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

Differential cross sections of the reactions $pp\to d{\pi }^{+}$ and $pp \to pn\pi^+$havebeenmeasuredat$T_p = 400$ MeV by detecting the charged ejectiles in the angular range $4^0\le {\mathrm{\Theta }}_{Lab}\le {21}^{\circ }$. 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 $\mathrm{\Delta }$ production and $NN$ final-state interaction. The data are well described by Monte Carlo simulations including both these effects. The ratio of $pp\to pn{\pi }^{+}$ and $pp\to d{\pi }^{+}$ cross sections also compares favourably to a recent theoretical prediction which suggests a dominance of $np$-production in the relative ${}^3{S}_1$-state.