Partonic structure of neutral pseudoscalars via two photon transition form factors

TL;DR

This paper calculates the transition form factors for neutral pseudoscalar mesons using a relativistic quantum field theory approach, confirming that heavy mesons' parton distributions are more compressed than the conformal limit predicts.

Contribution

It provides a unified continuum approach to compute transition form factors for heavy mesons, connecting light and heavy quark systems and enhancing understanding of meson structure.

Findings

The form factor $G_{\eta_c}(Q^2)$ aligns with experimental data.

Heavy-heavy meson PDAs are more compressed than the conformal limit.

The study links meson structure to fundamental QCD and the Standard Model.

Abstract

The $\gamma \gamma^\ast \to \eta_{c,b}$ transition form factors are computed using a continuum approach to the two valence-body bound-state problem in relativistic quantum field theory, and thereby unified with equivalent calculations of electromagnetic pion elastic and transition form factors. The resulting $\gamma \gamma^\ast \to \eta_c$ form factor, $G_{\eta_c}(Q^2)$, is consistent with available data: significantly, at accessible momentum transfers, $Q^2 G_{\eta_c}(Q^2)$ lies well below its conformal limit. These observations confirm that the leading-twist parton distribution amplitudes (PDAs) of heavy-heavy bound-states are compressed relative to the conformal limit. A clear understanding of the distribution of valence-quarks within mesons thus emerges; a picture which connects Goldstone modes, built from the lightest-quarks in Nature, with systems containing the heaviest valence-quarks that can now be studied experimentally, and highlights basic facts about manifestations of mass within the Standard Model.