Protons Are Probably Actually Smaller Than Long Thought

A few years ago, a novel measurement technique showed that protons are probably smaller than had been assumed since the 1990s. The discrepancy surprised the scientific community; some researchers even believed that the Standard Model of particle physics would have to be changed. Physicists at the University of Bonn and the Technical University of Darmstadt have now developed a method that allows them to analyze the results of older and more recent experiments much more comprehensively than before. This also results in a smaller proton radius from the older data. So there is probably no difference between the values — no matter which measurement method they are based on. The study appeared in Physical Review Letters.

[…] Using this method, the physicists reanalyzed readings from older, as well as very recent, experiments — including those that previously suggested a value of 0.88 femtometers. With their method, however, the researchers arrived at 0.84 femtometers; this is the radius that was also found in new measurements based on a completely different methodology. So the proton actually appears to be about 5 percent smaller than was assumed in the 1990s and 2000s. At the same time, the researchers’ method also allows new insights into the fine structure of protons and their uncharged siblings, neutrons. So it’s helping us to understand a little better the structure of the world around us — the chair, the air, but also the stars in the night sky.