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.
The radius of a protons, according to current measurements, is 0.84 femtometers. Until a few years ago, however, they were thought to be 0.88 femtometers.
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.
To determine the radius of a proton, one can bombard it with an electron beam in an accelerator. When an electron collides with the proton, both change their direction of motio. In physics, this process is called elastic scattering. The larger the proton, the more frequently such collisions occur. Its expansion can therefore be calculated from the type and extent of the scattering.
The higher the velocity of the electron beam, the more precise the measurements. However, this also increases the risk that the electron and proton will form new particles when they collide. Therefore, for measurements of the proton size, one has so far only used accelerator data in which the electrons had a relatively low energy.
In principle, however, collisions that produce other particles also provide important insights into the shape of the proton. The same is true for another phenomenon that occurs at high electron beam velocities, so-called electron-positron annihilation.(ScitechDaily)