Single-pion production in pp collisions at 0.95 GeV/c (II)

The single-pion production reactions pp \( \rightarrow\) d \( \pi^{+}_{}\) , pp \( \rightarrow\) np \( \pi^{+}_{}\) and pp \( \rightarrow\) pp \( \pi^{0}_{}\) were measured at a beam momentum of 0.95GeV/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. The main emphasis is put on the presentation and discussion of the np \( \pi^{+}_{}\) 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 pp \( \pi^{0}_{}\) channel we observe in the np \( \pi^{+}_{}\) channel a strong influence of the \( \Delta\) excitation. In particular, the pion angular distribution exhibits a (3 cos2 \( \Theta\) + 1) -dependence, typical for a pure s -channel \( \Delta\) excitation and identical to that observed in the d \( \pi^{+}_{}\) channel. Since the latter is understood by a s -channel resonance in the 1 D 2 pn partial wave, we discuss an analogous scenario for the pn \( \pi^{+}_{}\) channel.