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. 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. The study appeared in Physical Review Letters.
Our office chair,
the air we breathe, the stars in the night sky: they are all made of atoms,
which in turn are composed of electrons, protons, and neutrons. Electrons are
negatively charged; according to current knowledge, they have no expansion but
are point-like. The positively charged protons are different - according to
current measurements, their radius is 0.84 femtometers (a femtometer is a
quadrillionth of a meter).
Until a few years
ago, however, they were thought to be 0.88 femtometers - a tiny difference that
caused quite a stir among experts. Because it was not so easy to explain. Some
experts even considered it to be an indication that the Standard Model of
particle physics was wrong and needed to be modified. "However, our
analyses indicate that this difference between the old and new measured values
does not exist at all," explains Prof. Dr. Ulf Meißner from the Helmholtz
Institute for Radiation and Nuclear Physics at the University of Bonn.
"Instead, the older values were subject to a systematic error that has
been significantly underestimated so far."
Playing billiards
in the particle cosmos
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
motion - similar to the collision of two billiard balls. 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.
"At high velocities 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, the elastic scattering events are becoming rarer. Therefore, for measurements of the proton size, one has so far only used accelerator data in which the electrons had 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. "We have developed a theoretical basis with which such
events can also be used to calculate the proton radius," says Prof. Dr.
Hans-Werner Hammer of TU Darmstadt. "This allows us to take into account
data that have so far been left out."
Five percent smaller than assumed 20 years
Using this method,
the 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.
So the proton
actually appears to be about 5 percent smaller than was assumed in the 1990s
and 2000s. At the same time, the researchers' method also allows new insights
into the fine structure of protons and their uncharged siblings, neutrons. So
it's helping us to understand a little better the structure of the world around
us - the chair, the air, but also the stars in the night sky.
Reference:
Yong-Hui Lin, Hans-Werner Hammer and Ulf-G. Meißner: New insights into the nucleon's electromagnetic structure; Physical Review Letters, DOI: 10.1103/PhysRevLett.128.052002