Study of the ReactionK−+p→Λ+π++π−from 1.2 to 1.7 BeV/c

A sample of events of the reaction $K^{-}+p\to \Lambda +{\pi }^{+}+{\pi }^{-}$ was selected from an exposure of the Laboratory's 72-in. hydrogen bubble chamber to a separated $K^{-}$ beam of the Bevatron. The laboratory momentum of the $K^{-}$ in this sample was set at six values, ranging from 1.22 to 1.69 BeV/c. A sample at lower energy was also used in part of the analysis. Dalitz plots and effective-mass distributions show that the $\Lambda -\pi$ resonance, ${Y_1}^{*}\mathrm{}\left(1385\right)\mathrm{}$, is the dominant final state; the $\pi -\pi$ resonance, $\rho \mathrm{}\left(750\right)\mathrm{}$, also important, has a maximum cross section at 1.5 BeV/c; and ${Y_1}^{*}\mathrm{}\left(1660\right)\mathrm{}$ is produced weakly. We studied the angular correlations of the decay products of the ${Y_1}^{*}\mathrm{}\left(1385\right)\mathrm{}$ and determined that the spin-parity of this resonance is $P_{\frac{3}{2}}$, with a slight chance that it is $D_{\frac{5}{2}}$. With this spin-parity assignment, all of the observed properties of the ${Y_1}^{*}\mathrm{}\left(1385\right)\mathrm{}$ are consistent with assignment to a decuplet in the eightfold-way theory. There is evidence of the initiation of peripheral production of ${Y_1}^{*}\mathrm{}\left(1385\right)\mathrm{}$ above 1.5 BeV/c, but this process is not yet dominant at 1.7 BeV/c.