Study of CP Violation in the Neutral Kaon System

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We ran KTeV Experiment at Fermilab from 1996 to 1997, and obtained following results.1. We found that the CP is violated in the decay itself (direct CP violation), by measuring the double ratio of the branching fractions of K_L, Ks * <product> + <product> -and <product>^0 <product>^0 decays. Based on the 20% of the data, we obtained a preliminary result, Re(e'/e) * (28 0*4.1) x 10^<-4> which is more than 6 standard deviations away from 0. This result rejects the Superweak model which was one of the theories explaining the CP violation mechanism.2. We observed the first CP violation in a kinematical variable, the asymmetry in the distribution of the angle between <product> + <product>- and e+e-planes, in the K_L * <product> + <product> e+e- decay.3. We searched for the decay K_L * <product>^Onunu, which is dominated by direct CP violation, and set the best upper limit on the branching ratio, <5.9x10^7 (90%CL) (preliminary).We also measured the detection inefficiency of photon detectors due to photo-nuclear interactions by using tagged photon beam at KEK Tanashi. We found that pure CsI and lead/scintillator sandwich detectors satisfy the specs necessary for the future K_<1L> * _<1@ : S1O>nunu experiments.We had an international workshop on the CP violation in K, and had an extensive discussion on the future experiments to measure the branching ratio of K_L * ^Onunu decay.