According to recent findings, the proton is more elastic than previously believed. While this discrepancy has been seen, physicists are split on whether it will be replicated in future experiments or if our current model of the proton’s structure needs to be revised.
Quarks are the subatomic particles that make up protons; they are bound together by gluons and other particles, including “virtual” particles that exist very briefly. Protons deform or stretch when subjected to external electric and magnetic forces because their components, which are charged, move around as needed.
The electric and magnetic polarisabilities of the proton govern how far it may be stretched in this manner. The multiple measurements of these two values provide insight into the composition of the proton.
One of the earliest observations of this kind was made in 2000, and it revealed that the proton becomes stretchier in reaction to magnetic and electric fields for a short period of time before becoming stiffer, or harder to deform.
Recent tests, however, have contradicted these inaccurate findings by showing that the proton simply becomes stiffer as you zoom in on smaller areas, as predicted by the proton’s conventional model.
Now, researchers led by Nikolaos Sparveris of Temple University in Pennsylvania have measured the stretchability of protons with more accuracy and confirmed that, similar to the conclusion from 2000, the proton gets stretchier to electric and magnetic fields at specific length scales.
Sparveris claims, “We perceive it with a higher precision” after compiling additional information. Now the [standard model] hypothesis has the upper hand.
By directing a stream of low-energy electrons at a liquid-hydrogen target, Sparveris and his team were able to determine the proton’s stretch. In this setup, the proton is deformed as an electron goes past it within the hydrogen, creating a photon, or effectively an electromagnetic field.
Researchers can determine the degree to which individual protons are deformed by individual photons by measuring the extent to which electrons and protons scatter away from each other.
Although the anomalous finding looks similar to the study done in 2000, Judith McGovern of the University of Manchester, UK, claims that the extent of the impact has decreased by more than half. It is difficult to quantify proton polarisabilities at low energies with great accuracy, she adds, and there is no clear explanation from existing theories for why the value would rise as it does in Sparveris’s discovery.
I don’t believe most people took [the 2000 result] seriously, I think they expected that it would go away, and if I’m being completely honest, I think everyone will still assume that it will go away.
McGovern suggests that future tests utilising positrons (the antimatter equivalent to the electron) might help determine whether or not this anomaly exists. There will be more testing conducted by Sparveris and company. We need to rule out the idea that this is the result of some sort of experimental parameter or artefact, so we do want to repeat the experiment and take further data, he says.
However, if the anomaly persists, our current knowledge of the proton’s structure will need to be revised. According to Juan Rojo of the Vrije Universiteit Amsterdam in the Netherlands: “Other measurements will illuminate whether or not this has an experimental cause, but there looks to be a true mismatch between theory and experiment.”
What does this disparity tell us?” And, more specifically, what do these phenomena tell us about the proton structure?