Measuring the phase of the J/ψ strong decay amplitudes

It is shown that the interference between pp̄ → J/ψ → e+e− and pp̄ → e+e− can be measured w good accuracy at Fermilab. Therefore the phase of the strong decay amplitude of the J/ψ with respect to the proton magnetic form factor will be obtained. There are hints, from J/ψ → nn̄ and other J/ψ decays, of an unexpected ≈ 90° phase.