Proton is Smaller Than Thought, New Measurement Finds

This laser apparatus used to test the size of the proton found that the particle's radius is about 4 percent smaller than thought. CREDIT: F. Reiser and A. Antognini, Paul-Scherrer-Institute

The proton, one of the most well-known and basic building blocks of matter, turns out to be holding on to a few secrets. A new measurement found that the radius of the proton is about 4 percent smaller than previously thought.

Protons are positively charged elementary particles. Together with neutrons and electrons, they make up the atoms that build our universe.

Scientists discovered the surprising anomaly by shooting laser beams at an exotic version of a hydrogen atom, which most often consists of one proton and one electron. The new measurement has improved the accuracy of the known proton radius by a factor of ten, the researchers said.

The finding means that either the theory governing how light and matter interact (called quantum electrodynamics, or QED) must be revised, or that a constant used in many fundamental calculations is wrong, the researchers said.

The scientists detailed their discovery in the July 8 issue of the journal Nature.

"The authors’ measurement uses a novel method that is more sensitive than any of the earlier methods," wrote Jeff Flowers of the U.K.'s National Physical Laboratory in an accompanying essay in the same issue of Nature. "But it gives a result that is significantly discrepant from that obtained by the next most accurate method, throwing doubt on the QED calculations that underlie both methods."

Flowers was not involved in the new measurement.

Exotic hydrogen

In the experiment, the researchers used a special version of hydrogen that contains one proton and one muon – an exotic cousin of the electron that weighs about 200 times more than an electron. The muon, just like an electron, is a point-like particle that orbits around the more extended proton.

"Think of the proton as a fuzzy cloud of charge," described the lead researcher, Randolf Pohl of Germany's Max-Planck Institute of Quantum Optics. "It's not a hard surface, not a ball."

In fact, the muon can even pass straight through the proton, which contains lots of open space between its constituent building blocks – three particles called quarks.

The muon can exist in different energy states that affect the way it orbits the proton. The size of the proton affects these states and how much energy is required to knock a muon out of one and into another.

And these effects are amplified by the larger mass of the muon compared to an electron, allowing the researchers a chance to peer into the orbital mechanics of the atom.

Blasting with lasers

To home in on the size of the proton, the scientists finely tuned a laser beam to blast their hydrogen atoms with very specific amounts of energy, hoping to stimulate the muons to jump from one energy state to another.

For a long time, they observed no effect in the range they expected, and assumed their laser was faulty. Finally the researchers tried an energy range completely removed from the expected region, and found exactly the transition they were looking for.

"When it wasn't in the reasonable region, we extended our search region to the unreasonable, and then we had this indication of a signal," Pohl told LiveScience. "We were really stunned."

If the new value is confirmed, it could mean some rewriting of basic physics is in order.

Perhaps the value of the so-called Rydberg constant, which is used to calculate the proton's size, is off. If that is the case, other fundamental calculations will need revising, too.

Or, perhaps the entire theory describing this and other particles – quantum electrodynamics – is misunderstood.

"If experimental discrepancies are confirmed rather than errors being found, high-accuracy work such as that by Pohl and colleagues, not the high-energy collisions of giant accelerators, may have seen beyond the standard model of particle physics," Flowers wrote.

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