Production and Decay of Cascade Hyperons

Abstract

The production of cascade hyperons by ${K}^{\ensuremath{-}}$ incident on hydrogen has been studied at beam momenta of 1.7, 2.1, and 2.4-2.7 GeV/c. A sample of $3028{\ensuremath{\Xi}}^{\ensuremath{-}}$ and $934{\ensuremath{\Xi}}^{0}$ was obtained. Cross sections and polarization for ${\ensuremath{\Xi}}^{\ensuremath{-}}{K}^{+}$ and ${\ensuremath{\Xi}}^{0}{K}^{0}$ production are presented. The data are compatible with dominance by $I=0$ baryon exchange in ${\ensuremath{\Xi}}^{\ensuremath{-}}{K}^{+}$ production, but also provide strong evidence for resonance formation in the $s$ channel compatible with ${{Y}_{0}}^{*}(2100)$. Copious production of ${\ensuremath{\Xi}}^{*}(1530)$ and ${K}^{*}(890)$ is observed in the three-and four-body final states. A broad $\ensuremath{\Xi}\ensuremath{\pi}$ enhancement is observed in the ${\ensuremath{\Xi}}^{\ensuremath{-}}{K}^{+}{\ensuremath{\pi}}^{0}$ and ${\ensuremath{\Xi}}^{0}{K}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}$ final states at a mass near 1894 MeV/${\mathit{c}}^{2}$ and with a width of about 98 MeV/${\mathit{c}}^{2}$. This enhancement is identified with the ${\ensuremath{\Xi}}^{*}(1930)$ first observed by Badier et al. Lifetime measurements give ${\ensuremath{\tau}}_{{\ensuremath{\Xi}}^{\ensuremath{-}}}=(1.61\ifmmode\pm\else\textpm\fi{}0.04)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$ sec and ${\ensuremath{\tau}}_{{\ensuremath{\Xi}}^{0}}=({{3.07}_{\ensuremath{-}0.20}}^{+0.22})\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$ sec. A decay-parameter analysis assuming spin \textonehalf{} yields ${\ensuremath{\alpha}}_{{\ensuremath{\Xi}}^{\ensuremath{-}}}=\ensuremath{-}0.391\ifmmode\pm\else\textpm\fi{}0.045$, ${\ensuremath{\alpha}}_{{\ensuremath{\Xi}}^{0}}=\ensuremath{-}0.43\ifmmode\pm\else\textpm\fi{}0.09$, ${\ensuremath{\Phi}}_{{\ensuremath{\Xi}}^{\ensuremath{-}}}\ensuremath{\equiv}invtan{(\frac{\ensuremath{\beta}}{\ensuremath{\gamma}})}_{{\ensuremath{\Xi}}^{\ensuremath{-}}}=\ensuremath{-}(14\ifmmode\pm\else\textpm\fi{}11)\ifmmode^\circ\else\textdegree\fi{}$, and ${\ensuremath{\Phi}}_{{\ensuremath{\Xi}}^{0}}=(38\ifmmode\pm\else\textpm\fi{}19)\ifmmode^\circ\else\textdegree\fi{}$ if ${\ensuremath{\alpha}}_{\ensuremath{\Lambda}}=0.647$ is used. These results are in agreement with $T$ invariance and the $|\ensuremath{\Delta}I|=\frac{1}{2}$ rule. A compilation of LRL results for ${\ensuremath{\Xi}}^{\ensuremath{-}}$ and ${\ensuremath{\Xi}}^{0}$ yields ${\ensuremath{\alpha}}_{\ensuremath{\Xi}}=\ensuremath{-}0.380\ifmmode\pm\else\textpm\fi{}0.034$ and ${\ensuremath{\Phi}}_{\ensuremath{\Xi}}=\ensuremath{-}(1\ifmmode\pm\else\textpm\fi{}7)\ifmmode^\circ\else\textdegree\fi{}$, implying $\ensuremath{\Delta}=invtan{(\ensuremath{-}\frac{\ensuremath{\beta}}{\ensuremath{\alpha}})}_{\ensuremath{\Xi}}=(178\ifmmode\pm\else\textpm\fi{}16)\ifmmode^\circ\else\textdegree\fi{}$. Hence the final-state $\ensuremath{\Lambda}\ensuremath{\pi}$ phase difference ${\ensuremath{\delta}}_{s}\ensuremath{-}{\ensuremath{\delta}}_{p}=\ensuremath{-}(2\ifmmode\pm\else\textpm\fi{}16)\ifmmode^\circ\else\textdegree\fi{}$ if $T$ is strictly conserved in the decay. Two examples of ${\ensuremath{\Xi}}^{\ensuremath{-}}\ensuremath{\rightarrow}\ensuremath{\Lambda}{e}^{\ensuremath{-}}\overline{\ensuremath{\nu}}$ were observed. Upper limits \ensuremath{\approx} 1\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}3}$ have been set for the branching fractions of other $|\ensuremath{\Delta}S|=1$ and $|\ensuremath{\Delta}S|=2$ leptonic and nonleptonic decays of ${\ensuremath{\Xi}}^{\ensuremath{-}}$ and ${\ensuremath{\Xi}}^{0}$.