  Using $(448.1\pm2.9)\times10^6$ $ \psi(3686)$ events collected with the
BESIII detector at the BEPCII collider, the decay
$\psi(3686)\to\Sigma^-\bar\Sigma^+$ is observed for the first time with a
branching fraction of $(2.82\pm0.04_{\rm stat.}\pm0.08_{\rm
syst.})\times10^{-4}$, and the angular parameter $\alpha_{ \Sigma^-}$ is
measured to be $0.96\pm0.09_{\rm stat.} \pm 0.03_{\rm syst.}$.
