Two-Body Strange-Particle Final States in π - p Interactions at 4.5 and 6 GeV/c

Results on the following ${\ensuremath{\pi}}^{\ensuremath{-}}p$ reactions involving a hyperon are studied at 4.5 and 6.0 GeV/c from a high-statistics bubble-chamber experiment. (1) ${\ensuremath{\pi}}^{\ensuremath{-}}p\ensuremath{\rightarrow}(\ensuremath{\Lambda}, {\ensuremath{\Sigma}}^{0}){K}^{0}$: Differential cross sections and hyperon polarizations are presented. Comparison with the line-reversed reactions $\overline{K}N\ensuremath{\rightarrow}(\ensuremath{\Lambda}, {\ensuremath{\Sigma}}^{0})\ensuremath{\pi}$ indicates the failure of the predictions of ${K}^{*}(890)$ and ${K}^{*}(1420)$ exchange degeneracy. Effective trajectories for these two reactions are compared. Shrinkage is observed in $\overline{K}N\ensuremath{\rightarrow}\ensuremath{\Lambda}\ensuremath{\pi}$ and not in ${\ensuremath{\pi}}^{\ensuremath{-}}p\ensuremath{\rightarrow}\ensuremath{\Lambda}{K}^{0}$. (2) ${\ensuremath{\pi}}^{\ensuremath{-}}p\ensuremath{\rightarrow}(\ensuremath{\Lambda}, {\ensuremath{\Sigma}}^{0}){K}^{*}{(890)}^{0}$: Differential cross sections, hyperon polarizations, and ${K}^{*}{(890)}^{0}$ density-matrix elements are determined. $\ensuremath{\Lambda}{K}^{*}{(890)}^{0}$ 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, $\ensuremath{\Lambda}$ polarizations. This behavior cannot be explained by a simple exchange model utilizing $K$ and the exchange-degenerate ${K}^{*}(890)$ and ${K}^{*}(1420)$ only. Additional trajectories or absorption effects are required to obtain the observed $\ensuremath{\Lambda}$-polarization effects. Comparison of $\ensuremath{\Lambda}{K}^{*}{(890)}^{0}$ and ${\ensuremath{\Sigma}}^{0}{K}^{*}{(890)}^{0}$ indicates the greater importance of unnatural-parity exchange in the former reaction. We observe no evidence for deviations from isospin predictions in $\ensuremath{\Lambda}{K}^{*}{(890)}^{0}$ production where ${K}^{*}{(890)}^{0}\ensuremath{\rightarrow}{K}^{+}{\ensuremath{\pi}}^{\ensuremath{-}} \mathrm{and} {K}_{S}^{0}{\ensuremath{\pi}}^{0}$. (3) ${\ensuremath{\pi}}^{\ensuremath{-}}p\ensuremath{\rightarrow}\ensuremath{\Lambda}{K}^{*}{(1420)}^{0} \mathrm{and} \ensuremath{\Lambda}{K}^{*}{(1300)}^{0}: {K}^{*}{(1420)}^{0}$ density-matrix elements satisfying positivity constraints are determined allowing for $s$-wave interference effects. Evidence of the existence of a narrow ${K}^{*}{(1300)}^{0}\ensuremath{\rightarrow}K\ensuremath{\pi}\ensuremath{\pi}$ with a dominant ${K}^{+}{\ensuremath{\rho}}^{\ensuremath{-}}$ decay mode is observed in the 4.5- and 6-GeV/c data. (4) $\ensuremath{\Sigma}(1385)$, $\ensuremath{\Lambda}(1405)$, $\ensuremath{\Lambda}(1520)$ production: Differential cross sections for the quasi-two-body reactions ${\ensuremath{\pi}}^{\ensuremath{-}}p\ensuremath{\rightarrow}{Y}^{0}{K}^{0}$, where ${Y}^{0}$ is $\ensuremath{\Lambda}(1405)$, $\ensuremath{\Lambda}(1520)$, or $\ensuremath{\Sigma}{(1385)}^{0}$, are presented and found to have a very similar flat slope in the forward direction. Data for forward ${K}^{+}$ scattering in the reaction ${\ensuremath{\pi}}^{\ensuremath{-}}p\ensuremath{\rightarrow}\ensuremath{\Sigma}{(1385)}^{\ensuremath{-}}{K}^{+}$ 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.