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

The near-forward cross section for the pp --> pn pi (+) 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-Julich. Protons and pions emitted near zero degrees were detected in coincidence, and those with theta (pi) and theta (p) 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 --> d pi (+). The smallness of the singlet contribution is consistent with trends seen in lower energy data. (C) 2001 Elsevier Science B.V. All rights reserved.