Protons Are Smaller

MIT Universal Cloud press
MIT Universal Cloud press

Research by the University of Bonn and TU Darmstadt shows that there are errors in the interpretation of old measurements. A few years ago, a new measurement technique showed that protons were probably smaller than had been assumed since the 1990s. The inconsistency baffled the scientific community.

Some researchers even believed that the Standard Model of particle physics needed to be changed. Physicists at the University of Bonn and the Technical University of Darmstadt have developed a method that allows them to analyze the results of older and newer experiments much more thoroughly than before.

This also resulted in a smaller proton radius than the old data.

Therefore, it turned out that no matter what measurement method they were based on, there would probably be no difference between the values.

The study was published in Physical Review Letters.

Everything we see around us is the air we breathe, the stars in the night sky are all made of atoms made up of electrons, protons and neutrons.

Electrons are negatively charged. According to available information, they also do not expand. They have mass. They are considered in the category of elementary particles. They do not have any subparticles. We can also express them as dot-like.

The positively charged protons are different – ​​their radius is 0,84 femtometers (a femtometer is one quadrillionth of a meter), according to current measurements.

Proton Quarks and Gluons x
The proton (red) has a radius of 0.84 femtometers (fm). The figure also shows the three quarks that make up the proton and the gluons that hold them together. Credits: Dr. Yong-Hui Lin/Bonn University

However, until a few years ago these were thought to be 0,88 femtometers. It was a small difference that caused quite a stir among the experts. Because it wasn't that easy to tell. Some experts even thought it was an indication that the Standard Model of particle physics was wrong and needed to be changed.

Professor of the Helmholtz Institute for Radiation and Nuclear Physics, University of Bonn. Dr. “However, our analyzes show that this difference between the old and newly measured values ​​does not exist at all,” explains Ulf Meißner. “Instead, the old values ​​were subject to a systematic error that has hitherto been significantly underestimated.”

So we can say like this. Since we are not measuring anything, one might think that this confusion about measuring something extraordinarily small should be normal. Because the particle is our building block. After this short break, let's go back to our article.

How is the Radius of the Proton Determined?

To determine the radius of a proton, it can be bombarded with a beam of electrons in particle accelerators.

When an electron collides with a proton, both change their direction of motion.

We can compare this to two billiard balls colliding with each other. In physics, this process is called elastic scattering. It is covered in the topic of momentum. Of course, calculations in collisions that occur in subjects covered at high school level are made by considering Newtonian Mechanics.

The larger the proton, the more often such collisions occur. Therefore, its expansion can be calculated from the type and size of the scattering.

The higher the velocity of the electron beam, the more precise the measurements. But this also increases the risk of electron and proton forming new particles when they collide. Because the purpose here is to measure on the existing proton before new particles are formed. It is also known that the proton consists of quarks with half-lives. It is not a fundamental particle like the electron.

Elastic Collision of Electron and Proton

“At higher speeds or energies, this happens more and more often,” explains Meißner, who is also a member of the Interdisciplinary Research Areas “Mathematics, Modeling and Simulation of Complex Systems” and “The Building Blocks of Matter and Fundamental Interactions.”

“In contrast, elastic scattering events are becoming rarer. Therefore, only accelerator data, where electrons have a relatively low energy, have been used so far for measurements of proton size.

However, in principle, collisions that produce other particles also provide important information about the shape of the proton.

The same is true for another phenomenon that occurs at high electron beam velocities. We might call this the electron-positron annihilation.

prof. "We've developed a theoretical basis on which such events can also be used to calculate the proton radius," says Hammer. “This allows us to account for data that has been left out so far.”

Using this method, the physicists reanalyzed readings from earlier and very recent experiments, including those that previously suggested a 0.88 femtometer value. However, the researchers achieved 0.84 femtometers with their method.

This measurement is the radius that is also found in the new measurements based on a completely different methodology.

So the proton actually looks about 1990 percent smaller than was assumed in the 2000s and 5s. At the same time, the researchers' method opens up new insights into the fine structure of protons and their neutral siblings, neutrons.

So it helps us understand the structure of the world around us a little better. We will be able to better understand the air we breathe and our universe from the chair we sit on.

Source: Physics Review Letters

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