Proton’s stretchiness is a puzzle for particle physicists

The particles inside a proton move around when exposed to electric and magnetic fields, causing it to deform, but this behaviour isn’t well understood

By Alex Wilkins

An artist’s impression of a proton

ARSCIMED/SCIENCE PHOTO LIBRARY

The proton is stretchier than we thought, according to new measurements. But physicists are divided on whether this anomaly will persist in future measurements or if our fundamental understanding of the proton’s structure will need to change.

Protons contain three smaller particles called quarks, which are held together by other particles called gluons, as well as very short-lived “virtual” particles. When a proton is exposed to electric and magnetic fields, these internal constituents move about according to their charge, causing the proton to deform, or stretch.

The extent to which the proton can be stretched in this way is determined by its electric and magnetic polarisabilities. These two quantities, which have been measured many times, tell us about the proton’s internal structure. In 2000, one of the first measurements of these found that as you examine ever smaller sections of the proton, it gets briefly stretchier in response to magnetic and electric fields, before becoming stiffer, or harder to deform.

However, these results were imprecise and more recent experiments disagreed, finding that the proton just gets stiffer as you zoom in on smaller sections, which is also what the standard model of the proton predicts.

Now, Nikolaos Sparveris at Temple University in Pennsylvania and his colleagues have measured the proton’s stretchability to a higher level of precision and also found that, at certain length scales, it becomes stretchier to both electric and magnetic fields, as in the 2000 result.

“We see it with a higher precision,” says Sparveris, thanks to gathering more data. “So, the ball now is on the side of the [standard model] theory.”

To measure the proton’s stretch, Sparveris and his team fired a beam of low-energy electrons at a liquid-hydrogen target. In their set-up, when an electron moves past a proton within the hydrogen, it produces a photon, effectively an electromagnetic field, which distorts the proton. By measuring how the electrons and protons scatter away from each other, the team can calculate how much each proton is distorted by each photon.

While the anomalous result appears similar to the 2000 work, the size of the effect has more than halved, says Judith McGovern at the University of Manchester, UK. It is very difficult in general to measure the proton’s polarisabilities at low energies with high precision, she says, and there is no obvious explanation from current theories for why it should spike as it does in Sparveris’s result. “I don’t think most people took [the 2000 result] really seriously, I think they assumed that it would go away, and, if I’m quite honest, I think everyone will still assume that it will go away.”

Different future experiments, like using a beam of positrons – the antimatter counterpart to the electron – could shed light on whether this anomaly is really there or not, says McGovern. Sparveris and his team intend to do further experiments. “We have to eliminate any possibility that this is due to an experimental parameter or artefact, so we do plan to go back and perform more measurements,” he says.

However, if the anomaly remains, there will have to be a revision to our understanding of the proton’s structure. “Other measurements will elucidate whether or not this has an experimental origin, but it seems to be a genuine discrepancy between theory and experiment,” says Juan Rojo at Vrije University Amsterdam in the Netherlands. “The question is, what does this discrepancy tell us? And, especially, what we can learn about the proton structure by understanding these things?”

Journal reference: Nature, DOI: 10.1038/s41586-022-05248-1

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