6533b836fe1ef96bd12a1c33

RESEARCH PRODUCT

Accessing the real part of the forwardJ/ψ−pscattering amplitude fromJ/ψphotoproduction on protons around threshold

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We provide an updated analysis of the forward $J/\ensuremath{\psi}$-p scattering amplitude, relating its imaginary part to $\ensuremath{\gamma}p\ensuremath{\rightarrow}J/\ensuremath{\psi}p$ and $\ensuremath{\gamma}p\ensuremath{\rightarrow}c\overline{c}X$ 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 $J/\ensuremath{\psi}$-p s-wave scattering length ${a}_{\ensuremath{\psi}p}=0.046\ifmmode\pm\else\textpm\fi{}0.005\text{ }\text{ }\mathrm{fm}$, which can be translated into a $J/\ensuremath{\psi}$ binding energy in nuclear matter of ${B}_{\ensuremath{\psi}}=2.7\ifmmode\pm\else\textpm\fi{}0.3\text{ }\text{ }\mathrm{MeV}$. We estimate the forward-backward asymmetry to the $\ensuremath{\gamma}p\ensuremath{\rightarrow}{e}^{\ensuremath{-}}{e}^{+}p$ process around the $J/\ensuremath{\psi}$ resonance, which results from interchanging the leptons in the interference between the $J/\ensuremath{\psi}$ production and the Bethe-Heitler mechanisms. We show that to good approximation this asymmetry depends linearly on ${a}_{\ensuremath{\psi}p}$, and can reach values around $\ensuremath{-}25%$ for forthcoming $J/\ensuremath{\psi}$ threshold production experiments at Jefferson Lab. Its measurement can thus provide a very sensitive observable for a refined extraction of ${a}_{\ensuremath{\psi}p}$.