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 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. "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
Tags:
Physics