Study of CP Violation in Neutral Kaon System

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We ran KTeV experiment in 1996-1997, and have obtained the following results.* We have established the existance of direct CP violation, CP violation in the decay itself, instead of in the K^0-K^0 mixing. This is based on the result that the double ratio between the four branching ratios of K_L, S→π^+π^-, π^0π^0 decays is deviated from unity by 7 standard deviations. Based on this result and the result by CERN experiments, Super Weak model was rejected as the sole source of CP violation.* We have observed the CP violation in the distribution of angle between ee and ππ planes in the K_L→ππee decays. This is the first time that the CP violation was found in the kinematical distributions.* New upper limits on the branching ratios were set for the CP violating rare decays, K_L→π^0ee, K_L→π^0μμ, and K_L→π^0vv.* In addition, we have measured branching ratios and form factors for other rare K decays and hyperon decays.In 1999-2000, we had the second run, and collected the same amount of data for K_<L,S>→ππ events, and 2-3 times the data for rare decays.We are currently designing a new experiment to measure the CP violation parameter by using K_L→π^0vv decays. For this experiment, we measured the kaon and neutron flux and energy distributions by bombarding the target by 150GeV protons.