The decays $J/\psi \to p\overline{p}$ and $J/\psi \to n\overline{n}$ have been investigated with a sample of $225.2\times {10}^6$ $J/\psi$ events collected with the BESIII detector at the BEPCII $e^{+}{e}^{-}$ collider. The branching fractions are determined to be $\mathcal{B}(J/\psi \to p\overline{p})=(2.112\pm 0.004\pm 0.031)\times {10}^{-3}$ and $\mathcal{B}(J/\psi \to n\overline{n})=(2.07\pm 0.01\pm 0.17)\times {10}^{-3}$. Distributions of the angle $\theta$ between the proton or antineutron 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 $J/\psi \to p\overline{p}$ and $\alpha =0.50\pm 0.04\pm 0.21$ for $J/\psi \to n\overline{n}$. Our branching-fraction results suggest a large phase angle between the strong and electromagnetic amplitudes describing the $J/\psi \to N\overline{N}$ decay.

• Received 4 May 2012

DOI:https://doi.org/10.1103/PhysRevD.86.032014