A precise proton measurement helps put a core theory of physics to the test

For over a decade, confusion over the size of the proton has held scientists back. Disagreeing measurements of the subatomic particle’s radius meant that scientists couldn’t test one of their key theories with the extreme precision they aimed for.

A new measurement pegs the radius of the proton precisely enough to enable a test of the standard model of particle physics, which describes subatomic particles and their interactions. The theory agreed with the experiment to better than a tenth of a billionth of a percent, physicist Lothar Maisenbacher and colleagues report February 11 in Nature.

The researchers studied hydrogen atoms, measuring the frequency of the radiation needed to make the atom jump between two different energy levels. That information, combined with other measurements, revealed the proton’s radius was about 0.84 trillionths of a millimeter. 

That figure agrees with a host of measurements that suggest the proton’s size is smaller than once thought, and the measurement is precise enough to rule out the approximately 4 percent larger radius found by some earlier experiments.

That confirmation that the proton’s radius is small allowed the researchers to use their data to test the standard model. The standard model can predict the frequency of the radiation needed to make the atom jump between the energy levels in their experiment. But an independent measurement of the proton radius is needed. Now that the researchers had differentiated between the large and small values, they were free to use another measurement of the proton’s radius that favors the smaller size, made with an exotic type of hydrogen called muonic hydrogen. This is a proton bound to a heavy cousin of the electron, called a muon.

The standard model prediction matched the experiment, vindicating the theory. Specifically, it verified a pillar of the theory called quantum electrodynamics, which describes the interactions of electrically charged particles and light.

Scientists eventually expect to find a test that the standard model fails, says Maisenbacher, who performed the work at the Max Planck Institute of Quantum Optics in Garching, Germany. The theory doesn’t explain phenomena such as dark matter, the invisible substance that helps bind galaxies together. “These tests are important because we know that our understanding of the world is not complete.”