Dominance of the two-nucleon mechanism inO16(π+,pp) at 115 MeV

Singles ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$(${\mathrm{\pi }}^{+}$,p) and coincidence ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$(${\mathrm{\pi }}^{+}$,2p${)}^{14}$N measurements have been made at ${\mathit{T}}_{\mathrm{\pi }}$=115 MeV over an extended range of the phase space of the two final-state protons. The coincidence measurements have a missing-mass resolution of 4 MeV. The direct two-nucleon absorption cross section extracted from the data is 58±8 mb. After corrections for final-state interactions, it was found that the two-nucleon ${\mathrm{\pi }}^{+}$+np→pp process accounts for about 76% of the total absorption cross section, and thus is the dominant absorption mechanism. Evidence for absorption on s-p pairs is seen as a broad bump near 32 MeV of excitation. At backward angles the inclusive ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$(${\mathrm{\pi }}^{+}$,p) spectra show a distinct absorption peak, an important feature not previously observed. Comparisons of distorted wave impulse approximation calculations to both the coincidence data and inclusive data are presented.