| 摘要: |
The excitations of nucleon resonances with hidden charm, N-c (c) over bar*, in gamma p reactions are investigated by using the predictions from the available meson-baryon (MB) coupled-channel models of N-c (c) over bar*, with MB = rho N, omega N, J/psi N, (D) over bar Lambda(c), (D) over bar*Lambda(c), (D) over bar Sigma(c), (D) over bar*Sigma(c), (D) over bar Sigma(c)* . For the gamma p -> J/psi 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 gamma p -> N-c (c) over bar* -> J/psi p amplitudes to examine the effects of N-c (c) over bar* excitations on the cross sections of gamma p -> J/psi p in the near threshold energy region covered by the recent experiments at Jefferson Lab. The N-c (c) over bar* -> MB transition matrix elements are determined from the partial decay widths predicted by the considered meson-baryon coupled-channel models of N-c (c) over bar*. The gamma p -> N-c (c) over bar* transition amplitudes are calculated from the vector meson dominance (VMD) model as gamma p -> Vp -> N-c (c) over bar* with V = rho, omega, J/psi. The total gamma p -> J/psi p amplitudes then depend on an off-shell form factor, parametrized as F-V (q(2)) = Lambda(4)(V)/[Lambda(4)(V) + (q(2) - m(V)(2))(2)], which is needed to account for the q(2) dependence of the photon-vector meson coupling constant e m(V)(2)/f(V) of the VMD model. It was found that, with Lambda(V) = 0.55 GeV, the predicted total cross sections are within the range of the data in the energy region near the J/psi production threshold. We then demonstrate that the N-c (c) over bar* 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) over bar*, we also calculate the resonant amplitudes for the gamma p -> Vp -> N-c (c) over bar* -> (D) over bar (0)Lambda(+)(c)((D) over bar*(0)Lambda(+)(c)) processes. By adding the nonresonant amplitudes due to the exchange of (D) over bar*(0) ((D) over bar (0)), we then predict the cross sections of gamma p -> (D) over bar (0)Lambda(+)(c)((D) over bar*(0)Lambda(+)(c)) for additional experimental tests of the available meson-baryon coupled-channel models of N-c (c) over bar*. |
