Analysis ofK−p→Λπ+π−in the Region of theΛ(1520)

Abstract

The rate for the decay $\Lambda \mathrm{}\left(1520\right)-\Sigma \mathrm{}\left(1385\right)\mathrm{}\pi$ has been measured in a study of the reaction sequence $K^{-}p\to \Lambda \mathrm{}\left(1520\right)\mathrm{}\to \Sigma \mathrm{}\left(1385\right)\mathrm{}\pi \to \Lambda {\pi }^{+}{\pi }^{-}$. A sample of 9200 events of the type $K^{-}p\to \Lambda {\pi }^{+}{\pi }^{-}$ has been obtained in the Berkeley 25-in. hydrogen bubble chamber. The incident momenta range from 300 to 470 MeV/c. An energy-independent partial-wave analysis was performed using an isobar model. The model included a coherent mixture of six partial waves to describe all mass distributions, angular distributions, and the polarization of the $\Lambda$. The $\Sigma \mathrm{}\left(1385\right)\mathrm{}\pi$ decay mode was found to dominate the $\Lambda {\pi }^{+}{\pi }^{-}$ decay of the $\Lambda \mathrm{}\left(1520\right)\mathrm{}$, in disagreement with the results of a production reaction study of the same decay. The width for the decay was measured to be 1.40 ± 0.26 MeV. Mixing between the $\Lambda \mathrm{}\left(1520\right)\mathrm{}$ and the $\Lambda \mathrm{}\left(1690\right)\mathrm{}$ has been used in the past to describe the $J^P={\frac{3}{2}}^{-}$ baryons as SU(3) singlet and octet. Combining our rate for $\Lambda \mathrm{}\left(1520\right)\mathrm{}$ with a recently measured rate for $\Lambda \mathrm{}\left(1690\right)\mathrm{}\to \Sigma \mathrm{}\left(1385\right)\mathrm{}\pi$, we calculate the mixing angle $\left|\theta \right|=63\mathrm{}° \mathrm{to} 83°$. This is in strong disagreement with the mixing angle derived from the two-body $D$-wave decays of the singlet and octet of -25 ± 6°. Thus, some modification of the usual SU(3) description of these states needs to be made.