J/ psi longitudinal polarization from pi N interactions.

The spin alignment of 2.2\ifmmode\times\else\texttimes\fi{}${10}^{6}$ J/\ensuremath{\psi} particles produced in 252-GeV \ensuremath{\pi}N interactions has been measured via their decay to ${\ensuremath{\mu}}^{+}$${\ensuremath{\mu}}^{\mathrm{\ensuremath{-}}}$ in an experiment at Fermilab. Whereas J/\ensuremath{\psi}'s are produced unpolarized over most of the kinematic range, they are found to become longitudinally polarized as ${x}_{F}$\ensuremath{\rightarrow}1.