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

We observe the decay $\psi (3686)\to n\bar{n}$ for the first time and measure $\psi (3686)\to p\bar{p}$ with improved accuracy by using $1.07\times 10^8$ $\psi (3686)$ events collected with the BESIII detector. The measured branching fractions are $$\mathcal{B}$(\psi(3686) \to n \bar{n}) = (3.06 \pm 0.06 \pm 0.14)\times 10^{-4}$and$$\mathcal{B}$(\psi(3686) \to p \bar{p}) = (3.05 \pm 0.02 \pm 0.12) \times 10^{-4}$. Here, the first uncertainties are statistical and the second ones 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 $\alpha$ parameters to be ${\alpha }_{n\bar{n}}=0.68\pm 0.12\pm 0.11$ and $\alpha_{p\bar{p}} = 1.03 \pm 0.06 \pm 0.03$for$\psi(3686)\to n\bar{n}$and$\psi(3686)\to p\bar{p}$, respectively.