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).