Two-Body Strange-Particle Final States inπ−pInteractions at 4.5 and 6 GeV/c

Results on the following ${\pi }^{-}p$ reactions involving a hyperon are studied at 4.5 and 6.0 GeV/c from a high-statistics bubble-chamber experiment. (1) ${\pi }^{-}p\to (\Lambda , {\Sigma }^0)K^0$: Differential cross sections and hyperon polarizations are presented. Comparison with the line-reversed reactions $\overline{K}N\to (\Lambda , {\Sigma }^0)\pi$ indicates the failure of the predictions of $K^{*}\mathrm{}\left(890\right)\mathrm{}$ and $K^{*}\mathrm{}\left(1420\right)\mathrm{}$ exchange degeneracy. Effective trajectories for these two reactions are compared. Shrinkage is observed in $\overline{K}N\to \Lambda \pi$ and not in ${\pi }^{-}p\to \Lambda K^0$. (2) ${\pi }^{-}p\to (\Lambda , {\Sigma }^0)K^{*}{\mathrm{}\left(890\right)\mathrm{}}^0$: Differential cross sections, hyperon polarizations, and $K^{*}{\mathrm{}\left(890\right)\mathrm{}}^0$ density-matrix elements are determined. $\Lambda K^{*}{\mathrm{}\left(890\right)\mathrm{}}^0$ decay correlations are found to impose strong constraints on the scattering amplitudes. The data indicate that both natural- and unnatural-parity exchanges contribute large, but opposite, $\Lambda$ polarizations. This behavior cannot be explained by a simple exchange model utilizing $K$ and the exchange-degenerate $K^{*}\mathrm{}\left(890\right)\mathrm{}$ and $K^{*}\mathrm{}\left(1420\right)\mathrm{}$ only. Additional trajectories or absorption effects are required to obtain the observed $\Lambda$-polarization effects. Comparison of $\Lambda K^{*}{\mathrm{}\left(890\right)\mathrm{}}^0$ and ${\Sigma }^0{K}^{*}{\mathrm{}\left(890\right)\mathrm{}}^0$ indicates the greater importance of unnatural-parity exchange in the former reaction. We observe no evidence for deviations from isospin predictions in $\Lambda K^{*}{\mathrm{}\left(890\right)\mathrm{}}^0$ production where $K^{*}{\mathrm{}\left(890\right)\mathrm{}}^0\to K^{+}{\pi }^{-} \mathrm{and} K_S^0{\pi }^0$. (3) ${\pi }^{-}p\to \Lambda K^{*}{\mathrm{}\left(1420\right)\mathrm{}}^0 \mathrm{and} \Lambda K^{*}{\mathrm{}\left(1300\right)\mathrm{}}^0: K^{*}{\mathrm{}\left(1420\right)\mathrm{}}^0$ density-matrix elements satisfying positivity constraints are determined allowing for $s$-wave interference effects. Evidence of the existence of a narrow $K^{*}{\mathrm{}\left(1300\right)\mathrm{}}^0\to K\pi \pi$ with a dominant $K^{+}{\rho }^{-}$ decay mode is observed in the 4.5- and 6-GeV/c data. (4) $\Sigma \mathrm{}\left(1385\right)\mathrm{}$, $\Lambda \mathrm{}\left(1405\right)\mathrm{}$, $\Lambda \mathrm{}\left(1520\right)\mathrm{}$ production: Differential cross sections for the quasi-two-body reactions ${\pi }^{-}p\to Y^0{K}^0$, where $Y^0$ is $\Lambda \mathrm{}\left(1405\right)\mathrm{}$, $\Lambda \mathrm{}\left(1520\right)\mathrm{}$, or $\Sigma {\mathrm{}\left(1385\right)\mathrm{}}^0$, are presented and found to have a very similar flat slope in the forward direction. Data for forward $K^{+}$ scattering in the reaction ${\pi }^{-}p\to \Sigma {\mathrm{}\left(1385\right)\mathrm{}}^{-}{K}^{+}$ are presented and discussed. It is argued that this forward peak cannot be explained by kinematic reflection or an $s$-channel effect and therefore must be due to either two-particle exchange or a single exotic exchange in the $t$ channel.