Physicists have puzzled over the anomaly of the proton’s structure

Fast Moving Color Atom Physics Model

A new precise measurement of the proton’s electric polarization confirmed an anomaly, raising questions about its source.

Precise measurement of how a proton’s structure is distorted in an electric field has revealed new details about an unexplained spike in the proton data.

Nuclear physicists have confirmed that the current description of the proton’s structure is not perfect. A bump in data on proton structure sensors has been detected by a new precise measurement of proton electric polarization performed at the US Department of Energy’s Thomas Jefferson National Accelerators Facility. When this was seen in previous measurements, it was widely believed to be a coincidence. However, this new, more accurate measurement has confirmed the presence of the anomaly and raises important questions about its origin. The research was published October 19 in the journal temper nature.

“There is something that we are clearly missing at this point. The proton is the only compound building block in stable nature. So, if we miss something fundamental there, it will have implications or consequences for all of physics.” – Nikos Sparviris

Measurements of a proton’s electrical polarization reveal how much a proton is subjected to distortion or stretching in an electric field, according to Ronan Lee, first author on the new paper and a graduate student at Temple University. Like size or charge, electric polarization is a fundamental property of the proton’s structure.

Moreover, accurate determination of the electric proton polarization can help to bridge the various descriptions of the proton. Depending on how it is sounded, the proton may 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. And we can imagine it as a model with the three balanced quarks in the middle.” “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.”

Virtual Compton scattering interaction for a real photon

The real photon produced in the hypothetical Compton scattering reaction provides the electromagnetic turbulence of the proton and allows its generalized electromagnetic polarization to be measured. Credit: Image courtesy of Nikos Sparveris, Temple University

Nuclear physicists used a process called hypothetical Compton scattering to probe this distortion. This process begins with a carefully controlled beam of energetic electrons from the Jefferson Laboratory Continuous Electron Beam Accelerator Facility, a DOE Office of Science user facility. Electrons are sent bumping 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.

The strong nuclear force (also called the strong force) is one of the four fundamental forces of nature (the others are gravity, electromagnetic force, and the weak nuclear force). It is the strongest of the four, as its name suggests. However, it also has the shortest range, which means that the particles have to be extremely close before their effects can be felt. Its main function is to assemble the subatomic particles of the nucleus (protons, which carry a positive charge, and neutrons, which do not carry a charge. These particles are collectively called nucleons).

Low-energy photons may bounce off the surface of a 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 plotted from lower to higher energies, they will form a smooth curve.

This simple picture did not stand up to scrutiny, said Nikos Sparveris, an assistant professor of physics at Temple University and a spokesperson for the experiment. Measurements instead revealed a bump that has yet to be explained.

“What we see is that there is some local optimization of the magnitude of the polarization. The susceptibility to polarization decreases with increasing energy as expected. At some point, it appears that it will rise temporarily again before decreasing.” Based on our current theoretical understanding, it should follow a very simple behaviour. We see something deviating from this simple behavior. And that is the truth that baffles us at the moment.”

The theory predicts that the most energetic electrons directly probe 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 aspect of the strong force may be at work.

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

The physicists said the next step is to further elicit details of this anomaly and conduct careful investigations to verify other points of anomaly and to provide more information about the source of the anomaly.

“We want to measure more points at various energies to give a clearer picture and to see if there is any other structure out there,” he told me.

Sparveris agreed.

“We also need to accurately measure what this improvement looks like. The shape is important to clarify the theory further,” he said.

Reference: “The measured electromagnetic structure of the proton deviates from theoretical predictions” by R.Li, N. Sparveris, H. Atac, M. K. Jones, M. Paolone, Z. Akbar, C. Ayerbe Gayoso, V. Berdnikov, D. Biswas, M . Boer, A. Camsonne, J.-P. Chen, M. Deventhaler, P. Doran, D. Dutta, De Gaskell, O. Hansen, F. Hauenstein, N. Heinrich, W. Henry, T. Horn, J.M. Huber, S. J., S. Justin, A. Karki, S.J.D. Kay, V. Kumar, X. Li, W. B. Li, A. H. Liyanage, S. Malace, P. Markowitz, M. McCaughan, Z.-E. Meziani, H. Mkrtchyan, C. Morean, M. Muhoza, A. Narayan, B. Pasquini, M. Rehfuss, B. Sawatzky, G. R. Smith, A. Smith, R. Trotta, C. Yero, X. Zheng and J . Zhou, Oct 19, 2022, Available here. temper nature.
doi: 10.1038/s41586-022-05248-1

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