Photopion p-wave multipoles near threshold from 12C( gamma, pi 0) and 1H( gamma, pi 0).

Abstract

Experimental cross sections for the reaction $^{12}\mathrm{C}$(\ensuremath{\gamma},${\mathrm{\ensuremath{\pi}}}^{0}$) at low energy have been used to deduce the isovector-multipole combination [2${\mathit{M}}_{1+}$(v)+${\mathit{M}}_{1\mathrm{\ensuremath{-}}}$(v)]/qk for the nucleon near ${\mathrm{\ensuremath{\pi}}}^{0}$ 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 $^{1}\mathrm{H}$(\ensuremath{\gamma},${\mathrm{\ensuremath{\pi}}}^{0}$) data, near-threshold values are obtained for the reduced proton p-wave (\ensuremath{\gamma},${\mathrm{\ensuremath{\pi}}}^{0}$) multipoles ${\mathit{M}}_{1+}$/qk,${\mathit{M}}_{1\mathrm{\ensuremath{-}}}$/qk, and ${\mathit{E}}_{1+}$/qk which are in excellent agreement with the VPI multipoles as given by the said code but disagree with the findings of other authors. Combined with a conservative theoretical estimate of the isoscalar amplitude ${\mathit{M}}_{1\mathrm{\ensuremath{-}}}$(s), these results permit a decomposition of ${\mathit{M}}_{1\mathrm{\ensuremath{-}}}$(p) into isoscalar and isovector components in the near-threshold region.