Single-Pion Production in pp Collisions at 0.95 GeV/c (II)

Abstract

The single-pion production reactions $pp\to d\ddot{I}{€}^{+}$, $pp\to np\ddot{I}{€}^{+}$ and $pp\to pp\ddot{I}{€}^0$ 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 $np\ddot{I}{€}^{+}$ 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 $pp\ddot{I}{€}^0$ channel we find in the $np\ddot{I}{€}^{+}$ channel a strong influence of the $\hat{I}”$ excitation already at this energy close to threshold. In particular we find a $(3co{s}^2\hat{I}˜+1)$ dependence in the pion angular distribution, typical for a pure s-channel $\hat{I}”$ excitation and identical to that observed in the $d\ddot{I}{€}^{+}$ 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 $pn\ddot{I}{€}^{+}$ channel.