Production of Strange Particles byπ-Nucleon and Photonucleon Interactions near Threshold

An investigation of the behavior of the $\pi$-nucleon reaction cross sections near the strange particle threshold shows that the variation with energy of the ${\Lambda }^0-K$ cross section near the $\Sigma -K$ threshold depends strongly upon the relative parities of the $\Sigma$ and the ${\Lambda }^0$ hyperons. If the elements of the Wigner $R$ matrix are nearly stationary near the $\Sigma -K$ threshold, and if the decay of the compound system is nearly independent of its mode of formation, the variation of the ${\Lambda }^0$ and $\Sigma$ cross sections is almost uniquely established from cross sections measured at one energy. In particular, the ${\Lambda }^0$ cross section exhibits strong cusps at the $\Sigma -K$ thresholds. Angular distributions and polarizations are discussed from the viewpoint of these assumptions. The photonucleon production of strange particles is shown to be closely related to the $\pi$-nucleon production, essentially because the same final-state interactions are important. The strength and characteristics of these interactions lead to the conclusion that the matrix elements for photonucleon production of strange particles will, in general, be strongly affected in magnitude by the final-state interactions and they will not be real, even at threshold. The analysis of cusps, which are predicted in the $\gamma +p\to \pi +\mathrm{nucleon}$ cross section at the ${\Lambda }^0+K$ threshold and the $\Sigma +K$ threshold, and in the $\gamma +p\to {\Lambda }^0+K$ reaction at the $\Sigma +K$ threshold, should provide a means of determining the relative parities of the strange particles, in a manner almost independent of the dynamics of the reaction.