Observation of psi (3686) -> n(n)over-bar and improved measurement of psi (3686) -> p(p)over-bar

We observe the decay psi (3686) -> n (n) overbar for the first time and measure psi (3686) -> p (p) overbar with improved accuracy by using 1.07 x 10(8) psi (3686) events collected with the BESIII detector. The measured branching fractionsare B(psi(3686) -> n (n) overbar) = (3.06 +/- 0.06 +/- 0.14) x 10(-4) and B(psi(3686) -> p (p) overbar) = (3.05 +/- 0.02 +/- 0.12) x 10(-4). Here, the first uncertainties are statistical, and the second ones are systematic. With the hypothesis that the polar angular distributions of the neutron and proton in the center-of-mass system obey 1 + alpha cos(2) theta, we determine the a parameters to be alpha(n (n) overbar) = 0.68 +/- 0.12 +/- 0.11 and alpha(p (p) overbar) = 1.03 +/- 0.06 +/- 0.03 for psi(3686) -> n (n) overbar and psi(3686) -> p (p) overbar, respectively.