Study of J/ψ→pp̄ and J/ψ→nn̄

Abstract

The decays J/ψ→pp̄ and J/ψ→nn̄ 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→pp̄)=(2.112±0. 004±0.031)×10 -3 and B(J/ψ→nn̄)=(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+α cos2θ, and we find α=0.595±0.012±0.015 for J/ψ→pp̄ and α=0.50±0.04±0.21 for J/ψ→nn̄. Our branching-fraction results suggest a large phase angle between the strong and electromagnetic amplitudes describing the J/ψ→NN̄ decay. © 2012 American Physical Society.