The incredible shrinking proton

Subatomic particle may be smaller than theory dictates

Nothing is immune to downsizing in these tough economic times — not even subatomic particles. New measurements published in the July 8 Nature suggest that the proton has a radius about 4 percent smaller than previously thought.

MEASURE TWICE Measurements made with lasers suggest the proton is smaller than previously thought. F. Reiser and A. Antognini/Paul Scherrer Institute

The result could just be a mistake. But if confirmed, a smaller proton could have enormous implications, scientists say.

“If this result holds up there’s something drastically wrong,” says Jeff Flowers of the National Physical Laboratory in England.

It could be that there’s a problem with quantum electrodynamics, a major unifying theory that’s been called the jewel of physics. QED basically describes how light and matter interact by incorporating Einstein’s special relativity into the realm of quantum mechanics.

“That opens the door for a major advancement in theory,” Flowers says.

To get the new measurements, a team led by Randolf Pohl of the Max Planck Institute of Quantum Optics in Garching, Germany, created an exotic form of hydrogen in which the atom’s lone electron is replaced by a particle known as a muon. Muons have the same charge as electrons but are about 200 times heavier, so they orbit much closer to the hydrogen atom’s nucleus. This coziness enhances the muon’s interaction with the proton at the atom’s center, allowing researchers to probe the proton in greater detail than they could with ordinary hydrogen.

In their experiments, the scientists aimed a beam of muons at hydrogen gas, creating muonic hydrogen. Whenever a muon was detected in the gas, the team fired a laser at the muon, hoping to bump it up to a higher energy level. Measuring the gap between the muon’s first and second energy levels — known as the Lamb shift — would allow the team to calculate the size of the proton’s radius.

Yet after years of fiddling with the muon beam and laser arrangements, the team still wasn’t having any luck. The laser had been tuned so that it could measure the proton’s radius if the value fell within the range of 0.87 to 0.91 femtometers, in line with QED. But by tuning the laser to work with a smaller proton, the team finally started seeing results. Their estimate of the proton’s radius: just over 0.84184 femtometers.

“There was no signal till the last three weeks before the experiment would have been stopped,” says study coauthor Aldo Antognini of the Paul Scherrer Institute in Villigen, Switzerland. “It was like in a Hollywood movie where everything goes bad till five minutes before the end.” 

The new proton radius is 10 times more precise than previous estimates but well outside their range, which puzzles physicists.

“Presumably somebody made a mistake,” says Pohl. “But everybody’s convinced that nobody made a mistake, so it’s really intriguing. The measurements conflict with each other, but the question is now, how do you solve this problem?”

Physicists are already doing experiments with the hope of resolving the discrepancy, and theorists may have to revisit their numbers.

“Either one of the experiments is wrong, or the calculations are wrong,” says Pohl. “If it turns out that none of these is wrong, then one has to, maybe at some point in the far future, declare that QED is wrong, which would be really interesting. But we are not that far yet.”

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