Extraction of the D13(1520) photon-decay couplings from pion- and eta-photoproduction data

We compare results for the D13(1520) photon-decay amplitudes determined in analyses of etaand pion-photoproduction data. The ratio of helicity amplitudes (A3/2/A1/2), determined from eta-photoproduction data, is quite different from that determined in previous analyses of pion-photoproduction data. We consider how strongly the existing pion-photoproduction data constrain both this ratio and the individual photon-decay amplitudes. Recent precise measurements of eta-photoproduction observables have spawned a number of analyses, focused mainly on the properties of the S11(1535) resonance[1]. These studies have found values for the photo-decay amplitude, A 1/2, which are significantly larger than those found in previous analyses of pion-photoproduction data[2]. As the S11(1535) resonance is masked by a strong ηN threshold cusp in pion photoproduction, etaphotoproduction holds the promise of a less model-dependent analysis. Attempts to fit pionand eta-production data in coupled-channel approachs[3, 4] have generally found values between those extracted from single-channel fits. Two studies[5, 6] have gone beyond the S11(1535) and have considered the sensitivity of eta-photoproduction data to the nearby D13(1520) resonance. In both of these analyses, values for the ratio of photo-decay amplitudes, A3/2/A1/2, were found to be consistently far smaller than those inferred from pion-photoproduction analyses[7]. This discrepancy is certainly unexpected[8], as the D13 state appears to have a clean Breit-Wignerlike signal in the associated multipoles E 1/2 2− and M 1/2 2− extracted from pionphotoproduction data. This ratio, as determined from eta-photoproduction data, has the value −2.5± 0.2± 0.4[5] or −2.1± 0.2[6], as compared to the PDG estimate[9] of −6.9 ± 2.1. Eta-photoproduction has the advantage of