Single-Pion Production in pp Collisions at =.95 GeV/c (I)

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 implementation of a central calorimeter provided particle identification, energy determination and neutron detection in addition to time-of-flight and angle measurements. Thus all pion production channels were recorded with 1-4 overconstraints. The total and differential cross sections obtained are compared to previous data and theoretical calculations. Main emphasis is put on the discussion of the $pppi^0$ channel, where we obtain angular distributions different from previous experimental results, however, partly in good agreement with recent phenomenological and theoretical predictions. In particular we observe very large anisotropies for the $pi^0$ angular distributions in the kinematical region of small relative proton momenta revealing there a dominance of proton spinflip transitions associated with $pi^0$ $s$- and $d$-partial waves and emphasizing the important role of $pi^0$ d-waves.