The gluon-less pion? Insights to pion structure from leading neutron data

As mediators of the strong force interactions between protons and neutrons in nuclei, pions play an essential role in nuclear physics. Simultaneously, pions are the lightest bound states in quantum chromodynamics (QCD) and are composed of quarks and gluons (partons). The JAM Collaboration has performed several global QCD analyses to extract the pion parton distribution functions (PDFs) from pion-nucleus Drell-Yan (DY) and leading neutron (LN) electroproduction data. However, a recent analysis by the Fantomas Collaboration, which adopts differing treatments of LN data, reported that the gluon and sea quark PDFs are not well constrained, and even a zero gluon distribution is allowed. Motivated by this result, we examine the impact of LN data on the gluon PDF in pions at the input scale. Using a Bayesian Monte Carlo framework, we simultaneously fit DY and LN data while adopting various treatments of the LN data. We observe a clear preference for nonzero gluon PDFs, and the JAM data selection in fact excludes the possibility of zero gluon. Nonetheless, vanishing gluon distributions do indeed arise from particular data treatments used in the Fantomas analysis. This study demonstrates the impact of LN data on extracting pion PDFs and provides strong evidence for the presence of nonzero gluon. Further constraints on pion PDFs are expected from future experiments, such as tagged deep-inelastic scattering at Jefferson Lab and the Electron-Ion Collider.