ar X iv : n uc l - ex / 0 10 50 02 v 1 8 M ay 2 00 1 Spin - triplet final - state dominance in the pp → pnπ + reaction at 492 MeV

The cross section for the pp → pn π + 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. The produced proton and pion were detected in coincidence in a region close to the forward direction such that the excitation energy in the np system was below 3 MeV. This is the region of the np final-state-interaction peak which could be 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 π +. The smallness of the singlet contribution is consistent with trends seen in lower energy data. The two-body pp → dπ + reaction has long been used as a test bed for both experimental measurements and theoretical models of intermediate energy pion production [1]. Unfortunately, there is no corresponding bound state in the spin-0 np system to enable a similar study for the other isospin channel. On the other hand, the nearby virtual 1 S 0 state induces strong S-wave final-state-interaction (fsi) peaks in nuclear reactions leading to both spin-singlet and triplet np channels. To simulate a two-body reaction, the np system should be confined within a few MeV of excitation energy, E np. Furthermore the singlet and triplet contributions should be separated, which is possible with good resolution because the singlet fsi peak has a width of a fraction of an MeV as compared to the 2-3 MeV of the triplet. Inclusive pp → π + X reactions at intermediate energies show an np enhancement at the edge of phase space, but the energy resolution achieved and the contamination from the much stronger pp → dπ + channel makes it very difficult to study the fsi region at low E np [2]. The data nevertheless suggest that the triplet fsi provides the major signal [3]. To avoid the feed-through from the dπ + final state, it is useful to measure the proton and pion of the pnπ + channel in coincidence and this can also lead to better determination of E np. Such an experiment was carried out at LAMPF at 800 MeV [4]. Although fsi peaks …