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The reaction gammap-->pi(+)pi(0)n has been measured at MAMI for photon energies up to 820 MeV. Invariant mass spectra of the particles in the final state (pi(+)n), (pi(0)n), (pi(+)pi(0)) have been determined for several bins of incident photon energy. Differences in pi(+)pi(0) and simultaneously measured pi(0)pi(0) invariant mass distributions are assigned to a rho branch of the D13(1520) nucleon resonance.

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The photon asymmetry in the reaction 1H(--> gamma,pi(0))(1)H close to threshold has been measured for the first time with the photon spectrometer TAPS using linearly polarized photons from the tagged-photon facility at the Mainz Microtron MAMI. The total and differential cross sections were also measured simultaneously with the photon asymmetry. This allowed determination of the S-wave and all three P-wave amplitudes. The values obtained at threshold are E(0+) = [-1.33+/-0.08(stat)+/-0.03(syst)] x 10(-3)/m(pi(+)), P(1) = [9.47 +/- 0.08(stat) +/- 0.29(syst)] x 10(-3)q/m(2)(pi(+)), P(2) = [-9.46 +/- 0.1(stat) +/- 0.29(syst)] x 10(-3)q/m(2)(pi(+)), and P(3) = [11.48 +/- 0.06(stat) +/- 0.35(syst)] x 10(-3)q/m(2)(pi(+)).

In the results published in our Letter, the contributions from carbon and oxygen in the Kapton target windows were taken into account for the cross sections but they were neglected in the determination of the polarized beam asymmetries AE. The nuclei in the target windows have J ¼ 0 þ and therefore an asymmetry of AE ¼ 1 for the coherent 0 production that dominates in the threshold region. The contribution of the target-window contamination in each channel from threshold to 166 MeV is between 5% and 7% [1]. We therefore estimate the target-window contribution to the asymmetry to be 0:06 AE 0:01 in absolute units. Thus, the asymmetries should be corrected by subtracting %0:06 AE 0:01 from each datum. The published and corrected asymmetries are plotted in Fig. 1. The errors were increased by adding the 0.01 uncertainty in quadrature with the previously published values. The total and differential cross sections reported were corrected for the target-window contributions and thus remain valid. This correction to AE does not change the values of the S-wave amplitude E 0þ , the P-wave amplitude P 1 , or the P-wave combination P 23 . It will, however, modify the individual extraction of the P 2 and P 3 amplitudes. This is shown in Table I. / deg 0 π cms θ 0 20 40 60 80 100 120 140 160 180 Σ -0.3 -0.2 -0.1 -0.0 0.1 0.2 0.3 0.4 ChPT [2] DR [3] fit to the data published corrected FIG. 1. Published (filled squares) and corrected asymmetries (open squares) for E ¼ 159:5 MeV as a function of pion centerof-mass angle. The solid line is a fit to the published data, the dotted line is a prediction of ChPT [2], and the dashed line is a DR calculation [3].TABLE I. Results of the P-wave multipoles with statistical and systematic errors from a qk fit using AE pub and AE cor along with the original total and differential cross sections. Comparisons are made with predictions of ChPT [2] and a dispersion theoretical approach [3]. a AE pub AE cor ChPT DR P 1 9:44 AE 0:06 AE 0:28 9:44 AE 0:06 AE 0:28 9:14 AE 0:5 9.55 P 2 À9:48 AE 0:12 AE 0:28 À9:90 AE 0:12 AE 0:28 À9:7 AE 0:5 À10:37 P 3 11:32 AE 0:12 AE 0:34 10:96 AE 0:12 AE 0:34 10.36 9.27 P 23 10:45 AE 0:07 10:44 AE 0:07 11.07 9.84a P-wave combinations converted into the unit q  10 À3 =m 2 þ .