Symmetry, symmetry breaking, and pion parton distributions

Pion valence, glue, and sea distributions are calculated using a continuum approach to the two valence-body bound-state problem. Since the framework is symmetry preserving, physical features of the distributions are properly expressed. The analysis reveals that the emergent phenomenon of dynamical chiral symmetry breaking causes a hardening of the valence-quark distribution function, qπ(x). Nevertheless, this distribution exhibits the x ≃ 1 behavior predicted by quantum chromodynamics (QCD). At the scale ζ2 ≔ 2 GeV, the following momentum fractions are predicted: ⟨xvalence⟩ = 0.48 (3), ⟨xglue⟩ = 0.41 (2), ⟨xsea⟩ = 0.11 (2). Evolving to ζ = 5.2 GeV, the result for qπ(x) agrees with that computed using lattice QCD. These outcomes should both spur improved analyses of existing experiments and stimulate efforts to obtain new data on the pion distribution functions using available and envisioned facilities.