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Nuclear physicists have confirmed that the current description of the proton structure is not all smooth sailing. A new precision measurement of the proton’s electrical polarizability, conducted at the US Department of Energy’s Thomas Jefferson National Accelerator Facility, has revealed a bump in the data in probes of the proton’s structure.
Although it is widely believed to be a fluke on previous measurements, this new, more accurate measurement has 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, lead 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 deformation or stretching in an electric field. Like size or charge, electrical polarizability is a fundamental property of the proton structure.
In addition, an accurate determination of the electrical polarizability of the proton can help to bridge the different descriptions of the proton. Depending on how it is examined, a 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 imagine it as a model with the three balanced quarks in the middle,” explains Li. “Now put the proton in the electric field. The quarks have positive or negative charges. They will move in opposite directions. So the electrical polarizability reflects how easily the proton will be distorted by the electric field.”
To investigate 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. The electrons are sent to 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 proton’s surface, while more energetic photons will shoot into the proton to interact with one of its quarks. The 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 spokesperson for the experiment, said this simple photo didn’t hold up under close scrutiny. The measurements instead revealed a hitherto unexplained bump.
“What we see is that there is some local improvement in the magnitude of the polarizability. The polarizability decreases as the energy increases as expected. And at some point it seems to come back up temporarily before going down,” he said. . “Based on our current theoretical understanding, it should follow a very simple behavior. We see something different from this simple behavior. And this is the fact that surprises us right now.”
The theory predicts that the more energetic electrons sense the strong force more directly because it binds the quarks together to make the proton. This strange spike in stiffness that nuclear physicists have now confirmed in the proton’s quarks indicates that an unknown facet of the strong force is at work.
“There’s something we’re clearly missing right now. The proton is the only composite building block in nature that is stable. So if we miss something fundamental there, it has implications or consequences for all of physics,” says Sparveris. confirmed.
The physicists said the next step is to further tease the details of this anomaly and run precision probes to check for other points of anomaly and provide more information about the source of the anomaly.
“We want to measure more points at different energies to give a clearer picture and to see if there is further structure there,” Li said.
Sparveris agreed. “We also need to precisely measure the shape of this improvement. The shape is important to further clarify 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. www.nature.com/articles/s41586-022-05248-1 Provided by Thomas Jefferson National Accelerator Facility
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