Decoding the Proton’s Response to an External Electromagnetic Field

New measurements show the proton’s electromagnetic structure deviates from theoretical predictions.

Image courtesy of Nikos Sparveris, Temple University
A beam of electrons (e-) scatters from a proton (the sphere with two “up” quarks in red and one “down” quark in green) with enough energy to produce a photon (γ ). This exposes the proton’s internal structure to the electromagnetic field of the photon.

The Science

The visible world is founded on the proton, the only composite building block of matter that is stable in nature. This means that scientists seeking to understand the formation of matter need to explain the fundamental properties of the proton. A team of physicists measured the proton’s electric polarizability. This is a fundamental structural property of the proton. It characterizes the proton’s susceptibility to deformation, or its “stretchability,” in the presence of a photon’s electromagnetic field. The results reveal a puzzling new structure—a bump in the polarizability that nuclear theory cannot explain.

The Impact

The proton is a cornerstone of the visible matter in the universe. By measuring how the proton responds to an external electromagnetic field, physicists gain insight into the dynamics of the strong force. They can also understand how the proton emerges from its fundamental quark and gluon constituents. The new data contradict the theoretical predictions. They offer valuable input towards an improved theory that will be able to successfully describe this fundamental proton property.


In a new experiment conducted at the Thomas Jefferson National Accelerator Facility, a collaboration of nuclear physicists from 18 institutions have measured the proton’s electromagnetic generalized polarizabilities. The electric polarizability characterizes the susceptibility to deformation, or “stretchability,” of the proton in the presence of an external electric field, while the magnetic polarizability offers insight into the magnetic contributions in the proton. The experiment employed the virtual Compton scattering process, during which a real photon is produced and provides the electromagnetic perturbation to the proton that is needed to measure its polarizabilities.

The current theory predicts that the proton electric polarizability has a monotonic dependence as a function of the distance scale in the proton. That fact is a direct consequence of the increasingly dominant role of the quark degrees-of-freedom, which leads to the strong binding of the nucleon at short distance scales. The new results reveal a puzzling structure in this fundamental proton property that corresponds to a local enhancement of its magnitude as a function of the distance-scale in the nucleon. This striking contradiction to the theoretical predictions suggests that scientific understanding of the strong interaction that binds the proton’s elementary quark and gluon constituents together may be missing a key dynamical element. This presents a significant challenge to the prevalent theory.


Nikos Sparveris
Temple University


This work has been supported by the Department of Energy Office of Science, Nuclear Physics program.


Li, R., et al., Measured proton electromagnetic structure deviates from theoretical predictions. Nature 611, 265–270 (2022). [DOI: 10.1038/s41586-022-05248-1]

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