EXCHANGE CURRENTS IN PHOTOPRODUCTION OF BARYON RESONANCES

We calculate photoexcitation amplitudes for all nucleon and Delta reso nances up to root s = 1.6 GeV. We use a chiral quark model including t wo-body exchange currents. The two-body currents give important contri butions. For the Delta(1232) and the D-13(1520) we observe that the in dividual exchange current contributions considerably cancel each other while for the P-11(1440), the P-33(1600), and the S-11(1535) we get a reinforcement of the two-body amplitudes. In comparison with present experimental data, we obtain both for the S-11(1535) and for the P-11( 1440) amplitudes an improvement with respect to the impulse approximat ion. (C) 1997 Elsevier Science B.V.