Protons Are Smaller Than We Think

Energy atom close up

Protons Are Smaller Than We Think

The chair we sit on, the air we breathe, the stars in the sky… They are all made up of atoms. Atoms are also made up of electrons, protons and neutrons. Electrons are negatively charged; point-like structures that do not exhibit expansion according to the available information. The positively charged protons are different: according to current measurements, their diameter is 0.84 femtometers (a femtometer is one quadrillionth of a metre).

But until a few years ago, protons were thought to be 0.88 femtometers. This slight difference has created a lot of controversy among experts. Because it is not easy to explain. Some experts even thought that this showed that the standard model of particle physics was flawed and needed to be edited. Working at the Helmholtz Institute of Radiation and Nuclear Physics, University of Bonn, Prof. Dr. “But our analyzes show that there is no difference at all between the old and new measurement values,” says Ulf Meißner. “Older measurements were subject to a systematic error that has hitherto been significantly overlooked.”

Playing pool in the particle universe

In determining the diameter of a proton, the proton needs to be bombarded with electrons in a particle accelerator. When an electron collides with a proton, the direction of motion of both bodies changes; just like billiard balls. In physics, this process is known as flexible scattering. The larger the proton, the more often such collisions occur. Therefore, the expansion rate of the proton can be calculated from the type and size of the scattering.

The higher the speed of the electron beam, the more precise the measurements. But this situation also increases the danger of electron and proton forming new particles when they collide. “We see this much more often at higher speeds or energies,” Meißner says. “In turn, flexible scattering events are rarer. That’s why measurements at the proton size have only used acceleration data, where electrons have lower energy, so far.”

But collisions, which in principle produce other particles, also provide important determinations of the shape of the proton. The same is true of another phenomenon called electron-positron annihilation, caused by high-speed electron beams. Working at the Technical University of Darmstadt, Prof. Dr. “We’ve developed a theoretical basis that could allow events like these to be used in calculating the proton diameter as well,” says Hans Werner Hammer. “So we also took into account data that had been neglected until now.”

5 percent smaller

Using a newly developed method, the researchers reanalyzed the readings from both previous and recent experiments. These include experiments in which a value of 0.88 was previously suggested. But scientists have reached 0.84 femtometers in the new method they used. This diameter is also found in new measurements based on a completely different methodology.

So the proton appears to be actually about 5 percent smaller than the value assumed in the 1990s and 2000s. The researchers’ method also provides new insights into the delicate nature of protons and their uncharged siblings, neutrons. Therefore, the world around us; it helps us understand a little better the structure of the chair, the air, and the stars in the sky.



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