Kinematically complete measurement of pp → pn π + near threshold

Cross sections for $\mathrm{pp}\ensuremath{\rightarrow}\mathrm{pn}{\ensuremath{\pi}}^{+}$ have been measured at incident proton energies of 294.3, 299.3, 306.3, 314.1, and 319.2 MeV $(0.11l\ensuremath{\eta}l0.42)$ to investigate hadronic ${\ensuremath{\pi}}^{+}$ production near threshold. Pion angular distributions are presented for 294.3, 299.3, and 319.2 MeV. At 294.3 and 299.3 MeV they are consistent with isotropy, indicating $s$-wave dominance for the angular momentum of the ${\ensuremath{\pi}}^{+}\mathrm{pn}$ system. The shapes of the $\mathrm{pn}$, $n{\ensuremath{\pi}}^{+}$, and $p{\ensuremath{\pi}}^{+}$ invariant mass distributions provide further evidence for dominance of $s$-wave mechanisms close to threshold. At 319.2 MeV significant anisotropy indicates contributions from higher partial waves. The total cross sections show an energy dependence consistent with an early partial conservation of axial current based calculation and with a recent calculation that considers only $s$-wave contributions. Tentative values for the strongest contributing partial wave amplitudes are presented.