### Speaker

Mr
Oleksii Gryniuk
(JGU Mainz)

### Description

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 $\gamma p \to c\bar{c}$ cross sections 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 $B_\psi = 2.7 \pm 0.3$ MeV.
We estimate the forward-backward asymmetry to the $\gamma p \to e^- e^+$ 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.
To a good approximation this asymmetry depends linearly on $a_{\psi p}$, and can reach values around -25 % for forthcoming J/$\psi$ threshold production experiments at JLab. Its measurement can thus provide a very sensitive observable for a refined extraction of $a_{\psi p}$.
We also made a preliminary analysis of the case of $\phi$-p forward scattering. A lack of inclusive cross section data, as well as the absence of a consistent dataset for the exclusive total and differential cross section of the $\phi$ photoproduction off the proton, does not allow us to draw precise constraints on the fit parameters at this point.
By fixing the value of Regge slope, for both elastic and inelastic total $\phi$-p scattering cross sections, $\alpha_{el} = \alpha_{inel} = 1.092$ taken from fits of total photoproduction off the proton, we estimate the value of the subtraction constant of the forward elastic amplitude $T_{\phi p}(0) \approx 46.3$, which is approximately two times higher than what we obtained for the J/$\psi$ case. The estimated forward-backward asymmetry was shown to reach values of about -20% for the beam energies $E = 2.2, 4.4$ GeV.
The suitable kinematics were investigated for the specific setup of HMS and SHMS detectors of Hall C at JLab. Studies for the other JLab experiments (e.g. GlueX, SOLID) are ongoing.

### Primary author

Mr
Oleksii Gryniuk
(JGU Mainz)

### Co-author

Prof.
Marc Vanderhaeghen
(Univ. Mainz)