Partonic Pressure in the Nucleon: Results and Future Plans

Protons and neutrons, generally referred to as nucleons, are the fundamental building blocks of nuclei and make up nearly 100% of the mass of normal matter in the universe. They are composed of elementary objects, quarks and 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. Mechanical properties of the proton such as pressure, forces between quarks and the angular momentum distributions are largely unknown.

These properties are encoded in the proton's matrix element of the energy-momentum tensor and are expressedin scalar gravitational form factors (GFFs).  Recent theoretical development showed that the GFFs may be probed in deeply virtual Compton scattering (DVCS). In this process two photons couple to the proton and mimic the graviton-proton interaction, and hence probe its mechanical properties. Here we present the extraction of the pressure and its spatial distribution inside the proton. We will also present preliminary results on DVCS with CLAS12 and discuss the future Jefferson Laboratory data with a positron beam to provide the strongest constraints and minimize systematic uncertainties.