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20 years on, physicists are still figuring out anomaly in proton experiment

Anyone got the technical reference manual for this simulation we're living in?

Physicists in America 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 the standard model of proton composition isn't quite right and indicates that scientists still don't fully understand protons quite as well as assumed.

Today it's understood that protons and other subatomic particles are, generally speaking, comprised of quarks, even smaller particles that carry fractional charges. The simplified, standard model contends protons contain two positively charged quarks, and one negatively charged quark. Sounds straightforward, right?

But more realistically, the proton is a jumbled mess of countless quarks and antiquarks interacting with one another by exchanging gluons – a separate type of particle representing the strong force that holds quarks together to make up a proton.

However, that's not quite the whole picture either. There's something strange going on within the subatomic particle and we're a couple decades into figuring out just what that 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 hydrogen's protons, ultimately causing the proton's quarks to emit a photon. Detectors measure how the electrons and photons scatter, to figure out the quarks' position and momentum. The information gives researchers an idea of the proton's internal structure, and a way to measure the proton's electric polarizability.

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

"And we can imagine it like 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 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. Under conventional theories, protons should become stiffer as they are distorted by electric fields at higher energies. A graph plotting the electric polarizability against the strength of an electric field should be smooth – but the researchers observed a characteristic bump.

That bump is the strange measurement the Temple team has confirmed.

"What we actually see is that the electric polarizability decreases monotonically at the beginning, but at some point there is a local enhancement of this property before it will go down again," Nikos Sparveris, co-author of the paper and an associate professor of physics at Temple University, told The Register.

It is not clear at this point what could be the cause of this effect

"It is not clear at this point what could be the cause of this effect."

The team reckons the bump shows that some unknown mechanism may be affecting the strong force somehow.

"The first hint for such an anomaly was reported 20 years ago (that was an experiment at the MAMI Microtron in Germany), but the results came with rather large uncertainty and were not independently confirmed in the meantime. In this work we were able to measure more precisely. In our new experiment, we indeed find evidence for a structure in the electric polarizability, but we observe half the magnitude compared to what was originally reported," he added.

The electric polarizability gives scientists a way to probe 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 dynamical mechanism in the proton and present notable challenges to the nuclear theory," according to the team's paper [Arxiv preprint].

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

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