Nucleon resonances with hidden charm in γ p reactions

The excitations of nucleon resonances with hidden charm, N_c^{c ¯ *}, in γ p reactions are investigated by using the predictions from the available meson-baryon (M B ) coupled-channel models of N_c^{c ¯ *}, with M B =ρ N ,ω N ,J /ψ N ,D ¯Λ_c,D^¯*Λ_c,D ¯Σ_c,D^¯*Σ_c,D ¯Σ_c^* . For the γ p →J /ψ p process, we first apply the model of Donnachie and Landshoff to calculate the Pomeron-exchange amplitudes with the parameters determined from fitting the available total cross section data up to invariant mass W =300 GeV. We then add the resonant γ p →N_c^{c ¯ *}→J /ψ p amplitudes to examine the effects of N_c^{c ¯ *} excitations on the cross sections of γ p →J /ψ p in the near threshold energy region covered by the recent experiments at Jefferson Lab. The N_c^{c ¯ *}→M B transition matrix elements are determined from the partial decay widths predicted by the considered meson-baryon coupled-channel models of N_c^{c ¯ *}. The γ p →N_c^{c ¯ *} transition amplitudes are calculated from the vector meson dominance (VMD) model as γ p →V p →N_c^{c ¯ *} with V =ρ ,ω ,J /ψ . The total γ p →J /ψ p amplitudes then depend on an off-shell form factor, parametrized as F_V(q²) =Λ_V⁴/[Λ_V⁴+(q^{2-m_V2) 2}] , which is needed to account for the q² dependence of the photon-vector meson coupling constant e/m_V² f_V of the VMD model. It was found that, with Λ_V=0.55 GeV, the predicted total cross sections are within the range of the data in the energy region near the J /ψ production threshold. We then demonstrate that the N_c^{c ¯ *} can be most easily identified in the differential cross sections at large angles where the contribution from Pomeron exchange becomes negligible. With the same VMD model and the same coupled-channel models of N_c^{c ¯ *}, we also calculate the resonant amplitudes for the γ p →V p →N_c^{c ¯ *}→D^¯0Λ_c⁺(D^¯*0Λ_c⁺) processes. By adding the nonresonant amplitudes due to the exchange of D^¯*0 (D^¯0), we then predict the cross sections of γ p →D^¯0Λ_c⁺(D^¯*0Λ_c⁺) for additional experimental tests of the available meson-baryon coupled-channel models of N_c^{c ¯ *}.