Fixed-target charmonium production and pion parton distributions

We investigate how charmonium hadroproduction at fixed-target energies can be used to constrain the gluon distribution in pions. Using nonrelativistic QCD (NRQCD) formulation, the J/psi and psi(2S) cross sections as a function of longitudinal momentum fraction xF from pions and protons colliding with light targets, as well as the psi(2S) to J/psi cross section ratios, are included in the analysis. The color-octet longdistance matrix elements are found to have a pronounced dependence on the pion parton distribution functions (PDFs). This study shows that the xF differential cross sections of pion-induced charmonium production impose strong constraints on the pion's quark and gluon PDFs. In particular, the pion PDFs with larger gluon densities provide a significantly better description of the data. It is also found that the production of the psi(2S) state is associated with a larger quark-antiquark contribution, compared with J/psi.