Future probes of the origin of CP violation

We review what has been learned about CP violation in the K system. It is natural to hypothesize that the observed CP-violating effects are caused by the Standard Model weak interaction. We describe the stringent future test of this hypothesis via experiments on the B system. Then, we see how new physics beyond the Standard Model could be revealed by this test. 1. What do we already know? Before discussing the future probes of the origin of CP violation, let us briefly recall some of the things we already know. All laboratory CP-violating effects seen so far occur in the neutral K system. We know that the neutral K mass eigenstates, KShort(KS) and KLong(KL), are not CP eigenstates, as they would be if the world were CP invariant. Rather, they are CP admixtures given by |KS〉 = |K1〉+ ǫ|K2〉 |KL〉 = |K2〉+ ǫ|K1〉. (1.1) Here, K1 and K2 are, respectively, the CP-even and CP-odd eigenstates of CP, and |ǫ| = O(10 ). We know also that both KL and KS can decay to the CP-even final state π π. The ratio of their decay amplitudes, η+− ≡ 〈π π|T |KL〉/〈π π|T |KS〉, has the measured value [1] η+− = [(2.285± 0.019)× 10 ] e 0 . (1.2) The CP-violating decay KL→π π can, in principle, result from two effects. First, it can occur because, as described by Eqs. (1.1), KL has a small K1 component, which can decay to π π without violating CP. This type of CP violation, which results from CP-noninvariance of the K −K0 mixing amplitudes which make KS and KL what they are, is referred to as “indirect CP violation.” In addition, it may be that even the K2 component of KL can decay to π π. This type of CP violation, in which a decay amplitude itself violates CP, is called “direct CP violation”. [2] The CP-violating decay of KL to the CP-even final state π π has also been seen, and it has been found that η00 ≡ 〈π π|T |KL〉/〈π π|T |KS〉 has the value [3] η00 = [(2.275± 0.019)× 10 ] e ◦ . (1.3) † To appear in the Proceedings of the Workshop ”Beyond the Desert—Accelerator and Non-Accelerator Approaches”, held in Tegernsee, Germany, June 1997.