Magnetics Business & Technology - September/October 2021 - 18

RESEARCH & DEVELOPMENT
Breakthrough Discovery: Physicists Cheer Major Muon Result
The storage-ring magnet used for the g − 2 experiment at Fermilab. Credit: Reidar Hahn/Fermilab
Muons keep on misbehaving. An experiment in the United States
has confirmed an earlier finding that the particles - massive, unstable
cousins of the electron - are more magnetic than researchers
originally expected. If the results hold up, they could ultimately
force major changes in theoretical physics and reveal the existence
of completely new fundamental particles.
The Muon g − 2 collaboration at the Fermi National Accelerator
Laboratory (Fermilab) outside Chicago, Illinois, reported the latest
measurements in a webcast on 7 April, and published them in
Physical Review Letters. The results are " extremely encouraging "
for those hoping to discover other particles, says Susan Gardner,
a physicist at the University of Kentucky in Lexington.
Muon g − 2 (pronounced 'g minus 2') first hinted that something
was amiss with the muon in 2001, when the experiment was running
at the Brookhaven National Laboratory in Upton, New York.
Physicists measured the strength of the particle's magnetic moment,
a property that makes it act like a tiny bar magnet. The standard
model of particle physics says that, in the appropriate units,
the muon's magnetic moment should be a number very close, but
not equal, to 2. The Brookhaven experiment measured that tiny
difference, known as g - 2, but found it to be slightly bigger than
theorists had predicted.
The magnetic moment of elementary particles is influenced by 'virtual'
versions of known elementary particles that continually pop
out of the vacuum only to disappear a fraction of a second later.
Physicists perform detailed and lengthy calculations of the contributions
from all known particles, so if the experimental results
differ significantly from the predicted value of g − 2, they reason
that previously unknown types of particle must be lurking in the
vacuum. The original Muon g − 2 experiment gave many physicists
hope that new particles would soon be discovered.
Secret frequency
To verify the Brookhaven results, researchers rebuilt the experiment
- which keeps muons running in circles around a superconducting
ring magnet 15 metres in diameter - at Fermilab. They
began collecting data in 2018, and have now presented the results
from the first year of operations.
To avoid biasing its data analysis, the collaboration had blinded
itself to a crucial parameter that is needed to calculate the g − 2
constant - the exact frequency of a digital clock in their instrumentation.
Two Fermilab physicists who are not collaboration members
were entrusted with the missing bit of information. As a result,
the team was able to conduct a lengthy study, but could initially
plot its findings only on a graph in which the axes had slightly
uncertain scales.
Then at a 25 February teleconference that included most of the
200-plus team members, two leading members of the experiment
opened an envelope that contained the secret clock frequency.
When they plugged the number into their computers, it revealed
the true value of their g − 2 measurement. It was immediately obvious
to the team that the result was consistent with the one recorded
at Brookhaven more than 20 years ago.
" The agreement is excellent, " says Lee Roberts at Boston University
in Massachusetts, one of the original Muon g − 2 team
members. " People were clapping and jumping up and down - as
much as you can do that on Zoom. " The joyful reactions were obvious,
even though " a lot of us were muted " , adds Brynn MacCoy,
a physicist at the University of Washington in Seattle. The result
vindicates the claim of the original experiment, Roberts says.
Other physicists agree. The latest announcement gives " a nice,
clear answer " to the riddle posed by the earlier results, says theoretical
physicist Gino Isidori at the University of Zurich in Switzerland.
" The experiment was correct. "
18 Magnetics Business & Technology * September/October 2021
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