One possibility is that protons are more elastic than standard physics dictates.

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Maybe protons have more give than they should.

Strong force is a powerful contact that holds the subatomic particles together, which are constructed from smaller particles called quarks. New tests, conducted by physicist Nikolaos Sparveris and colleagues, and published on October 19 in Nature, appear to reveal that quarks respond more than predicted to an electric field pushing on them. This finding may indicate that the strong force is weaker than previously thought.

This discovery runs counter to the accepted theory of elementary particles and the forces that bind them together to form living things and the physical universe. Some physicists are at a loss as to how to explain the outcome, or whether to even attempt.

According to Sparveris of Temple University in Philadelphia, "if this phenomenon endures, it is undoubtedly troubling for the physics of the strong interaction."

Similar elasticity has been shown in trials at other labs, but those results were less compelling, according to Sparveris. There was reduced experimental uncertainty in the measured stretchiness compared to prior tests because to his and his colleagues' careful measurement techniques. Researchers now have more faith that protons are truly more malleable than predicted by theory.

Researchers in Newport News, Virginia's Thomas Jefferson National Accelerator Facility shot electrons at a target of ultracold liquid hydrogen to investigate protons. The quarks of protons may be studied in response to electric fields by seeing how they scatter electrons in hydrogen (SN: 9/13/22). More information about the strong force's inner workings in protons was disclosed by the electrons as their energy increased.

When subjected to electromagnetic interactions that tugged in opposing ways, quarks behaved predictably. At one point, however, when the electron energy was increased, the quarks appeared to react more strongly to an electric field than was previously thought to be the case.

However, this humps in a plot of the proton's stretch over just a certain range of electron energies.

According to physicist Vladimir Pascalutsa of Germany's Johannes Gutenberg University Mainz, "usually, behaviors of these objects are fairly, let's say, smooth and there are no bumps."

Pascalutsa claims that he is normally keen to delve headfirst into challenging issues, but that the peculiar stretchiness of protons is now too uncertain for him to commit pen to paper. To explain the spike, he adds, "you need to be very, very innovative to come up with a complete framework that somehow gets you a new result." "I don't want to put a damper on things, but as a theorist, I'm rather dubious that this phenomenon is here to stay," he said.

Pascalutsa thinks that additional tests are needed before theoretical physicists like his can get enthusiastic about extraordinarily stretchable protons. If Sparveris's aspirations of redoing the experiment using positrons, the antimatter equivalent of electrons, dispersed from protons, come true, he may receive his desire.

Pascalutsa posits that a different kind of experiment might increase the appeal of stretchable protons. Perhaps a research to be released soon by the Paul Scherrer Institute in Villigen, Switzerland, will do the trick. Hydrogen atoms with muons instead of the customary electrons in the atomic nucleus's outermost orbit will be utilized. Muons are nearly 200 times heavier than electrons, yet their tighter orbit around the nucleus of an atom allows for a clearer view of the proton within (SN: 10/5/17). Instead than using lasers to deflect electrons or positrons from the "muonic hydrogen," the experiment would be designed to stimulate it.

Pascalutsa predicts that the accuracy of experiments with muonic hydrogen will much surpass that of scattering experiments. If the elasticity appears there as well, "then I would begin to look at this immediately."

CITATIONS

• R. Li et al. Measured proton electromagnetic structure deviates from theoretical predictions. Nature. Published online October 19, 2022. doi: 10.1038/s41586-022-05248-1.