Fixed-target charmonium production and pion parton distributions

TL;DR

This paper demonstrates that fixed-target charmonium production data can effectively constrain pion parton distribution functions, especially the gluon density, using NRQCD analysis of $J/$ and $(2S)$ cross sections.

Contribution

It introduces a method to use charmonium production data to refine pion PDFs, highlighting the impact of gluon density on the data description.

Findings

Larger gluon densities in pion PDFs improve data fit.

Charmonium production favors pion PDFs with higher gluon content.

$(2S)$ production involves more quark-antiquark contributions.

Abstract

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 $x_F$ 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 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 $\psi(2S)$ state is associated with a larger quark-antiquark contribution, compared with $J/\psi$.