Madrid, 20 (European press)
A new investigation of the electric polarization of the proton, conducted at the US Thomas Jefferson Accelerator Facility (Jefferson Laboratory), has revealed an increase in data from probes of proton structure. The results were published in the journal Nature.
According to Ruonan Li, first author of the new research paper and a graduate student at Temple University, measurements of the electric proton’s polarization reveal how sensitive the proton is to twisting or stretching in an electric field. Like size or charge, electric polarization is a fundamental property of the proton’s structure.
Moreover, accurate determination of the electric polarizability of the proton can help to bridge the different descriptions of the proton. Depending on how you test it, a proton can appear as a single opaque particle or as a complex particle of three quarks bound together by the strong force.
“We want to understand the proton’s infrastructure. We can imagine it as a model with the three balanced quarks in the middle,” Lee explained in a statement. “Now, put the proton in the electric field. Quarks have a positive or negative charge. They will move in opposite directions. So the electric polarization reflects how easy it is to distort the electric field of the proton.”
To test this distortion, nuclear physicists used a process called hypothetical Compton scattering. It starts with a carefully controlled beam of energetic electrons from the Jefferson Continuous Electron Accelerator Facility, where electrons are sent to collide into protons.
In hypothetical Compton scattering, electrons interact with other particles by emitting an energetic photon or particle of light. The energy of an electron determines the energy of the photon it emits, which also determines how the photon interacts with other particles.
Low-energy photons can bounce off the surface of the proton, while higher-energy photons explode inside the proton to interact with one of its quarks. The theory predicts that when photon-quark interactions are plotted from lower energies to higher energies, they will form a smooth curve.
This simple picture did not stand up to scrutiny, said Nikos Sparviris, an assistant professor of physics at Temple University and a spokesperson for the experiment. Instead, measurements revealed an unexplained bulge.
“What we see is that there are some local enhancements in the magnitude of the polarization. The depolarization decreases with increasing energy as expected. And at some point, it appears to go up again temporarily before it goes down,” he said. “According to our current theoretical understanding, it should follow a very simple behaviour. We see something deviating from this simple behaviour. And that is the fact that puzzles us at the moment.”
The theory predicts that the most energetic electrons directly verify the strong force, as they bind quarks together to form the proton. This strange rise in hardness, now confirmed by nuclear physicists in proton quarks, suggests that an unknown facet of the strong force may be at work.
“There’s something we’re clearly missing at this point. The proton is the only compound building block in stable nature. So if we’re missing something fundamental there, it will have repercussions or consequences for all of physics,” warns Sparveris.
The physicists said the next step is to clarify the details of this anomaly, run precision tests to check other points of anomaly, and provide more information about the source of the anomaly.
“We want to measure more points at various energies to give a clearer picture and see if there is any additional structure there,” Lee said.
Sparveris agreed. “We also need to accurately measure the shape of this reinforcement,” he said. “The shape is important to further clarify the theory.”
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