Study suggests errors in interpreting old measurements
A few years ago, a new measurement technique showed that protons are probably smaller than had been assumed since the 1990s. The discrepancy surprised the scientific community; some researchers even thought that the standard model of particle physics should be changed. Physicists from 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 newer experiments much more comprehensively than before. This also results in a smaller proton radius from the older data. There is therefore probably no difference between the values, regardless of the measurement method on which they are based. The study appeared in Physical Review Letters.
Our office chair, the air we breathe, the stars in the night sky: they are all made up of atoms, which are themselves made up of electrons, protons and neutrons. Electrons are negatively charged; according to current knowledge, they have no expansion, but are punctual. Positively charged protons are different – according to current measurements, their radius is 0.84 femtometers (a femtometer is one quadrillionth of a meter).
Until a few years ago, however, they were thought to be 0.88 femtometers – a small difference that caused quite a stir among experts. Because it was not so easy to explain. Some experts even considered it an indication that the Standard Model of particle physics was flawed and needed to be changed. “However, our analyzes indicate that this difference between the old and new measured values does not exist at all,” explains Professor Ulf Meißner from the Helmholtz Institute for Radiation and Nuclear Physics at the University of Bonn. “Instead, the old values were subject to systematic error that has been significantly underestimated until now.”
Play pool in the cosmos of particles
To determine the radius of a proton, it can be bombarded with an electron beam in an accelerator. When an electron collides with the proton, the two change direction of motion, like the collision of two billiard balls. In physics, this process is called elastic scattering. The larger the proton, the more frequent these collisions are. Its expansion can therefore be calculated from the type and extent of scattering.
The higher the speed of the electron beam, the more accurate the measurements. However, it also increases the risk of the electron and proton forming new particles when they collide. “At high speeds or energies, this happens more and more often,” explains Meißner, who is also a member of the transdisciplinary research areas “Mathematics, Modeling and Simulation of Complex Systems” and “Building Blocks of Matter and fundamental interactions”. “In turn, elastic scattering events become rarer. Therefore, for proton size measurements, only accelerator data in which the electrons have relatively low energy have been used so far. .”
In principle, however, 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, which is called electron-positron annihilation. “We have developed a theoretical basis with which such events can also be used to calculate the radius of the proton,” explains Professor Hans-Werner Hammer from TU Darmstadt. “It allows us to take into account data that has so far been overlooked.”
Five percent smaller than expected 20 years
Using this method, 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.
Thus, the proton actually appears to be around 5% smaller than was assumed in the 1990s and 2000s. At the same time, the researchers’ method also enables new insights into the fine structure of protons and their siblings and uncharged sisters, neutrons. So it helps us better understand the structure of the world around us — the chair, the air, but also the stars in the night sky.
The study was funded by the German Research Foundation (DFG), the National Natural Science Foundation of China (NSFC), the Volkswagen Foundation, the EU Horizon 2020 program and the German Federal Ministry of Education and Welfare. Research (BMBF).
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Material provided by University of Bonn. Note: Content may be edited for style and length.