Spin-triplet final-state dominance in the pp→pn π+ reaction at 492 MeV

The near-forward cross section for the pp→pnπ+ reaction has been measured at 492 MeV by using the large acceptance ANKE magnetic spectrometer placed at an internal target position of the storage ring COSY-Jülich. Protons and pions emitted near zero degrees were detected in coincidence, and those with θπ⩽2° and θp⩽2.5° were subjected to detailed analysis. Under these conditions, the excitation energy in the np system was below 3 MeV over the measured momentum range. This is the region of the np final-state-interaction peak, which was measured with a resolution of a fraction of an MeV. The shape of the peak allows one to conclude that the fraction of final spin-singlet np pairs is below about 10%. By using the results of scattering theory, this limit is confirmed through a comparison with the cross section for pp→dπ+. The smallness of the singlet contribution is consistent with trends seen in lower energy data.