Measurement of the asymmetry parameter for the decay $\bar\Lambda \to \bar p\pi^+$

Based on a sample of $58\times10^6J/\psi$ decays collected with the BESII detector at the BEPC, the $\bar\Lambda$ decay parameter $\alpha_{\bar\Lambda}$ for $\bar\Lambda\to \bar p \pi^+$ is measured using about 9000 $J/\psi\to\Lambda\bar\Lambda\to p \bar p \pi^+\pi^-$ decays. A fit to the joint angular distributions yields $\alpha_{\bar\Lambda}(\bar\Lambda\to \bar p\pi^+)=-0.755\pm0.083\pm0.063$, where the first error is statistical, and the second systematic.