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

The helicity dependence of the {gamma}p{yields}n{pi}{sup +} 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{sub 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{sub 13}(1520) and S{sub 11}(1535) resonances with improved accuracy.