The origin of proton mass from J/$${\Psi }$$ photo-production data
Rong Wang (Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou, 730000, China, University of Chinese Academy of Sciences, Beijing, 100049, China); Xurong Chen (Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou, 730000, China, Guangdong Provincial Key Laboratory of Nuclear Science, Institute of Quantum Matter, South China Normal University, Guangzhou, 510006, China); Jarah Evslin (Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou, 730000, China, University of Chinese Academy of Sciences, Beijing, 100049, China)
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 $$\hbox {J}/{\Psi }$$ photo-production data of the GlueX experiment, at JLab. We apply a vector meson dominance model to describe the photo-production of $$\hbox {J}/{\Psi }$$ 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 $$\mu ^2=0.41\hbox { GeV}^2$$ and $$\mu ^2=4\hbox { GeV}^2$$ .