Physicists Just Learned Something Major About the Proton

Physics at the smallest scales is a challenge of observation: Particles are often fleeting, and the forces that govern their behavior are nearly imperceptible. But now, by exploiting decades-old data and a 50-year-old prediction about gravity’s import on subatomic particles, a team of physicists has teased out a measurement for a second mechanical property in the proton.

Let’s slow down for a moment. A proton is a particle that, along with neutrons and electrons, makes up an atom. Protons themselves are made of even tinier particles called quarks. These quarks glom together thanks to the strong force, one of the four fundamental forces. (The other three are gravity, the weak force, and electromagnetism.) The recent team successfully measured the strong force’s distribution within the proton, revealing the shear stress on the proton’s quarks.

“At its peak, this is more than a four-ton force that one would have to apply to a quark to pull it out of the proton,” said Volker Burkert, principal staff scientist at Jefferson Lab and the study’s lead author, in a lab release.

Published in Reviews of Modern Physics, the work follows up on a 2018 measurement of the proton’s internal pressure. The data the team studied came out of experiments at Jefferson Lab’s Continuous Electron Beam Accelerator Facility, or CEBAF, and the researchers used something called ‘deeply virtual Compton scattering’ (DVCS) to take the measurement. In DVCS, a high-energy electron is beamed at a target hydrogen proton. A quark within the proton emits a photon, which carries information about the quark’s properties. DVCS also yields information on gravity’s effects on matter, an idea developed in the early 2000s by the physicist Maxim Polyakov.

“This breakthrough in theory established the relationship between the measurement of deeply virtual Compton scattering to the gravitational form factor. And we were able to use that for the first time and extract the pressure that we did in the Nature paper in 2018, and now the normal force and the shear force,” Burkert added.

The data collection was something of an accident. The team was trying to do three-dimensional imaging of the scattering, but in doing so, they collected data that revealed how the strong force exerted itself on the proton’s insides.

“In my view, this is just the beginning of something much bigger to come,” said study co-author Latifa Elouadhriri, a staff scientist at Jefferson Lab, in the same release. “It has already changed the way we think about the structure of the proton.”

The scattering method can clue researchers in to proton properties that are “encoded in gravitational form factors,” as described by the 2023 Long Range Plan of Nuclear Science. In other words, DVCS could be an avenue for the discovery of new physics. Next, the team plans to take new measurements of the size of the proton.

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