Pion Electroproduction Amplitude Relations in the 1/N_c Expansion

Abstract

We derive expressions for pion electroproduction amplitudes in the 1/N_c expansion of QCD, and obtain from them linear relations between the electromagnetic multipole amplitudes that hold at all energies. The leading-order relations in 1/N_c compare favorably with available data (especially away from resonances), but the next-to-leading-order relations tend to provide only small or no improvement.

45 pages, pdflatex, contains multiple figures but .pdf file is < 4 MB. This version to appear in Phys. Rev. D, but has much better figure resolution. References greatly expanded, some additional discussion