Protons may be more stretchy than they need to be.
Subatomic particles are made up of smaller particles called quarks. These particles are linked together by strong interactions, also known as the strong force. New experiments show that the quarks can be found in subatomic particles. Respond more than expected to an electrical field pulling on themNikolaos Sparveris, physicist, and his colleagues report on October 19, Nature. The result suggests that the strong force isn’t quite as strong as theory predicts.
It’s a finding at odds with the standard model of particle physics, which describes the particles and forces that combine to make up us and everything around us. The result has some physicists stumped about how to explain it — or whether to even try.
“It is certainly puzzling for the physics of the strong interaction, if this thing persists,” says Sparveris, of Temple University in Philadelphia.
Such stretchiness has turned up in other labs’ experiments, but wasn’t as convincing, Sparveris says. He and his colleagues found a less extreme stretchiness than previous experiments. However, there was also less experimental uncertainty. That increases the researchers’ confidence that protons are indeed stretchier than theory says they should be.
The team conducted a probe of protons at the Thomas Jefferson National Accelerator Facility, Newport News, Va. by firing electrons at ultracold liquid hydrogen. The hydrogen’s protons were revealed by electrons scattering. the protons’ quarksReact to electric fieldsSN: 9/13/22). Higher electron energies mean that researchers can see deeper into protons and more information is revealed by electrons about the strong force inside them.
The quarks moved in the expected manner when the electric interactions pulled the particles towards opposite directions. The quarks did respond to an electric field more strongly than was predicted by theory at one point.
But it only happened for a small range of electron energies, leading to a bump in a plot of the proton’s stretch.
“Usually, behaviors of these things are quite, let’s say, smooth and there are no bumps,” says physicist Vladimir Pascalutsa of the Johannes Gutenberg University Mainz in Germany.
Pascalutsa says he’s often eager to dive into puzzling problems, but the odd stretchiness of protons is too sketchy for him to put pencil to paper at this time. “You need to be very, very inventive to come up with a whole framework which somehow finds you a new effect” to explain the bump, he says. “I don’t want to kill the buzz, but yeah, I’m quite skeptical as a theorist that this thing is going to stay.”
Pascalutsa believes that it will take many more experiments to get theorists excited about strangely stretchy protons. He could get his wish if Sparveris’ hopes are fulfilled to try the experiment again with positrons, the antimatter version of electrons, scattered from protons instead.
Pascalutsa suggests that a different experiment might make stretchy protons seem more compelling. This could be done by a forthcoming study from Switzerland’s Paul Scherrer Institute, Villigen. It will use hydrogen atoms that have muons in place of the electrons that usually orbit atoms’ nuclei. Muons are about 200 times as heavy as electrons, and orbit much closer to the nucleus of an atom than do electrons — offering A closer look inside the proton (SN: 10/5/17). The experiment would involve stimulating the “muonic hydrogen” with lasers rather than scattering other electrons or positrons from them.
“The precision in the muonic hydrogen experiments will be much higher than whatever can be achieved in scattering experiments,” Pascalutsa says. If the stretchiness turns up there as well, “then I would start to look at this right away.”
