Asymmetry in the Decay ofΣHyperons

A search has been made for an asymmetry in the decay of ${\Sigma }^{\pm }$ hyperons. The ${\Sigma }^{\pm }$ were produced in the reactions ${\pi }^{\pm }+p\to {\Sigma }^{\pm }+K^{+}$. Detection and identification of the $K^{+}$ mesons by a counter technique selects the above reactions and establishes the plane of production. Additional counters, which detect the pions from the $\Sigma$-hyperon decay in coincidence with the $K^{+}$ mesons, measure the symmetry with respect to the plane of production. The results yield the parameter $\alpha \overline{P}$, where $\alpha$ measures the strength of parity nonconservation and $\overline{P}$ is the average polarization of the $\Sigma$ hyperon. The data give: for ${\Sigma }^{-}$ produced by 1.0-Bev ${\pi }^{-}$, ${\alpha }^{-}\overline{P}=+0.02\mathrm{}\pm 0.05$; for ${\Sigma }^{-}$ from 1.1-Bev ${\pi }^{-}$, ${\alpha }^{-}\overline{P}=-0.06\mathrm{}\pm 0.05$; for ${\Sigma }^{+}$ from 1.0-Bev ${\pi }^{+}$, the decay mode ${\Sigma }^{+}\to {\pi }^{+}+n$ gives ${\alpha }^{+}\overline{P}=+0.02\mathrm{}\pm 0.07$ and the mode ${\Sigma }^{+}\to {\pi }^0+p$ gives ${\alpha }^0\overline{P}=+0.70\mathrm{}\pm 0.30$. The value of ${\alpha }^0\overline{P}$ for ${\Sigma }^{+}\to {\pi }^0+p$ strongly indicates that parity is not conserved. For the ${\Sigma }^{+}$, $\left|\overline{P}\right|>~0.70\pm 0.30$; thus we can conclude $\left|{\alpha }^{+}\right|<~0.03\pm 0.11$. The results therefore indicate that the asymmetry parameter $\alpha$ depends upon the isotopic spin states in the final pion-nucleon system. The dependence on the isotopic spin is compatible with the $\Delta I=\pm \frac{1}{2}$ rule.