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

The legacy of the experimental hadron physics programme at COSY

“…In this approach it is assumed that the π + d final state is produced by a fusion of the spin-triplet proton and neutron pair in the threebody π + pn channel in a final state interaction. Starting from experimental data on pp → dπ + , an evaluation of the final state interaction model at 397 MeV gives b ≈ 3.1, which is very close to the COSY-TOF value [119].…”

“…The FSI model also predicts [106] a pp → pnπ + total cross section of about 600 µb at 397 MeV compared to an experimental value of (470 ± 20) µb [119], where the error bar is the authors' estimate of the systematic uncertainty. Given the uncertainties in both the measurement and the calculation, the experimental result could be consistent with the theoretical estimate.…”

“…This reinforces the COSY-TOF conclusion that it is the same intermediate state that governs both the dπ + and pnπ + channels. While accepting this, it may be a step too far to agree with the authors' suggestion that both production are associated with an isovector 2 + dibaryon [119]! To obtain information on pion production in the isospinzero channel requires data from neutron-proton collisions and, in the absence of neutron beams at COSY, this necessarily involves the use of a deuteron beam or target.…”

“…Though the experimental procedures were very similar for the pp → pnπ + reaction measured in parallel at 397 MeV [119], the results were significantly different because the data were dominated by the onset of the J P = 2 + ∆(1232)N intermediate state. A cut had to be imposed to stop leakage from the pp → dπ + reaction, where the deuteron broke up in a secondary reaction.…”

“…Pion production in unpolarized proton-proton collisions was also studied well away from threshold at a beam energy of 397 MeV [118,119]. Since the momenta of all three particles in the pp → ppπ 0 reaction were measured or reconstructed over a very large fraction of phase space, an intrinsic problem was choosing which variables to use in the presentation of the data.…”