The decays J/ψ→ppˉ and J/ψ→nnˉ have been
investigated with a sample of 225.2 million J/ψ events collected with the
BESIII detector at the BEPCII e+e− collider. The branching fractions are
determined to be B(J/ψ→ppˉ)=(2.112±0.004±0.031)×10−3 and B(J/ψ→nnˉ)=(2.07±0.01±0.17)×10−3. Distributions of the angle
θ between the proton or anti-neutron and the beam direction are well
described by the form 1+αcos2θ, and we find
α=0.595±0.012±0.015 for J/ψ→ppˉ and
α=0.50±0.04±0.21 for J/ψ→nnˉ. Our branching-fraction
results suggest a large phase angle between the strong and electromagnetic
amplitudes describing the J/ψ→NNˉ decay.Comment: 16 pages, 13 figures, the 2nd version, submitted to PR