  The positively charged pions produced in proton-proton collisions at a beam
momentum of 1640 MeV/c were measured in the forward direction with a high
resolution magnetic spectrograph. The missing mass distribution shows the bound
state (deuteron) clearly separated from the $pn$ continuum. Despite the very
good resolution, there is no evidence for any significant production of the
$pn$ system in the spin-singlet state. However, the $\sigma(pp\to
\pi^+pn)/\sigma(pp\to \pi^+d)$ cross section ratio is about twice as large as
that predicted from $S$-wave final-state-interaction theory and it is suggested
that this is due to $D$-state effects in the $pn$ system.

Nuclear Experiment