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We provide an updated analysis of the forward $J/\psi$-p scatteringamplitude, relating its imaginary part to $\gamma p \to J/\psi p$ and $\gamma p\to c \bar c X$ cross section data, and calculating its real part through aonce-subtracted dispersion relation. From a global fit to both differential andtotal cross section data, we extract a value for the spin-averaged $J/\psi$-ps-wave scattering length $a_{\psi p} = 0.046 \pm 0.005$ fm, which can betranslated into a $J/\psi$ binding energy in nuclear matter of $B_\psi = 2.7\pm 0.3$ MeV. We estimate the forward-backward asymmetry to the $\gamma p \toe^- e^+ p$ process around the $J/\psi$ resonance, which results frominterchanging the leptons in the interference between the $J/\psi$ productionand the Bethe-Heitler mechanisms. We show that to good approximation thisasymmetry depends linearly on $a_{\psi p}$, and can reach values around -25%for forthcoming $J/\psi$ threshold production experiments at Jefferson Lab. Itsmeasurement can thus provide a very sensitive observable for a refinedextraction of $a_{\psi p}$.

Accessing the real part of the forward $J/\psi$-p scattering amplitude from $J/\psi$ photo-production on protons around threshold