Charmonium rescattering effects in psi(2S) -> gamma eta(c) decay

Charmonium rescattering effects in the M1 transition of psi(2S) -> gamma eta(c) are investigated by modeling a chi(cJ) or J/psi rescattering into a eta(c) final state. The absorptive and dispersive part of the transition amplitudes for the rescattering loops of eta psi(gamma*) and gamma chi(psi) are separately evaluated. The numerical results show that the contribution from the gamma chi(psi) rescattering process is negligible. Compared with the virtual D (D) over bar (D*) rescattering processes, the eta psi(gamma*) process may be regarded as the next- leading order of the hadronic loop mechanism, which only offers the partial decay width of similar to 0.045 keV to the psi(2S) -> gamma eta(c).