Energy independent partial wave analysis of single pion photoproduction in the first resonance region

Abstract

We present results of an energy independent multipole analysis of π +, π 0 and π − photoproduction on nucleons for photon energies between 240 MeV and 500 MeV. New data which are included in the present analysis are the asymmetry by polarized photons for γn → π −p and the π −/ π + ratio recently measured at Tokyo. Each isospin part of the l ⩽ 1 amplitudes has been parametrized; the 2 ⩽ l ⩽ 3 amplitudes have been fixed to the values given by Berends, Donnachie and Weaver, and the l ≧ 4 amplitudes have been approximated by the Born terms. Imaginary parts of l ≦ 3 amplitudes have been calculated from the real parts by Watson's theorem. Without any isotensor current or anomalous energy dependence of non-resonant multipoles, we obtained a good fit to the π +, π 0 and π − photoproduction. The results are compared with dispersion theoretical calculations and phenomenological multipole analyses. M 1− (1 2) is small and consistent with the dispersion theory of Schwela but contradicts the previous results of Pfeil and Schwela. The strong energy dependence of E 0+ (0) given by Noelle and Pfeil is not justified. Discrepancies between our analysis and dispersion theories occur in the magnitude of M 1− (3 2) and in the energy dependences of E 0+ (0), E 0+ (1 2) and E 1+ (1 2) . Above 400 MeV two types of solution have been obtained. It is pointed out that the measurement of polarized target asymmetry for γp → π 0p will distinguish the two solutions.