Observation of ψ (3686) →n n ̄ and improved measurement of ψ (3686) →p p ̄

We observe the decay ψ(3686)→nn̄ for the first time and measure ψ(3686)→pp̄ with improved accuracy by using 1.07×108 ψ(3686) events collected with the BESIII detector. The measured branching fractions are B(ψ(3686)→nn̄)=(3.06±0.06±0.14)×10-4 and B(ψ(3686)→pp̄)=(3.05±0.02±0.12)×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+αcos2θ, we determine the α parameters to be αnn̄=0.68±0.12±0.11 and αpp̄=1.03±0.06±0.03 for ψ(3686)→nn̄ and ψ(3686)→pp̄, respectively.