The single-pion production reactions $ppto dpi^+$, $ppto nppi^+$ and $ppto pppi^0$ were measured at a beam momentum of 0.95 GeV/c ($T_p approx$ 400 MeV) using the short version of the COSY-TOF spectrometer. The central calorimeter provided particle identification, energy determination and neutron detection in addition to time-of-flight and angle measurements from other detector parts. Thus all pion production channels were recorded with 1-4 overconstraints. Main emphasis is put on the presentation and discussion of the $nppi^+$ channel, since the results on the other channels have already been published previously. The total and differential cross sections obtained are compared to theoretical calculations. In contrast to the $pppi^0$ channel we find in the $nppi^+$ channel a strong influence of the $Delta$ excitation already at this energy close to threshold. In particular we find a $(3 cos^2Theta + 1)$ dependence in the pion angular distribution, typical for a pure s-channel $Delta$ excitation and identical to that observed in the $dpi^+$ channel. Since the latter is understood by a s-channel resonance in the $^1D_2$ $pn$ partial wave, we discuss an analogous scenario for the $pnpi^+$ channel.
Download