Proton's stretchiness is a puzzle for particle physicists

The particles wrong a proton determination astir erstwhile exposed to electrical and magnetic fields, causing it to deform, but this behaviour isn't good understood

Physics 19 October 2022

By Alex Wilkins

An artist’s content of a proton

ARSCIMED/SCIENCE PHOTO LIBRARY

The proton is stretchier than we thought, according to caller measurements. But physicists are divided connected whether this anomaly volition persist successful aboriginal measurements oregon if our cardinal knowing of the proton’s operation will request to change.

Protons incorporate 3 smaller particles called quarks, which are held unneurotic by different particles called gluons, as good arsenic precise short-lived “virtual” particles. When a proton is exposed to electrical and magnetic fields, these interior constituents determination astir according to their charge, causing the proton to deform, oregon stretch.

The grade to which the proton tin beryllium stretched successful this mode is determined by its electrical and magnetic polarisabilities. These 2 quantities, which person been measured galore times, archer america astir the proton’s interior structure. In 2000, 1 of the archetypal measurements of these recovered that arsenic you analyse ever smaller sections of the proton, it gets concisely stretchier successful effect to magnetic and electrical fields, earlier becoming stiffer, oregon harder to deform.

However, these results were imprecise and much caller experiments disagreed, uncovering that the proton conscionable gets stiffer arsenic you zoom successful connected smaller sections, which is besides what the modular exemplary of the proton predicts.

Now, Nikolaos Sparveris astatine Temple University successful Pennsylvania and his colleagues person measured the proton’s stretchability to a higher level of precision and besides recovered that, astatine definite magnitude scales, it becomes stretchier to some electrical and magnetic fields, arsenic successful the 2000 result.

“We spot it with a higher precision,” says Sparveris, acknowledgment to gathering much data. “So, the shot present is connected the broadside of the [standard model] theory.”

To measurement the proton’s stretch, Sparveris and his squad fired a beam of low-energy electrons astatine a liquid-hydrogen target. In their set-up, erstwhile an electron moves past a proton wrong the hydrogen, it produces a photon, efficaciously an electromagnetic field, which distorts the proton. By measuring however the electrons and protons scatter distant from each other, the squad tin cipher however overmuch each proton is distorted by each photon.

While the anomalous effect appears akin to the 2000 work, the size of the effect has much than halved, says Judith McGovern astatine the University of Manchester, UK. It is precise hard successful wide to measurement the proton’s polarisabilities astatine debased energies with precocious precision, she says, and determination is nary evident mentation from existent theories for wherefore it should spike arsenic it does successful Sparveris’s result. “I don’t deliberation astir radical took [the 2000 result] truly seriously, I deliberation they assumed that it would spell away, and, if I’m rather honest, I deliberation everyone volition inactive presume that it volition spell away.”

Different aboriginal experiments, similar utilizing a beam of positrons – the antimatter counterpart to the electron – could shed airy connected whether this anomaly is truly determination oregon not, says McGovern. Sparveris and his squad mean to bash further experiments. “We person to destruct immoderate anticipation that this is owed to an experimental parameter oregon artefact, truthful we bash program to spell backmost and execute much measurements,” helium says.

However, if the anomaly remains, determination volition person to beryllium a revision to our knowing of the proton’s structure. “Other measurements volition elucidate whether oregon not this has an experimental origin, but it seems to beryllium a genuine discrepancy betwixt mentation and experiment,” says Juan Rojo astatine Vrije University Amsterdam successful the Netherlands. “The question is, what does this discrepancy archer us? And, especially, what we tin larn astir the proton operation by knowing these things?”

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

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• particle physics
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