Measurement of the helicity dependence for the γp→nπ+ channel in the second resonance region

The helicity dependence of the gamma p -> n pi(+) reaction has been measured for the first time in the photon energy range from 450 to 790 MeV. The experiment, performed at the Mainz Microtron (MAMI), used a 4 pi-detector system, a circularly polarized, tagged photon beam, and a longitudinally polarized frozen-spin target. Although these polarized data are mainly sensitive to mechanisms involving the D-13(1520) resonance, a cusp structure at the eta production threshold can be clearly observed. These data are significantly different from the predictions of the existing multipole analyses and are used to determine the helicity amplitudes of the D-13(1520) and S-11(1535) resonances with improved accuracy.