Near-threshold proton-induced neutral pion production from 12C.

Abstract

The total cross section and angular distributions for the differential cross section and the analyzing power of the exclusive two-body pion production reaction $^{12}\mathrm{C}$(p\ensuremath{\rightarrow},${\mathrm{\ensuremath{\pi}}}^{0}$) $^{13}\mathrm{N}_{\mathrm{g}.\mathrm{s}.}$ have been measured at 147 MeV, very near the threshold energy of 145 MeV. Both the large forward-backward asymmetry in the angular distribution of the differential cross section and the large values observed for the analyzing powers indicate strong interference between the amplitude for s-wave pion emission and the amplitudes for p-wave pion emission. A phase-shift analysis performed on these new data yields significant p-wave strength at this low energy. The measurements for ${\mathrm{\ensuremath{\pi}}}^{0}$ production from carbon are in agreement with results extracted from existing data for ${\mathrm{\ensuremath{\pi}}}^{+}$ production from carbon, after corrections for isospin invariance and Coulomb effects. The reduced s-wave strength is found to have a simple power law dependence on the total number of nucleons in the final bound state.