The mechanical radius of the proton

Abstract

In this letter we present the first determination of the proton's mechanical radius from data on deeply virtual Compton scattering cross sections and polarized electron beam spin asymmetries. This process involves the interaction of two spin J = 1 photons with the proton, and closely mimics the spin J = 2 interaction that is characteristic of gravity interaction with matter, involving the electromagnetic coupling strength rather than the many orders of magnitude smaller gravitational coupling. This allows for the mechanical or physical radius of the proton to be determined in deep inelastic electron scattering experiments with experimental counting rates that are typical for exclusive deep inelastic electromagnetic processes. We find that in comparison to the proton's charge radius, the mechanical properties are more concentrated near the proton's center than is the case of its electrical properties as reflected in the significantly larger charge radius of the proton.