Probing pion valence quark distribution with beam-charge asymmetry of pion-induced J/ψ production

We consider the beam-charge asymmetry of the J / ψ production cross sections in π − - versus π + -induced reactions on proton or nuclear targets. We show that the J / ψ production cross section difference between π − and π + beams impinging on a proton target has a positive sign with a magnitude proportional to the product of pion's valence quark distribution, V π , and proton's up and down valence quark distribution difference, u V − d V . The existing J / ψ production data for π − + p and π + + p at 39.5 and 200 GeV/c are consistent with the expected positive beam-charge asymmetry. The magnitude of the asymmetry is compared with calculations performed within two theoretical frameworks, the Color Evaporation Model (CEM) and the Non-Relativistic QCD (NRQCD) formalism. We also examine the beam-charge dependence for pion-induced J / ψ production cross sections measured on the neutron-rich platinum target, and find good agreement between the data and theory for both the negative sign and the magnitude of the beam-charge asymmetry. The comparison between data and theoretical calculations for both proton and platinum targets suggests that the beam-charge asymmetry in pion-induced J / ψ production is a viable method of accessing the valence quark distribution of the pion.