Speaker
Description
The mass radius is a fundamental property of the proton that so far has not been determined from experiment. Basing on my recent paper arXiv:2102:00110, I will 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. I will 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/ψ and Υ quarkonia, and use the recent GlueX Collaboration data to extract the r.m.s. mass radius of the proton R_m = 0.55 ± 0.03 fm. The extracted mass radius is significantly smaller than the r.m.s. charge radius of the proton R_C = 0.8409 ± 0.0004 fm. I will discuss the possible origin of this difference, and outline future measurements at JLab, RHIC and EIC that should enable a more precise determination.
