Photopionp-wave multipoles near threshold fromC12(γ,π0) andH1(γ,π0)

Experimental cross sections for the reaction ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$(γ,${\mathrm{\pi }}^0$) at low energy have been used to deduce the isovector-multipole combination [2${\mathit{M}}_{1+}$(v)+${\mathit{M}}_{1\mathrm{-}}$(v)]/qk for the nucleon near ${\mathrm{\pi }}^0$ threshold. The effects of pion-wave distortion (distorted-wave impulse approximation) and Fermi motion in ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ are addressed in detail. By incorporating additional constraints on certain combinations of the p-wave multipoles as deduced from the Mainz ${}_{}{}^1{}_{}{}^{}\mathrm{H}$(γ,${\mathrm{\pi }}^0$) data, near-threshold values are obtained for the reduced proton p-wave (γ,${\mathrm{\pi }}^0$) multipoles ${\mathit{M}}_{1+}$/qk,${\mathit{M}}_{1\mathrm{-}}$/qk, and ${\mathit{E}}_{1+}$/qk which are in excellent agreement with the VPI multipoles as given by the s a i d code but disagree with the findings of other authors. Combined with a conservative theoretical estimate of the isoscalar amplitude ${\mathit{M}}_{1\mathrm{-}}$(s), these results permit a decomposition of ${\mathit{M}}_{1\mathrm{-}}$(p) into isoscalar and isovector components in the near-threshold region.