A new method shows that protons are even smaller than we thought

Protons are incredibly small. One femtometer is the measurement for a quadrillionth of a meter, and recent measurements show that protons have a radius of 0.84 femtometers.

Only a few years ago, however, the small particles were believed to measure 0.88 femtometers. Though this tiny difference is so small it is practically imperceptible, it caused a great deal of discussion within the scientific community with some even calling for changes to the Standard Model of particle physics.

Now, physicists from the University of Bonn and the Technical University of Darmstadt developed a method that allowed them to revise old and new measurement studies with unprecedented accuracy, a press statement reveals. The results suggest there may have been errors in the interpretation of older data, meaning that both measurements were correct, but the newer results, from 1990, likely provided the correct interpretation. The researchers published their findings in Physical Review Letters.

"Our analyses indicate that [the] difference between the old and new measured values does not exist at all," explains Prof. Dr. Ulf Meißner from the Helmholtz Institute for Radiation and Nuclear Physics at the University of Bonn. "Instead, the older values were subject to a systematic error that has been significantly underestimated so far."

Measuring a microscopic particle

Protons, alongside neutrons, make up our everyday matter, meaning the new findings could have widespread implications when it comes to our understanding of particle physics. To measure the radius of a proton, researchers bombard the particle with an electron beam using an accelerator. Once an electron collides with the proton, both change direction in a process called elastic scattering. This occurs more often the larger a proton, meaning the particle's expansion can be calculated by measuring how much elastic scattering is taking place inside the accelerator.

The researchers formed a theoretical basis that takes into account the fact that the electron and proton can form new particles when they collide. This is a phenomenon that meant previous measurements could only be made using accelerator data in which electrons had relatively low energy. 

"We have developed a theoretical basis with which such events can also be used to calculate the proton radius," says Prof. Dr. Hans-Werner Hammer of TU Darmstadt. "This allows us to take into account data that have so far been left out." Using their new method, they reanalyzed older readings as well as the newer ones. They confirmed that the proton appears to be roughly 5 percent smaller than was previously believed in the 1990s and the 2000s.