Proton's radius revised downward

Only in physics can a few quintillionths of a meter be cause for uneasy excitement. A new measurement finds that the proton is about 4 percent smaller than previous experiments suggest. The study, published in the Jan. 25 issue of Science, has physicists cautiously optimistic that the discrepancy between experiments will lead to the discovery of new particles or forces.

“Poking at small effects you can’t explain can be a way of unraveling a much bigger piece of physics,” says Carl Carlson, a theoretical physicist at the College of William and Mary in Williamsburg, Va., who was not involved in the study. “And this case is particularly intriguing.”

For years, physicists have used two indirect methods to determine the size of the proton. (Unfortunately, there is no such thing as a subatomic ruler.) They can fire an electron beam at protons and measure how far the flying particles get deflected. Alternatively, physicists can study the behavior of electrons in hydrogen atoms. They shoot a laser at an atom so that the one electron jumps to a higher, unstable energy level; when the electron returns to a low-energy state, it releases X-rays whose frequency depends on the size of the proton. Both methods suggest the proton has a radius of about 0.88 femtometers, or 0.88 quadrillionths of a meter.

That measurement was not in doubt until 2010, when physicist Aldo Antognini at ETH Zurich and his team developed a new technique to probe proton size. They also used hydrogen atoms, but replaced the electrons with muons — particles similar to electrons but more than 200 times as massive. Muons’ additional heft enhances their interaction with protons and makes their behavior more dependent on proton size. After measuring the X-rays emitted by muons shifting between energy states, Antognini’s team published a paper in Nature saying that the proton radius is 0.84 femtometers — about 4 percent less than previous estimates (SN 7/31/10, p. 7).

Now, two-and-a-half years later, the team has reexamined muon-containing hydrogen and measured the X-ray frequencies resulting from two energy level shifts. Both emissions yielded the same, slimmed-down value for the size of the proton. The new study eliminates the possibility of certain systematic errors and reduces the measurement’s uncertainty by 40 percent.

“This shows that our experiment is consistent and that there were no mistakes,” Antognini says.

Carlson agrees, although he says physicists may still be overlooking an error in either the muon or electron experiments. Researchers are on the case, scouring the details of each experiment in the hopes of a consistent value for proton size.

Yet as a theorist Carlson can’t help but entertain the possibility that new physics, not human error, causes the varying size measurements. According to the standard model of physics, electrons and muons should differ only in mass, but Carlson and other theorists are exploring the possibility that there is a yet-undiscovered particle that interacts only with muons. “It would certainly shake things up,” he says.

Researchers are desperate to discover new physics because, while successful in describing most of what we see in everyday life, the standard model is terrible at describing phenomena such as gravity at small scales and the accelerating expansion of the universe.

The best test for theorists’ ideas could come in two or three years, when physicists hope to introduce yet another independent method of determining proton size. John Arrington, a physicist at Argonne National Laboratory in Illinois, is helping to develop a muon beam that would be fired at protons. If such an experiment yields similar results to the muonic hydrogen one, Arrington says, then it’s likely that new physics is at work. “That,” he says, “is the most intriguing possibility.”