Physicists in America have confirmed a strange measurement first discovered by scientists investigating the internal structure of protons two decades ago.
This latest experiment — conducted at the Thomas Jefferson National Accelerator Facility by a team of academics, mainly from Temple University in Philadelphia — shows that the Standard Model of proton composition isn’t quite right and indicates that scientists still don’t understand protons as well as supposed.
Today it is understood that protons and other subatomic particles are generally made up of quarks, even smaller particles that carry fractional charges. The simplified Standard Model states that protons contain two positively charged quarks and one negatively charged quark. Sounds obvious, right?
More realistically, though, the proton is a jumble of countless quarks and antiquarks interacting with each other by exchanging gluons — a distinct type of particle that represents the strong force that holds quarks together to form a proton.
That’s not quite the whole picture, however. There’s something strange going on in the subatomic particle, and we’ve spent a few decades figuring out exactly what that is.
In 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 hydrogen’s protons, eventually causing the proton’s quarks to emit a photon. Detectors measure how the electrons and photons scatter to determine the position and momentum of the 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, lead 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 positive or negative charges. They will move in opposite directions. So the electrical polarizability reflects how easily the proton will be distorted by the electric field.”
The deformation shows how much a proton can stretch under an electric field. According to conventional theories, protons should become stiffer because they are distorted by electric fields at higher energies. A graph plotting electrical polarizability against the strength of an electric field should be smooth, but the researchers saw a characteristic bulge.
That bump is the odd reading the Temple team confirmed.
“What we actually see is that the electrical polarizability decreases monotonously at the beginning, but at some point there is a local improvement in this property before it goes back down,” Nikos Sparveris, co-author of the paper and a researcher professor of physics at Temple University, told The Register.
It is currently not clear what could be the cause of this effect
“It is not clear at this time what may be the cause of this effect.”
The team thinks the bump shows that an unknown mechanism could somehow affect the strong force.
“The first hint of such an anomaly was reported 20 years ago (that was an experiment at the MAMI Microtron in Germany), but the results came with quite a lot of uncertainty and were not independently confirmed in the meantime. In this work, we could measure more accurately In our new experiment, we do indeed find evidence for a structure in the electrical polarizability, but we observe half the magnitude compared to what was originally reported,” he added.
The electrical polarizability gives scientists a way to investigate the internal structure of a proton and the force that binds it together. “The reported measurements suggest the presence of a new, not yet understood dynamic mechanism in the proton and present remarkable challenges to nuclear theory,” the team’s paper said. [Arxiv preprint].
The group plans to conduct more follow-up experiments to study the anomalous bump in more detail. “We need to identify the shape of such a structure as precisely as possible (it is an important input to the theory, to explain the cause of the effect) and we need to eliminate any possibility that this effect could be an experimental artifact”, concludes Sparveris.®