Photoproduction of excited baryons in the 1 /Nc expansion of QCD

Abstract

Baryon photo-couplings have been the subject of many studies over the last forty years, and are key elements in the understanding of baryon structure and dynamics. In this talk I will present the results of the photoproduction helicity amplitudes for excited baryons in the context of the 1=Nc expansion of QCD [1]. Such an expansion has been shown to provide a useful and systematic framework for the analysis of various baryon properties. This is mostly due to the existence of a contracted spin-flavor symmetry in the large Nc limit[2, 3]. Although SU(2N f) is not an exact symmetry in the excited baryon sector[4], its N 0 c breaking is small as several analyses of the baryon spectrum have shown[5‐7]. In this context the photoproduction helicity amplitudes for the negative parity excited baryons have been first analyzed in [8]. Here we report on an extension of such analysis which includes a systematic building of a complete basis of current operators to sub-leading order in the 1=Nc expansion, for both negative and positive parity resonances[9, 10].