Color-Octet $ψ'$ Production at Low $p_\perp$

We study contributions from color-octet quarkonium formation mechanisms to $p_\perp$-integrated $\psi'$ production cross sections in pion-nucleon reactions. The observed polarization of the $\psi'$ is not reproduced by the lowest-order leading-twist color-singlet and color-octet mechanisms. This suggests that there are important quarkonium production mechanisms beyond leading twist.