Observation of Direct<i>CP</i>Violation in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="italic">K</mml:mi></mml:mrow><mml:mrow><mml:mi mathvariant="italic">S</mml:mi><mml:mo>,</mml:mo><mml:mi mathvariant="italic">L</mml:mi></mml:mrow></mml:msub></mml:mrow><mml:mspace/><mml:mo>→</mml:mo><mml:mspace/><mml:mi mathvariant="italic">π</mml:mi><mml:mi mathvariant="italic">π</mml:mi></mml:math>Decays

We have compared the decay rates of ${K}_{L}$ and ${K}_{S}$ to ${\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}$ and ${\ensuremath{\pi}}^{0}{\ensuremath{\pi}}^{0}$ final states using a subset of the data from the KTeV experiment (E832) at Fermilab. We find that the direct-CP-violation parameter $\mathrm{Re}({\ensuremath{\epsilon}}^{\ensuremath{'}}/\ensuremath{\epsilon})$ is equal to $[28.0\ifmmode\pm\else\textpm\fi{}3.0(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}2.8(\mathrm{syst})]\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$. This result definitively establishes the existence of CP violation in a decay process.