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

Abstract

The decays J/ψ→pp¯ and J/ψ→nn¯ have been investigated with a sample of 225.2×106 J/ψ events collected with the BESIII detector at the BEPCII e+e− collider. The branching fractions are determined to be B(J/ψ→pp¯)=(2.112±0.004±0.031)×10−3 and B(J/ψ→nn¯)=(2.07±0.01±0.17)×10−3. Distributions of the angle θ between the proton or antineutron and the beam direction are well described by the form 1+αcos2θ, and we find θ=0.595±0.012±0.015 for J/ψ→pp¯ and θ=0.50±0.04±0.21 for J/ψ→nn¯. Our branching-fraction results suggest a large phase angle between the strong and electromagnetic amplitudes describing the J/ψ→NN¯ decay.