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 /ψ and ψ (2 S ) cross sections as a function of longitudinal momentum fraction x_F from pions and protons colliding with light targets, as well as the ψ (2 S ) to J /ψ cross section ratios, are included in the analysis. The color-octet long-distance matrix elements are found to have a pronounced dependence on the pion parton distribution functions (PDFs). This study shows that the x_F 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 ψ (2 S ) state is associated with a larger quark-antiquark contribution, compared with J /ψ .