The Origin of Proton Mass from J/${\rm \Psi}$ Photo-production Data

TL;DR

This paper estimates the QCD trace anomaly from near-threshold J/Ψ photo-production data, providing insights into the origin of proton mass and its decomposition at different energy scales.

Contribution

It introduces a method to extract the QCD trace anomaly from experimental data using a vector meson dominance model and a running coupling with nonperturbative effects.

Findings

Experimental data favors a small trace anomaly parameter b=0.07±0.17.

Proton mass decompositions are reported at two different energy scales.

The approach links J/Ψ photo-production data to fundamental QCD mass contributions.

Abstract

The trace of the stress tensor characterizes the transformation of a theory under rescaling. In quantum chromodynamics (QCD), this trace contains contributions from the bare masses of the quarks and also from a purely quantum effect, called the QCD trace anomaly. It affects all masses in the theory. We present an estimation of the QCD trace anomaly from the near-threshold J/${\rm \Psi}$ photo-production data of the GlueX experiment, at JLab. We apply a vector meson dominance model to describe the photo-production of J/${\rm \Psi}$ and a running strong coupling which includes the nonperturbative effects in the low $\mu^2$ region. Despite the large uncertainty, we find that the experimental data favors a small value of the trace anomaly parameter $b=0.07\pm 0.17$. We report the resulting proton mass decompositions at $\mu^2=0.41$ GeV$^2$ and $\mu^2=4$ GeV$^2$.