Physicists confirm a complication in the structure of protons

Oct. 20 () –

More precise measurements have confirmed that the current description of the structure of protons, fundamental to nuclear physicsis more complex than established.

New investigation of the electrical polarizability of the proton, conducted at the US Thomas Jefferson Accelerator Facility (Jefferson Lab), has revealed an increase in data from the proton structure probes. The findings are published in the journal Nature.

According to Ruonan Li, first author of the new paper and a graduate student at Temple University, measurements of the proton’s electrical polarizability reveal how susceptible the proton is to warping or stretching in an electric field. Like size or charge, electrical polarizability is a fundamental property of the proton’s structure.

Furthermore, a precise determination of the electrical polarizability of the proton can help bridge the different descriptions of the proton. Depending on how it is tested, a proton can appear as a single opaque particle or as a composite particle made of three quarks held together by the strong force.

“We want to understand the substructure of the proton. And we can imagine it as a model with the three balanced quarks in the middle,” Li explained. it’s a statement. “Now, put the proton in the electric field. The quarks have positive or negative charges. They will move in opposite directions. So the electric polarizability reflects how easily the electric field will distort the proton.”

To test for this distortion, nuclear physicists used a process called virtual Compton scattering. It begins with a carefully controlled beam of energetic electrons from Jefferson Lab’s continuous electron beam accelerator facility. The electrons are sent colliding with the protons.

In virtual Compton scattering, electrons interact with other particles by emitting an energetic photon or light particle. The energy of the electron determines the energy of the photon it emits, which also determines how the photon interacts with other particles.

Lower energy photons can bounce off the surface of the proton, while higher energy photons will explode inside the proton to interact with one of its quarks. Theory predicts that when these photon-quark interactions are plotted from lower to higher energies, they will form a smooth curve.

Nikos Sparveris, an associate professor of physics at Temple University and a spokesman for the experiment, said this simple image did not stand up to scrutiny. Instead, measurements revealed a still unexplained bulge.

“What we see is that there is some local enhancement in the magnitude of the polarizability. The polarizability decreases as the energy increases as expected. And, at some point, it appears to temporarily rise again before it decreases,” he said. “According to our current theoretical understanding, it should follow a very simple behavior. We see something that deviates from this simple behavior. And this is the fact that puzzles us at the moment.”

The theory predicts that more energetic electrons more directly probe the strong force, as it binds quarks together to form the proton. This strange spike in stiffness that nuclear physicists have now confirmed in the quarks of the proton indicates that an unknown facet of the strong force may be at work.

“There is something clearly missing at this point. The proton is the only composite building block in nature that is stable. So if we’re missing something fundamental there, it has implications or consequences for all of physics.“, warns Sparveris.

Physicists said the next step is to further tease out the details of this anomaly and perform precision tests to check other deviation points and provide more information about the anomaly’s source.

“We want to measure more points at various energies to present a clearer picture and see if there is any additional structure there,” Li said.

Sparveris agreed. “We also need to accurately measure the shape of this enhancement. The shape is important to further elucidate the theory,” he said.