisely in order to distinguish between the two cases.