Observation of Direct CP Violation in K_(S,L) to π π Decays

We have compared the decay rates of K_L and K_S to \pi^+ \pi^- and \pi^0 \pi^0 final states using a subset of the data from the KTeV experiment (E832) at Fermilab. We find that the direct-CP-violation parameter Re(\epsilon'/\epsilon) is equal to (28.0 \pm 3.0 (stat) \pm 2.8 (syst)) \times 10^{-4}. This result definitively establishes the existence of CP violation in a decay process.