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 $$\mu ^2=4\hbox { GeV}^2$$ .
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"value": "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 $$\\hbox {J}/{\\Psi }$$ <math><mrow><mtext>J</mtext><mo>/</mo><mi>\u03a8</mi></mrow></math> photo-production data of the GlueX experiment, at JLab. We apply a vector meson dominance model to describe the photo-production of $$\\hbox {J}/{\\Psi }$$ <math><mrow><mtext>J</mtext><mo>/</mo><mi>\u03a8</mi></mrow></math> and a running strong coupling which includes the nonperturbative effects in the low $$\\mu ^2$$ <math><msup><mi>\u03bc</mi><mn>2</mn></msup></math> region. Despite the large uncertainty, we find that the experimental data favors a small value of the trace anomaly parameter $$b=0.07\\pm 0.17$$ <math><mrow><mi>b</mi><mo>=</mo><mn>0.07</mn><mo>\u00b1</mo><mn>0.17</mn></mrow></math> . We report the resulting proton mass decompositions at $$\\mu ^2=0.41\\hbox { GeV}^2$$ <math><mrow><msup><mi>\u03bc</mi><mn>2</mn></msup><mo>=</mo><mn>0.41</mn><mspace width=\"0.333333em\"></mspace><msup><mtext>GeV</mtext><mn>2</mn></msup></mrow></math> and $$\\mu ^2=4\\hbox { GeV}^2$$ <math><mrow><msup><mi>\u03bc</mi><mn>2</mn></msup><mo>=</mo><mn>4</mn><mspace width=\"0.333333em\"></mspace><msup><mtext>GeV</mtext><mn>2</mn></msup></mrow></math> ."
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