How hard is a proton?

A proton is a composite particle made up of the basic building blocks of quarks and gluons. These components and their interactions determine the structure of the proton, including its electrical charges and currents. This structure deforms when exposed to external electric and magnetic (EM) fields, a phenomenon known as polarizability. EM polarizability is a measure of rigidity against deformation caused by EM fields. By measuring the electromagnetic polarization, scientists learn about the internal structure of the proton.

This knowledge helps to confirm scientific understanding how nucleons (protons and neutrons) are formed, comparing the results with theoretical descriptions of gamma-ray scattering by nucleons. Scientists call this the scattering process nucleon Compton scattering.

When scientists investigate proton at a distance and scale dominated by EM responses, they can determine EM polarizability values ​​with high precision. To do this, they use the theoretical framework of effective field theories (EFT). EFT promises to reconcile the description of the structure of the nucleon at low energies with the current theory of the strong nuclear force, called quantum chromodynamics (QCD). In this study, scientists confirmed EFT using proton Compton scattering. This approach also validated the foundation and methodology behind EFT.

Compton scattering of protons is a process in which scientists scatter circularly or linearly polarized gamma rays from a hydrogen target (in this case a liquid target), then measure the angular distribution of the scattered gamma rays. High-energy gamma rays carry sufficiently strong electromagnetic fields that the reaction of charges and currents in the nucleon becomes significant.

In this study, published now in Review of physical sheets, scientists made new measurements of Compton scattering from a proton at the High Intensity Gamma-ray Source (HIGS) at the Triangle Universities Nuclear Laboratory. This work provided a new experimental approach to proton Compton scattering at low energies using polarized gamma rays. The study highlights the need for new high-precision measurements at HIGS to improve the accuracy of proton and neutron polarizabilities. These measurements support theories linking the low-energy description of nucleons to QCD.