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

Abstract

We provide an updated analysis of the forward $J/\psi$-p scattering amplitude, 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 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 $a_{\psi p}=0.046\pm 0.005$ fm, which can be translated 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 $\gamma p\to e^{-}{e}^{+}p$ 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 $a_{\psi p}$.

References in corpus (5)

• Charmonium
• High-precision determination of the electric and magnetic form factors of the proton
• First lattice study of low-energy charmonium-hadron interaction
• Charmonium-nucleon potential from lattice QCD
• $J/\Psi$-nuclear bound states

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