Study of J/ψ→pp̄ and J/ψ→nn̄

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 α=0.595±0.012±0.015 for J/ψ→pp and α=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.