A Model for the gamma p --> pi+ pi- p Reaction

[EN] We have studied the gammap--> pi+ pi- p reaction using a model which includes N, DELTA(1232), N*(1440) and N*(1520) intermediate baryonic states and the rho-meson as intermediate 2pi-resonance. The model reproduces fairly well experimental cross sections below E(gamma) = 800 MeV and invariant-mass distributions even at higher energies. One of the interesting findings of the study is that the gammaN --> N* (1520) --> DELTApi process is very important and interferes strongly with the dominant DELTA Kroll-Ruderman term to produce the experimental peak of the cross section. We show that the study of the reaction can provide information on some couplings, concretely the N*(1520) --> DELTApi. We also find that the pion energy distributions for different photon energies, which have not been measured so far, contain very valuable information on the dynamics of the reaction. Finally, the analogies and differences with respect to the piN --> 2piN reaction are also discussed.We are indebted to S.M. Singh who provided us with relevant information on the N*Ny coupling. Discussions with J.Bernabeu, F. Botella, P. González, S. Noguera, J. Peñarrocha and M.J. Vicente-Vacas are warmly acknowledged. This work has been partially supported by CICYT contract number AEN 90-049. One of us, J.A. Gómez Tejedor, wishes to acknowledge financial support from the Institució Valenciana d Estudis i InvestigacióGómez-Tejedor, J.; Oset, E. (1994). A Model for the gamma p --> pi+ pi- p Reaction. Nuclear Physics A. 571(4):667-693. https://doi.org/10.1016/0375-9474(94)90715-3S667693571