Protons are probably actually smaller than long thought
Study suggests errors in the interpretation of older measurements

Date:
February 6, 2022
Source:
University of Bonn
Summary:
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 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.



FULL STORY ==========================================================================
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.

Funding: 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 Research (BMBF).

========================================================================== Story Source: Materials provided by University_of_Bonn. Note: Content
may be edited for style and length.


========================================================================== Journal Reference:
1. Yong-Hui Lin, Hans-Werner Hammer, Ulf-G. Meissner. New Insights
into the
Nucleon's Electromagnetic Structure. Physical Review Letters,
2022; 128 (5) DOI: 10.1103/PhysRevLett.128.052002 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/02/220206154437.htm
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