Experimental Comparison of $\mathrm{CP}$ Violation in ${\overline{K}}^0\to {\pi }^{+}{\pi }^{-}$ and $K^0\to {\pi }^{+}{\pi }^{-}$ Decays

The time distributions of the decays $K^0\to {\pi }^{+}{\pi }^{-}$ and ${\overline{K}}^0\to {\pi }^{+}{\pi }^{-}$ are directly compared in the same apparatus. $K^0 \left({\overline{K}}^0\right)$ mesons were produced by charge exchange of $K^{+} \left(K^{-}\right)$ in a carbon target. Wire spark chambers with magnetostrictive readout were used to detect the decays. The observation of the difference in the decay distributions of $K^0$ and ${\overline{K}}^0$ mesons serves as a direct proof, independent of any theoretical conventions, that $\mathrm{CP}$ is violated. No significant deviation from the presently accepted theoretical formalism of $\mathrm{CP}$ violation was found. The values of the $\mathrm{CP}$-violating parameters for $K^0$ and ${\overline{K}}^0$ decays are compared.