ǫ/ǫ Results from KTeV

Since the 1964 discovery of KL → π π [1], understanding CP violation has been a major goal of particle physics. Subsequent experiments showed that the dominant mechanism of CP violation in neutral kaons (still the only particle system in which CP violation has been observed) is a small asymmetry between K → K 0 and K 0 → K mixing. This asymmetry, referred to as indirect CP violation, results in the KL and KS being states of mixed CP. The parameter ǫ, which is used to parametrize this effect, quantifies the CP impurity of the KL and KS states: KS ∼ Keven + ǫKodd KL ∼ Kodd + ǫKeven, where |ǫ| = 2.28× 10, CP |Keven >= +1|Keven >, and CP |Kodd >= −1|Kodd > . There has been considerable effort during the last 30 years to determine whether or not the CP symmetry is also violated in the decay amplitude (i.e., Kodd → ππ). This effect is referred to as direct CP violation and is parametrized by ǫ. The ratio ǫ/ǫ can be determined from the double ratio of the 2-pion decay rates of KL and KS: