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

Differential cross sections of the reactions $pp to dpi^+$ and $pp to pnpi^+$ have been measured at $T_p = 400$ MeV by detecting the charged ejectiles in the angular range $4^0 leq Theta_{Lab} leq 21^circ$. 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 $Delta$ production and $NN$ final-state interaction. The data are well described by Monte Carlo simulations including both these effects. The ratio of $pp to pnpi^+$ and $pp to dpi^+$ cross sections also compares favourably to a recent theoretical prediction which suggests a dominance of $np$-production in the relative $^3S_1$-state.