Researchers confirm an anomaly in the structure of the proton

Researchers confirm an anomaly in the structure of the proton

While making measurements of electrical polarization in protons, the researchers confirmed an anomaly detected a few years ago. The plotted polarization curve as a function of energy does not correspond to what the theory predicts. According to the team, this discovery challenges quantum chromodynamics that describes the strong interaction.

The visible matter that surrounds us is composed mainly of atoms. They themselves are divided into two parts, a nucleus made up of protons and neutrons, and electrons orbiting around it. If the electron is in fact an elementary particle, that is, not divisible into smaller components, the protons and neutrons are made up of, among other things, three valence quarks. The proton has two “up” quarks and one “down” quark. Added to this are the quark-antiquark pairs, and the gluons, vectors of the strong interaction, which maintain the cohesion between the quarks. The description of this strong interaction is done using quantum chromodynamics (term often abbreviated in QCD for quantum chromodynamics).

In a study published in Nature, researchers have focused on another element of the proton: its electrical polarizability. Designates its ability to deform when subjected to an electric field: the particle can deform, more or less depending on the strength of the applied field. A few years earlier, the study explains, an anomaly had been detected in the proton polarizability curve, attributed to measurement errors. But the new study confirms this anomaly which, for the time being, is unexplained, even by QCD.

Virtual photons entering the proton.

For their research, the team used the Thomas Jefferson National Accelerator Facility of the United States Department of Energy. This gigantic laboratory has an electron accelerator that generates powerful electron beams. They can then be used to bombard a stationary target, in this case a proton. Then there is an effect called virtual Compton scattering which consists of “the diffusion of an electron into a proton by exchange of a virtual photon and the re-emission of a real photon by the proton in the final channel”, explains the CEA. In other words, when an electron hits the proton, as in the beam case, it produces a virtual photon that will directly interact with the proton. It is the initial energy of the electron that imposes that of the virtual photon.

At low energy, it can bounce off the surface of the proton. But, beyond a certain threshold, it enters the substructure and can then interact with all the subparticles contained in it. “We want to understand the substructure of the proton. And we can imagine it as a model with the three balanced quarks in the middleexplained Ruobab Li in a press release, first author of the study. Now put the proton in the electric field. Quarks have positive or negative charges. They will move in opposite directions. Therefore, the electric polarizability reflects the ease with which the electric field will deform the proton.

Measures in disagreement with theory

The theory predicts a smooth polarizability curve, which gradually decreases as the incident energy increases. But the measurements, on the contrary, have revealed a “blow”: “What we see is that there is a local enhancement in the magnitude of the polarizability. The polarizability decreases as the energy increases as expected. And, at one point, it seems to go up temporarily before going down againdescribed Nikos Sparveris, co-author of the study and who led the experiment. Based on our current theoretical understanding, it should follow a very simple behavior. We see something that deviates from this simple behavior. And it is this fact that perplexes us at the moment.”

And this is not the first time such a result has been found. As the study explains, “a local enhancement of electrical polarizability as a function of distance scale in the system, was reported by a measurement (later repeated by the same group) at Qtwo = 0.33GeVtwo but with a large experimental uncertainty. Anomaly that had been questioned for years, attributed to measurement errors, added to the uncertainties. But, this time, the researchers made very precise measurements, confirming this strange phenomenon that is not taken into account in theory. “There is something that we are clearly missing at this stage. The proton is the only stable compound building block in nature. So if we’re missing something fundamental there, that has implications or consequences for all of physics.”confirmed N. Sparveris.

For the future, the researchers plan to conduct new experiments, aimed at better understanding this anomaly. “We want to measure more points at different energies to present a clearer picture and see if there is another structure there.”Ruobab Li concluded.

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