1432 17 DECEMBER 2021 • VOL 374 ISSUE 6574 science.org SCIENCE
PHOTO: REIDAR HAHN/FERMI NATIONAL ACCELERATOR LABORATORY2021 BREAKTHROUGH OF THE YEAR^ | RUNNERS-UP
At last, a crack in particle physics’ standard model?
It may be a sign of particle physicists’
desperation for something new that the big-
gest result in years confirms an oddity first
observed 2 decades ago. A particle called
the muon—a heavier, unstable cousin of the
electron—is slightly more magnetic than
physicists’ prevailing theory of fundamental
particles and forces predicts. Reported in
April, the 2.5-parts-per-billion discrepancy
could signal new particles lurking just over
the high energy horizon.
Developed in the 1960s and ’70s, the cur-
rent theory, known as the standard model,
accounts for three forces—electromagnetism,
the strong nuclear force, and the weak
nuclear force—and two dozen fundamental
particles. It cannot be the final description
of nature, as it leaves out both gravity and
dark matter, the mysterious stuff thought to
outweigh the universe’s ordinary matter. Yet,
so far, the standard model accounts for every
particle blasted into fleeting existence with
high energy particle accelerators.
The muon’s magnetism gives scientists
an indirect way to search for additional,
undiscovered particles. Thanks to quantum
uncertainty, empty space around the muon
roils with particle-antiparticle pairs popping
in and out of “virtual” existence too fast to
be directly observed. Those in the standard
model increase the muon’s magnetism by a
precise amount. New particles could change
that calculation in unpredicted ways.
To measure muon magnetism, scientists
fire a beam of them into a magnetic field,
where they twirl like compass needles at a
rate that depends on how magnetic they are.
Physicists ran just such an experiment from
1997 to 2001 at Brookhaven National Labora-
tory in New York, where they first detected
the anomaly. In 2003, they hauled their
15-meter-wide magnet to Fermi NationalAccelerator Laboratory in Illinois, to obtain
a purer muon beam. This year, they proved
their previous result was not a fluke.
But a proper comparison depends on the
precision of the standard model prediction.
On the same day experimenters released
their result, one team of theorists published
a calculation that, they argued, increases
the standard model prediction and closes
the observed gap. Other physicists say the
theoretical consensus still indicates that the
muon is extra magnetic.
Now, the question is why. Other searches
for tiny discrepancies from the standard
model’s predictions could yield more clues to
the hoped-for new physics. Or, if physicists
are lucky, the world’s biggest atom smasher,
Europe’s Large Hadron Collider, will blast
some new particle into plain view when it
comes back online next spring after 3 years
of upgrades. —Adrian ChoWithin this ring at the Fermi National Accelerator Laboratory, muons twirl like compass needles in a magnetic field mapped with an accuracy of 30 parts per billion.