Protons May Be Smaller Than Thought Upsetting Theory

The proton, a building block of matter, appears to be smaller than previously thought, according to researchers who say the finding may open new questions about how sub-atomic particles interact.

A team led by Aldo Antognini, a physicist at the Max Planck Institute for Quantum Optics in Zurich, used laser spectroscopy to study the proton’s charge radius in a hydrogen atom that had been reconstituted to allow a clearer look. They found the radius differed by about 4 percent from previous measurements, according to a report in the journal Science.

Five research groups worldwide are now working to determine why the different measurements exist and what they mean, Antognini said in a telephone interview. The result could spur a reassessment of certain keystone constants used since the late 1940s to outline the electromagnetic force controlling the actions of sub-atomic particles.

“It’s important to have the proton right to push the science farther,” Antognini said. The experiment confirms a similar result the scientists showed in 2010.

Hydrogen is a unique and important molecule for physics, because, with just a single electron and proton, it’s too small to hide anything, Antognini said. Quantum mechanics and electrodynamics both have emerged from physicists studying the interactions with hydrogen atoms.

Three methods have been used to measure protons in the past. Scientists first measured protons by using electron scattering, a strategy that shakes an electron beam into hydrogen gas. The collisions between the electrons and the protons, along with the scatter, allowed scientists to deduce the proton radius. A second method used hydrogen spectroscopy, which measured energy levels of electrons to deduce how large the proton might be.

Heavy Muons

The most recent method uses a particle called a muon, which is 200 times heavier than an electron but carried many of the same properties. By firing muons at the hydrogen, the team was sometimes able to get the muons to replace the electrons orbiting the proton. The researchers then extrapolated the proton’s size from the muon’s energy levels, with the result showing it was about 4 percent smaller than previously thought.

That discrepancy may seem small to laymen, but it’s significant to physicists, Antognini said, comparing it to standing a tiny crack in a dam filled with water. The amount of water creeping through the crack is “negligible compared to the water on the other side, but it may indicate a larger problem.”

If confirmed the new measurement may introduce new physics theories to be explored to explain the discrepancy, he said, adding that the finding may be due to some kind of an error, or it may be a basic problem with physics theories as they currently exist, he said. “We want to be very, very cautious,” he said.

(Corrects description of hydrogen in fifth paragraph of story published Jan. 24.)
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