Measurement of the Phase and Magnitude ofη00

Abstract

This Letter presents measurements of the phase and magnitude of the $\mathrm{CP}$-nonconservation parameter ${\eta }_{00}$ resulting from a study of the proper-time distribution of $K^0\to {\pi }^0{\pi }^0$ decays in a short-lived neutral beam. We find ${\varphi }_{00}=55.7\mathrm{}°\pm 5.8°$ and $\left|{\eta }_{00}\right|=(2.33\mathrm{}\pm 0.18)\times {10}^{-3\mathrm{}}$. A direct comparison to ${\eta }_{+-}$ measured in the same experiment gives ${\varphi }_{00}-{\varphi }_{+-}=12.6\mathrm{}°\pm 6.2°$ and $\left|\frac{{\eta }_{00}}{{\eta }_{+-}}\right|=1.00\mathrm{}\pm 0.09$.