Science Gazette

Protons are probably smaller than previously assumed


The findings show that previous measurements were misinterpreted.

A new measuring method revealed a few years ago that protons are likely smaller than previously thought since the 1990s. The difference astounded the scientific community, and several experts speculated that the Standard Model of particle physics would need to be revised. Physicists from the University of Bonn and the Technical University of Darmstadt have created a system that enables them to assess the findings of older and newer experiments in much more detail than previously possible. From the earlier data, this also results in a lower proton radius. As a result, regardless of the measuring technique used, there is likely no difference in the readings. Physical Review Letters published the research.

The atoms that make up our office chair, the oxygen we breathe, and the stars in the night sky are all made up of electrons, protons, and neutrons. Electrons are negatively charged particles with no expansion and a point-like shape, according to current understanding. Positively charged protons have a different radius, which according to current data is 0.84 femtometers (a femtometer is a quadrillionth of a meter).

They were supposed to be 0.88 femtometers until a few years ago, a little variance that generated quite a stir among academics. Because it wasn’t straightforward to convey. Some academics even thought it was proof that the Standard Model of particle physics was incorrect and needed to be changed. However, according to Prof. Dr. Ulf Meißner of the Helmholtz Institute for Radiation and Nuclear Physics at the University of Bonn, “our investigations suggest that this gap between the old and current observed values does not exist at all.” “Instead, the earlier figures were vulnerable to a systemic mistake that has been grossly underestimated to until point.”

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In the particle universe, billiards are being played

A proton’s radius may be determined by bombarding it with an electron beam in an accelerator. When an electron collides with a proton, both of their motions are reversed, analogous to two billiard balls colliding. This is known as elastic scattering in physics. The bigger the proton, the more likely it is to collide with another proton. The kind and extent of the scattering may therefore be used to determine its growth.

The more exact the readings, the greater the electron beam’s velocity. However, when the electron and proton meet, there is a greater chance that additional particles may arise. 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,” notes that “at high velocities or energy, this occurs more and more frequently.” “As a result, elastic scattering events are becoming more infrequent. As a result, for proton size studies, only accelerator data with very low energy electrons has been utilized so far.”

Collisions that create other particles, on the other hand, convey vital information about the geometry of the proton in theory. The same may be said for another event that happens at high electron beam speeds: electron-positron annihilation. Prof. Dr. Hans-Werner Hammer of TU Darmstadt states, “We have built a theoretical framework with which such occurrences may likewise be utilized to compute the proton radius.” “This enables us to include data that has previously been overlooked.”

5% less than expected after 20 years

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The physicists used this strategy to reexamine results from earlier and more current tests, including some that had previously indicated a value of 0.88 femtometers. The researchers got at 0.84 femtometers using their approach, which is the same radius discovered in fresh measurements using a totally different methodology.

As a result, the proton looks to be around 5% smaller than previously thought in the 1990s and 2000s. Simultaneously, the researchers’ approach reveals fresh details about the fine structure of protons and their uncharged brothers, neutrons. As a result, it’s assisting us in better comprehending the structure of the environment around us — the chair, the air, and even the stars in the night sky.


The German Research Foundation (DFG), the Chinese National Natural Science Foundation (NSFC), the Volkswagen Foundation, the EU Horizon 2020 initiative, and the German Federal Ministry of Education and Research all contributed to the research (BMBF).

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