Observation of strong final-state effects in π+ production in pp collisions at 400 MeV

Differential cross sections of the reactions pp→dπ+ and pp→pnπ+ have been measured at Tp=400 MeV by detecting the charged ejectiles in the angular range 4°≤ΘLab≤21°. The deduced total cross sections agree well with those published previously for neighbouring energies. The invariant mass spectra are observed to be strongly affected by Δ production and NN final-state interaction. The data are well described by Monte Carlo simulations including both these effects. The ratio of pp→pnπ+ and pp→dπ+ cross sections also compares favourably to a recent theoretical prediction which suggests a dominance of np-production in the relative 3S1-state.