Physicists still can't explain anomaly in proton experiment

Physicists still can’t explain anomaly in proton experiment

American physicists have confirmed a strange measurement that was first discovered by scientists probing the internal structure of protons two decades ago.

This latest experiment – conducted at the Thomas Jefferson National Accelerator Facility by a team of academics primarily from Temple University in Philadelphia – shows that the standard model of proton composition isn’t quite correct and indicates that scientists don’t understand still not so good protons. as assumed.

Today, it is understood that protons and other subatomic particles are, generally speaking, composed of quarks, even smaller particles that carry fractional charges. The simplified Standard Model holds that protons contain two positively charged quarks and one negatively charged quark. Sounds simple, right?

But more realistically, the proton is a jumbled mess of countless quarks and antiquarks interacting with each other by exchanging gluons – a distinct type of particle representing the strong force that holds quarks together to form a proton.

However, that’s not quite the picture either. Something strange is going on inside the subatomic particle and we have a few decades to figure out what it is.

At the Jefferson lab, the team bombarded liquid hydrogen with electrons to study the internal nature of the proton in each hydrogen atom, using virtual Compton scattering. The electrons interact with the protons in the hydrogen, eventually causing the proton’s quarks to emit a photon. Detectors measure how electrons and photons scatter, to determine the position and momentum of quarks. The information gives researchers an idea of ​​the proton’s internal structure and a way to measure the proton’s electrical polarizability.

“We want to understand the substructure of the proton,” Ruonan Li, first author of the study published in Nature and a graduate student at Temple University, said in a statement.

“And we can imagine it as a model with the three balanced quarks in the middle. Now put the proton in the electric field. The quarks have either positive or negative charges. They will move in opposite directions. So the electric polarizability reflects how easily the proton will be distorted by the electric field.”

The distortion shows how much a proton can stretch under an electric field. According to conventional theories, protons should become stiffer as they are deformed by electric fields at higher energies. A graph plotting electric polarizability versus electric field strength should be smooth – but the researchers observed a characteristic bump.

This bump is the strange measurement that the Temple team has confirmed.

“What we actually see is that the electrical polarizability monotonically decreases at first, but at some point there is a local improvement in this property before it goes down again,” Nikos Sparveris, co-author of the paper and an associate professor of physics at Temple University, said The register.

It is not known at this stage what could be the cause of this effect

“It is not clear at this stage what could be causing this effect.”

The team believes the bump shows that an unknown mechanism may affect the strong force in some way.

“The first hint of such an anomaly was reported 20 years ago (it was a MAMI microtron experiment in Germany), but the results came with quite a large uncertainty and have not been independently confirmed. In this work we were able to measure more precisely.In our new experiment we do indeed find evidence of a structure in the electrical polarizability, but we observe half the magnitude of what had been originally reported,” he added.

Electrical polarizability gives scientists a way to probe the internal structure of a proton and the force that binds it. “The reported measurements suggest the presence of a new dynamical mechanism not yet understood in the proton and present notable challenges to nuclear theory,” according to the team’s paper. [Arxiv preprint].

The group plans to perform further follow-up experiments to study the anomalous bump in more detail. “We have to identify the shape of such a structure as precisely as possible (this is an important contribution for the theory, in trying to explain the cause of the effect) and we have to eliminate any possibility that this effect could be a experimental artifact.”, concluded Sparveris. ®

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