Protons may be stretchier than physicists had thought

The quarks making up these particles seem to move more than they should in an electric field

An illustration of a proton with red, blue, and green quarks

A proton (illustrated) contains three particles called quarks (red, green and blue blobs). In electric fields, those quarks seem to move more than theories of physics predict. Basically, protons seem to be stretchier than scientists had ever imagined.


Protons might be stretchier than physicists had expected these building blocks of atoms to be.

These particles are made of even smaller building blocks called quarks. Quarks are bound together by what’s known as the strong force. New experiments used electric fields to tug on the quarks inside protons. And those quarks seemed easier to yank around than expected. This finding hints that the strong force isn’t quite as strong as predicted.

“It is certainly puzzling,” says Nikolaos Sparveris. This physicist works at Temple University in Philadelphia, Penn. He was part of the team that shared the new finding October 19 in Nature.

Other labs had also found it weirdly easy to stretch apart the quarks inside protons. But those data weren’t as convincing, Sparveris says. The stretchiness that his team measured was less extreme than in past experiments. But the new data also came with less uncertainty. That boosts confidence that protons are indeed strangely stretchy.

The new experiment took place at the Thomas Jefferson National Accelerator Facility. That’s in Newport News, Va. There, Sparveris’ team fired electrons at ultracold liquid hydrogen. The electrons scattered off protons in the hydrogen. That scattering revealed how quarks in those protons respond to electric fields.

For the most part, the quarks moved as expected when electric fields pulled them in opposite directions. But something odd happened when the electrons had higher energies. The quarks seemed to respond more strongly to an electric field than theory predicted. But this only happened for a small range of electron energies.

Explaining protons’ stretchiness would require coming up with new physics. “You need to be very, very inventive,” says Vladimir Pascalutsa. A theoretical physicist, he works at the Johannes Gutenberg University. That’s in Mainz, Germany.

Pascalutsa isn’t ready to rewrite the physics of protons just yet. He’s not sure if protons really are a stretchy as they seem in the new data. “I don’t want to kill the buzz,” he says. “But yeah, I’m quite skeptical.”

It will take more experiments to get theorists like Pascalutsa excited about stretchy protons. For instance, Sparveris hopes to do his experiment again but with one key difference. He would swap out electrons for positrons. (Positrons are just like electrons except they have a positive rather than negative electric charge.)

If electric fields made by positrons can tug quarks around as easily as fields made by electrons, that would provide more evidence of protons’ stretchiness.

James Riordon is a freelance science writer who covers physics, math, astronomy and occasional lifestyle stories.

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