Measurement of the Phase and Magnitude ofη+−

Abstract

The phase and magnitude of the $\mathrm{CP}$-nonconservation parameter ${\eta }_{+-}$ have been determined from the proper-time distribution of $K^0\to {\pi }^{+}{\pi }^{-}$ decays in a short-lived neutral beam. We find ${\varphi }_{+-}=41.7\mathrm{}°\pm 3.5°$ and $\left|{\eta }_{+-}\right|=(2.27\mathrm{}\pm 0.12)\times {10}^{-3\mathrm{}}$.