Study of $J\mathrm{/}\psi \to p\bar{p}$ and $J\mathrm{/}\psi \to n\bar{n}$

The decays $J\mathrm{/}\psi \to p\bar{p}$ and $J\mathrm{/}\psi \to n\bar{n}$ have been investigated with a sample of 225.2 million $J\mathrm{/}\psi$ events collected with the BESIII detector at the BEPCII $e^{+}{e}^{-}$ collider. The branching fractions are determined to be $$\mathcal{B}$(J/\psi\to p\bar{p})=(2.112\pm0.004\pm0.031)\times10^{-3}$and$$\mathcal{B}$(J/\psi\to n\bar{n})=(2.07\pm0.01\pm0.17)\times10^{-3}$\mathrm{.}Distributionsoftheangle$\theta$ between the proton or anti-neutron and the beam direction are well described by the form $1+\alpha {\cos }^2\theta$, and we find $\alpha =0.595\pm 0.012\pm 0.015$ for $J\mathrm{/}\psi \to p\bar{p}$ and $\alpha =0.50\pm 0.04\pm 0.21$ for $J\mathrm{/}\psi \to n\bar{n}$. Our branching-fraction results suggest a large phase angle between the strong and electromagnetic amplitudes describing the $J\mathrm{/}\psi \to N\bar{N}$ decay.