Studies ofK20→π+π−Decay and Interference

We describe a detailed study of the $\mathrm{CP}$-violating decay ${K_2}^0\to {\pi }^{+}+{\pi }^{-}$ in an experiment performed at the Brookhaven AGS. In a search for an energy dependence in the branching ratio, $\mathrm{BR}=\frac{\mathrm{rate}({K_2}^0\to {\pi }^{+}+{\pi }^{-})}{\mathrm{rate}({K_2}^0\to \mathrm{all}\mathrm{charged}\mathrm{modes})}$, we find between 0.95 and 2.45 GeV that if $\mathrm{BR}\propto E^n$ then $n=0.03\mathrm{}\pm 0.33$. Assuming $n=0\mathrm{}$, then BR = (1.97±0.16)×${10}^{-3\mathrm{}}$, yielding $\left|{\eta }_{+-}\right|=(1.91\mathrm{}\pm 0.09)\times {10}^{-3\mathrm{}}$. In an interference experiment at a mean momentum of 1.55 GeV/c, we observe $cos\alpha =1.00\mathrm{}\pm 0.21$, where $\alpha$ is the phase angle between the ${K_2}^0\to {\pi }^{+}+{\pi }^{-}$ decay amplitude and the ${\pi }^{+}{\pi }^{-}$ amplitude from coherent regeneration in uniformly distributed Be. A search for neutral currents yields a limit on the branching ratio of ${K_2}^0\to {\mu }^{\pm }+{\mu }^{\mp }$ of 3.5×${10}^{-5\mathrm{}}$ and ${K_2}^0\to {\mu }^{\pm }+e^{\mp }$ of 8×${10}^{-6\mathrm{}}$ (both 90% confidence levels). We measure the mass of the ${K_1}^0$ to be 497.44±0.50 MeV.