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The mass radius of the proton

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arxiv 2102.00110 v2 pith:VZVRVC5M submitted 2021-01-29 hep-ph hep-latnucl-exnucl-th

The mass radius of the proton

classification hep-ph hep-latnucl-exnucl-th
keywords radiusmassprotondataformfactortraceanomalyappropriate
verification ladder T0 review T1 audit T2 compute T3 formal T4 reserved
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The mass radius is a fundamental property of the proton that so far has not been determined from experiment. Here we show that the mass radius of the proton can be rigorously defined through the formfactor of the trace of the energy-momentum tensor (EMT) of QCD in the weak gravitational field approximation, as appropriate for this problem. We then demonstrate that the scale anomaly of QCD enables the extraction of the formfactor of the trace of the EMT from the data on threshold photoproduction of $J/\psi$ and $\Upsilon$ quarkonia, and use the recent GlueX Collaboration data to extract the r.m.s. mass radius of the proton ${\rm R_m = 0.55 \pm 0.03 \ fm}$. The extracted mass radius is significantly smaller than the charge radius of the proton ${\rm R_C = 0.8409 \pm 0.0004 \ fm}$. We attribute this difference to the interplay of asymptotic freedom and spontaneous breaking of chiral symmetry in QCD, and outline future measurements needed to determine the mass radius more precisely.

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Forward citations

Cited by 9 Pith papers

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