Near-threshold $J/ψ$ photoproduction off nuclei

Abstract

We study the $J/ψ$ photoproduction from nuclei near the kinematic threshold within the first collision model, based on the nuclear spectral function, for incoherent primary photon--nucleon charmonium creation processes. The model takes into account the final $J/ψ$ absorption, target nucleon binding and Fermi motion, the formation length of $J/ψ$ mesons as well as the effect of their nuclear mean-field potential on these processes. We calculate the A dependences of the absolute and relative (transparency ratio) charmonium yields as well as its absolute and relative excitation functions within the different scenarios for the $J/ψN$ absorption cross section, for the $J/ψ$ formation length and for $J/ψ$ in-medium modification. We demonstrate that the studied observables, on the one hand, are not practically affected by the charmonium formation length and mass shift effects and, on the other hand, they are appreciably sensitive to the genuine $J/ψN$ absorption cross section at above threshold beam energies, which means that these observables can be useful to help determine the $J/ψN$ absorption cross section from the comparison of the results of our calculations with the future data from the experiments in the Hall C at the upgraded up to 12 GeV CEBAF facility. We also show that the absolute and relative excitation functions for $J/ψ$ subthreshold production in $γA$ reactions reveal some sensitivity to adopted in-medium modification scenarios for $J/ψ$ mesons. Therefore, such observables, measured in the subthreshold energy domain, may be an important tool to get valuable information on the charmonium in-medium properties in cold nuclear matter.