The role of hidden-charm pentaquark resonance $P^+_c(4450)$ in $J/ψ$ photoproduction on nuclei near threshold

Abstract: We study the $J/\psi$ photoproduction from nuclei at near-threshold incident photon energies of 5--11 GeV within the nuclear spectral function approach by considering incoherent direct (${\gamma}N \to {J/\psi}N$) and two-step (${\gamma}p \to P^+_c(4450)$, $P^+_c(4450) \to {J/\psi}p$) photon--nucleon charmonium creation processes. We calculate the absolute and relative excitation functions within the different scenarios for in-medium modification of the directly photoproduced $J/\psi$ mesons. We show that the overall subthreshold $J/\psi$ production in ${\gamma}A$ reactions reveals some sensitivity to adopted in-medium modification scenarios for $J/\psi$ mesons only if branching ratio $Br[P^+_c(4450) \to {J/\psi}p]$ $\sim$ 1% and less. Our studies also demonstrate that the presence of the $P^+_c(4450)$ resonance in $J/\psi$ photoproduction produces above threshold additional enhancements in the behavior of the total $J/\psi$ creation cross section on nuclei, which could be also studied in the future JLab experiments at the upgraded up to 12 GeV CEBAF facility to provide further evidence for its existence.