Accessing the real part of the forward J/ψ - p scattering amplitude from J/ψ photoproduction on protons around threshold

We provide an updated analysis of the forward J/psi-p scattering amplitude, relating its imaginary part to gamma p -> J/psi p and gamma p -> ccX cross section data, and calculating its real part through a once-subtracted dispersion relation. From a global fit to both differential and total cross section data, we extract a value for the spin-averaged J/psi-p s-wave scattering length a(psi p) = 0.046 +/- 0.005 fm, which can be translated into a J/psi binding energy in nuclear matter of B-psi = 2.7 +/- 0.3 MeV. We estimate the forward-backward asymmetry to the gamma p -> e(-)e(+)p process around the J/psi resonance, which results from interchanging the leptons in the interference between the J/psi production and the Bethe-Heitler mechanisms. We show that to good approximation this asymmetry depends linearly on a(psi p), and can reach values around -25% for forthcoming J/psi threshold production experiments at Jefferson Lab. Its measurement can thus provide a very sensitive observable for a refined extraction of a(psi p).