Protons can be more elastic than they should be.
Subatomic particles are made up of smaller particles called quarks, which are held together by a powerful interaction known as the strong force. New experiments seem to show that quarks respond more than expected to an electric field by pulling on themPhysicist Nikolaos Sparveris and colleagues report Oct. 19 in Nature. The result suggests that the strong force is not as strong as theory predicts.
It’s a finding that stands in stark contrast to the standard model of particle physics, which describes the particles and forces that combine to form us and everything around us. The result has some physicists stumped on how to explain it, or even whether to try.
“It certainly is puzzling for the physics of the strong interaction, if this persists,” says Sparveris, of Temple University in Philadelphia.
Such elasticity has shown up in experiments from other labs, but it wasn’t nearly as convincing, says Sparveris. The elasticity that he and his colleagues measured was less extreme than in previous experiments, but it also came with less experimental uncertainty. That increases researchers’ confidence that protons are more elastic than theory says they should be.
At the Thomas Jefferson National Accelerator Facility in Newport News, Virginia, the team probed protons by shooting electrons at an ultracold liquid hydrogen target. Electrons scattering off protons in hydrogen revealed how the quarks of the protons respond to electric fieldsSerial number: 09/13/22). The higher the energy of the electrons, the deeper the researchers could see into the protons, and the more the electrons revealed about how the strong force inside protons works.
For the most part, the quarks moved as expected when electrical interactions pushed the particles in opposite directions. But at one point, as the energy of the electrons increased, the quarks seemed to respond more strongly to an electric field than the theory predicted.
But it only happened for a small range of electron energies, causing a bump in the graph of proton stretching.
“Usually the behavior of these things is quite, shall we say, smooth and there are no bumps,” says physicist Vladimir Pascalutsa of the Johannes Gutenberg University of Mainz in Germany.
Pascalutsa says he’s often itching to dive into puzzling problems, but the strange elasticity of protons is too sketchy for him to put pen to paper at this point. “You have to be very, very inventive to come up with a whole framework that somehow finds you a new effect” to explain the bump, he says. “I don’t want to kill the buzz, but yeah, I’m pretty skeptical as a theorist that this is going to stick.”
More experiments will be needed to get theorists like him excited about unusually elastic protons, says Pascalutsa. He might get his wish if Sparveris’s hopes of retrying the experiment with positrons, the antimatter version of electrons, scattered from protons are fulfilled.
An entirely different kind of experiment could make stretchy protons more convincing, says Pascalutsa. An upcoming study from the Paul Scherrer Institute in Villigen, Switzerland, could solve the problem. It will use hydrogen atoms that have muons instead of the electrons that normally orbit the nuclei of atoms. Muons are about 200 times heavier than electrons and orbit much closer to the nucleus of an atom than electrons do, offering a closer look at the proton inside (Serial number: 5/10/17). The experiment would involve stimulating “muonic hydrogen” with lasers rather than scattering other electrons or positrons from them.
“The precision in muonic hydrogen experiments will be much higher than anything that can be achieved in scattering experiments,” says Pascalutsa. If elasticity shows up there as well, “then I would start looking at this right away.”