Main menu


Physicists confirm disruption in proton structure


Credit: Pixabay/CC0 Public Domain

Nuclear physicists have confirmed that the current definition of proton structure is not entirely smooth. A new sensitive measurement of the electrical polarizability of the proton, performed at the U.S. Department of Energy’s Thomas Jefferson National Accelerator Facility, has revealed a bump in the data from the proton structure probes.

While earlier measurements were commonly thought to be random, this new, more precise measurement confirmed the existence of the anomaly and raised 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, measurements of the electrical polarizability of the proton reveal how susceptible the proton is to deformation or stretching in an electric field. Like size or charge, electrical polarizability is a fundamental property of proton structure.

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

“We want to understand the background of the proton. We can imagine it like a model with three balanced quarks in the middle,” Li explained. “Now, put the proton in the electric field. The quarks have a positive or negative charge. They will move in opposite directions. So, the electrical polarizability reflects how easily the proton will be disrupted by the electric field.”

To probe this decay, 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 sent by hitting 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 in turn determines how the photon interacts with other particles.

Lower energy photons can bounce off the surface of the proton, while more energetic photons explode inside the proton, interacting with one of its quarks. The theory predicts that when these photon-quark interactions are drawn from low energy to high energy, they will form a smooth curve.

This simple picture does not stand up to scrutiny, said Nikos Sparveris, associate professor of physics at Temple University and spokesperson for the experiment. Instead, the measurements revealed a yet unexplained lump.

“What we’re seeing is a local increase in magnitude of polarizability. As expected, polarizability decreases as energy increases. And at some point, it seems to rise again temporarily before it drops,” he said. aforementioned. “According to our current theoretical understanding, it should follow a very simple behavior. We see something deviating from this simple behavior. And that’s the truth that surprises us right now.”

The theory predicts that more energetic electrons probe the strong force more directly as they bind quarks together to form the proton. This strange increase in hardness, which nuclear physicists have now confirmed in the proton’s quarks, indicates that an unknown aspect of the strong force may be at work.

“There’s something we clearly overlooked at this point. The proton is the only stable compound building block in nature. So if we miss something fundamental there, it has implications or implications for all of physics,” Sparveris said. approved.

The physicists said the next step is to make sensitive probes to reveal more details of this anomaly and to check for other divergence points and provide more information about the source of the anomaly.

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

Sparveris agreed. “We also need to measure exactly 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.

Provided by the Thomas Jefferson National Accelerator Facility

Quotation: Physicists confirm disruption in proton structure, retrieved from on October 20, 2022 (2022, October 19)

This document is subject to copyright. No part of it may be reproduced without written permission, except in any fair dealing for private study or research purposes. The content is provided for informational purposes only.