Physicists in America have confirmed a strange measurement first discovered two decades ago by scientists studying the internal structure of protons.
This latest experiment — conducted at the Thomas Jefferson National Accelerator Facility by a team of academics, mostly from Temple University in Philadelphia — shows that the Standard Model of proton composition isn’t quite right and shows that scientists still don’t quite think of protons that way understand well as expected.
It is now understood that protons and other subatomic particles are generally made up of quarks, even smaller particles that carry partial charges. The simplified Standard Model states that protons contain two positively charged quarks and one negatively charged quark. Sounds easy right?
But more realistically, the proton is a jumble of myriad quarks and antiquarks that interact with each other by exchanging gluons — a separate type of particle that represents the powerful force that holds quarks together to form a proton.
However, that’s not quite the whole picture. Something strange is going on in the subatomic particle, and we’ve been spending a couple of decades trying to figure out what it is.
At the Jefferson lab, the team bombarded liquid hydrogen with electrons to probe the inner nature of the proton in each hydrogen atom using virtual Compton scattering. The electrons interact with the protons of the hydrogen, which ultimately causes the quarks of the proton to emit a photon. Detectors measure how the electrons and photons scatter to find the position and momentum of the quarks. The information gives the 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 think of it like a model with the three balanced quarks in the middle. Now bring the proton into the electric field. The quarks have positive or negative charges. They will move in opposite directions. So the electric polarizability reflects how easily the proton is distorted by the electric field.”
The distortion shows how much a proton can expand under an electric field. According to conventional theories, protons should become stiffer as they are distorted by electric fields at higher energies. A plot of electric polarizability versus the strength of an electric field should be smooth – but the researchers observed a characteristic bump.
This bump is the odd measurement confirmed by the Temple team.
“What we actually see is that the electrical polarizability decreases monotonically at first, but eventually there is a local amplification of this property before it decreases again,” Nikos Sparveris, co-author of the paper and associate professor of physics at Temple University , told The registry.
It is currently not clear what could be the cause of this effect
“At this point in time, it is not clear what could be causing this effect.”
The team believe the bump shows that an unknown mechanism could somehow affect the strong force.
“The first indication of such an anomaly was reported 20 years ago (that was an experiment at MAMI Microtron in Germany), but the results came with quite a lot of uncertainty and have not since been independently confirmed. In this work we were able to measure this more precisely. In our new experiment, while we find evidence of structure in the electrical polarizability, we observe half the order of magnitude compared to what was originally reported,” he added.
Electrical polarizability gives scientists the ability to study the internal structure of a proton and the force holding it together. ‘The reported measurements suggest the presence of a new, not yet understood dynamical mechanism in the proton and pose significant challenges to nuclear theory,’ the team’s paper reads [Arxiv preprint].
The group plans to conduct further follow-up experiments to further investigate the anomalous bump. “We need to identify as precisely as possible the shape of such a structure (it is an important input to the theory to explain the cause of the effect) and we need to rule out any possibility that this effect could be an experimental artifact,” concluded Sparveris . ®
https://www.theregister.com/2022/10/23/physicists_proton_structure/ Physicists still can’t explain anomaly in proton experiment • The Register