Observation of $\psi \left(3686\right)\to n\overline{n}$ and improved measurement of $\psi \left(3686\right)\to p\overline{p}$

We observe the decay $\psi \left(3686\right)\to n\overline{n}$ for the first time and measure $\psi \left(3686\right)\to p\overline{p}$ with improved accuracy by using $1.07×{10}^{8}$ $\psi \left(3686\right)$ events collected with the BESIII detector. The measured branching fractions are $B\left(\psi \left(3686\right)\to n\overline{n}\right)=\left(3.06±0.06±0.14\right)×{10}^{-4}$ and $B\left(\psi \left(3686\right)\to p\overline{p}\right)=\phantom{\rule{0ex}{0ex}}\left(3.05±0.02±0.12\right)×{10}^{-4}$. 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 {\mathrm{cos}}^{2}\theta $, we determine the $\alpha $ parameters to be ${\alpha }_{n\overline{n}}=0.68±0.12±0.11$ and ${\alpha }_{p\overline{p}}=1.03±0.06±0.03$ for $\psi \left(3686\right)\to n\overline{n}$ and $\psi \left(3686\right)\to p\overline{p}$, respectively.