First Measurement of Near- and Sub-Threshold J/ψ Photoproduction off Nuclei at Jefferson Lab

We report on the first measurement of J/ψ photoproduction from nuclei in the photon threshold energy region of E_γ < 10.8 GeV as well as below the threshold energy E_γ ∼ 8.2 GeV. These data were measured using a tagged photon beam incident on nuclear targets deuterium, helium, and carbon, searching for the semi-inclusive reaction A(γ,e⁺e⁻p) with dilepton mass M(e⁺e⁻) ∼ m_J/ψ = 3.1 GeV. We extract the cross-sections for incoherent J/ψ photoproduction across nuclei and place limits on substantial deviations from plane-wave predictions. In helium and carbon we observe “sub-threshold” production of J/ψ from photons with energies below the proton energy threshold 8.2 GeV, and we comment on the implications on the partonic structure of nuclei.