Observation of Direct [ital CP] Violation in K[sub S,L] [r arrow] [pi][pi] Decays

We have compared the decay rates of K[sub L] and K[sub S] to [pi][sup +][pi][sup [minus]] and [pi][sup 0][pi][sup 0] final states using a subset of the data from the KTeV experiment (E832) at Fermilab. We find that the direct-[ital CP]-violation parameter Re([epsilon][sup [prime]]/[epsilon] ) is equal to [28.0[plus minus]3.0(stat)[plus minus]2. 8(syst)][times]10[sup [minus]4] . This result definitively establishes the existence of [ital CP] violation in a decay process. [copyright] [ital 1999] [ital The American Physical Society]