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

We observe the decay ψ (3686 )→n n ¯ for the first time and measure ψ (3686 )→p p ¯ with improved accuracy by using 1.07 ×10⁸ ψ (3686 ) events collected with the BESIII detector. The measured branching fractions are B (ψ (3686 )→n n ¯ )=(3.06 ±0.06 ±0.14 )×10^-4 and B (ψ (3686 )→p p ¯ )=(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 +α cos²θ , we determine the α parameters to be α_{n n ¯}=0.68 ±0.12 ±0.11 and α_{p p ¯}=1.03 ±0.06 ±0.03 for ψ (3686 )→n n ¯ and ψ (3686 )→p p ¯, respectively.