Observation of Direct CP Violation in K_{S,L} to \pi \pi Decays

  title={Observation of Direct CP Violation in K\_\{S,L\} to \pi \pi Decays},
  author={KTeV collaboration},
We have compared the decay rates of K L and K S to π + π − and π 0 π 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 ( ǫ ′ /ǫ ) is equal to (28 . 0 ± 3 . 0 (stat) ± 2 . 8 (syst)) × 10 − 4 . This result definitively establishes the existence of CP violation in a decay process. 

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