Mechanical Properties of the Proton

Protons and neutrons are composed of elementary objects, quarksand gluons. The latter are the carrier of the strong force that governs the dynamics binding quarks and gluons together. It is well established that quarks do not exist in isolation but only in the confines of nucleons and mesons(hadrons) of finite size. Although the proton is an absolutely stable particle, the distribution of forces between the quarks in theproton,whichareresponsibleforthatstabilityarelargelyunknown.

The mechanical properties of the proton, including pressure, forces and mechanical size are encoded in the pro- ton’s matrix element of the energy-momentum tensor and are expressed in scalar gravitational form factors (GFFs). Recent theoretical developments showed that the GFFs can be accessed in deeply virtual Compton scattering (DVCS). In this process two photons couple to the proton and mimic the graviton-proton interaction, and hence can probe its mechanical properties, such as the normal and shear stress, the internal pressure distribution, and the mechanical radius. This new direction of nucleon structure research has already resulted in thefirstdeterminationofthepressuredistributioninsidetheproton. Here, wediscusstheextractionofshearforces and their spatial distribution and the protons mechanical size, and compare with model predictions.