Physicists confirm a deadlock in the structure of protons

Physicists confirm a deadlock in the structure of protons

Physicists confirm a deadlock in the structure of protons

Physicists confirm a deadlock in the structure of protons

Credit: Pixabay/CC0 Public Domain

Nuclear physicists have confirmed that the current description of the proton’s structure is not all easy. A new precise measurement of the electrical polarizability of protons performed at the US Department of Energy’s Thomas Jefferson National Accelerator Facility has revealed a bump in the data in proton structure probes.

Although thought to be a coincidence when seen in earlier measurements, this new, more precise measurement confirmed the presence of the anomaly and raises questions about its origin. The research has just been published in the journal Nature.

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

Moreover, a precise determination of the electrical polarizability of protons can help to bridge different descriptions of protons. Depending on how it is examined, the proton can appear as an opaque single 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 think of it as a model with three balanced quarks in the middle,” Li explained. “Now, put a proton in an electric field. Quarks have positive or negative charges. They will move in opposite directions. So the electric polarizability reflects how easily the proton will be distorted by the electric field.”

To examine 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, a DOE Office of Science user facility. Electrons are broken into protons.

In virtual Compton scattering, electrons interact with other particles by emitting an energetic photon, or particle of light. 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 proton’s surface, while higher energy photons will explode inside the proton to interact with one of its quarks. The theory predicts that when these photons-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 picture did not stand up to scrutiny. Instead, the measurements revealed an as yet unexplained bump.

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

The theory predicts that more energetic electrons more directly probe the strong force as it binds quarks together to create a proton. This strange jump in rigidity that nuclear physicists have now confirmed in proton quarks signals an unknown aspect strong force maybe at work.

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

Physicists said the next step is to further uncover the details of this anomaly and conduct precise probes to check for other points of deviation and provide more information about the source of the anomaly.

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

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

More information:
Nikolaos Sparveris, Measured proton electromagnetic structure deviates from theoretical predictions, Nature (2022). DOI: 10.1038/s41586-022-05248-1.

Citation: Physicists confirm hitch in proton structure (2022, October 19) Retrieved October 19, 2022 from

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