It’s a subatomic mystery with big implications. Six years after
physicists announced a bafflingly too small measurement of the size of
the proton, we’re still not sure what’s going on. With the release of
new data today, the mystery has, if anything, got deeper.
Protons
are particles found inside the nucleus of atoms. For years, the
proton’s radius seemed pinned down at about 0.877 femtometres, or less
than a quadrillionth of a metre.
But in 2010, Randolf Pohl at the
Max Planck Institute of Quantum Optics in Garching, Germany, got a
worryingly different answer using a new measurement technique.
Pohl’s team altered the one proton, one electron composition of a
hydrogen atom by switching the electron for a heavier particle called a
muon. They then zapped this altered atom with a laser. Measuring the
resulting change in its energy levels allowed them to calculate the size
of its proton nucleus. To their surprise, it came out 4 per cent
smaller than the traditional value measured via other means.
A 2013 measurement strengthened the finding, sending physicists searching for an explanation to the “proton radius puzzle“.
Pohl’s experiment also applied the new technique to deuterium, an
isotope of hydrogen that has one proton and one neutron – collectively
known as a deuteron – at its nucleus. Accurately calculating the size of
the deuteron took plenty of time, however.
“The only thing that’s going to allow us to solve it is new data”
Today, the team have published their measurements, revealing that like the proton, the deuteron comes up short: in this case by 0.8 per cent.
Today, the team have published their measurements, revealing that like the proton, the deuteron comes up short: in this case by 0.8 per cent.
These new numbers show that the proton radius problem isn’t going away,
says Evangeline J. Downie at the George Washington University in
Washington DC. “It tells us that there’s still a puzzle,” says Downie.
“It’s still very open, and the only thing that’s going to allow us to
solve it is new data.”
Several more experiments, at Pohl’s lab
and others, are already under way. One will return to the same muon
technique to measure the size of heavier atomic nuclei, like helium.
Another plans to simultaneously measure the scattering of muons and
electrons.
Pohl suspects the culprit may not be the proton
itself, but an incorrect measurement of the Rydberg constant, a number
that describes the wavelengths of light emitted by an excited atom. But
this constant is well established through other precision experiments,
so something drastic would have to have gone wrong.
Another
explanation proposes new particles that cause unexpected interactions
between the proton and the muon, without changing its relationship with
the electron.
That could mean the puzzle is taking us beyond the
standard model of particle physics. “If at some point in the future,
somebody will discover something beyond the standard model, it would be
like this,” says Pohl, with first one small discrepancy, then another
and another, slowly building to a more monumental shift.
http://science.sciencemag.org/content/353/6300/669
https://www.newscientist.com/…/2100834-how-big-is-a-proto…/…
http://arstechnica.com/…/researchers-orbit-a-muon-around-a…/
https://www.newscientist.com/…/dn23105-shrinking-proton-pu…/
http://www.physicscentral.com/explore/pictures/deuteron.cfm
https://www.psi.ch/media/the-psi-proton-accelerator
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