By Michelle Nguyen
New evidence from subatomic particles ‘wobbles’ our understanding of unseen particles and forces in the universe
Muons – a heavier, unstable counterpart to electrons – challenge one of science’s most watertight theories, the Standard Model of particle physics.
Photo by Brett Jordan on Unsplash
Like electrons, muons are also negatively charged and have a quantum property called spin. In a magnetic field, spin allows muons to wobble like spinning tops. A muon wobbles faster under a stronger magnetic field.
Additionally, when muons mingle with other short-lived particles – imagine bubbles popping up and disappearing from soapy water – it can wobble ~0.1% faster than expected. This mysterious boost is called the anomalous magnetic moment, which can be further tweaked by the presence of unknown heavy particles…
The Theory of Almost Everything:
The Standard Model attempts to explain how all particles in the entire universe behave. With excellent precision, it guesses how frequently a particle wobbles. It’s currently humankind’s best mathematical explanation and considerably the most successful scientific theory.
However, it’s incomplete. Since a toolbox is only as handy as its tools, the accuracy of the model depends on being sure about every. Single. Particle. In the universe.
In 2001, the Brookhaven National Laboratory in New York found that muons may wobble slightly faster than model’s prediction.
Witnessing this anomaly again is said to be a 1 in 40,000 fluke.
Fast-forward 20 years to just last Wednesday, researchers at Fermilab announced that their Muon g-2 experiment have defied these odds!
The Muon g-2 experiment:
its potential & its controversy
How did the Fermilab researchers do this? By shooting particles into a 14-ton circular tube and seeing what happens. Sort of.
Before firing a beam of muons into a magnetised ring at high speed, the subatomic particles were gathered by filtering from debris resultant of pairs of protons smashing together. As muons whirl inside the ring, the powerful magnetic field gradually rotates the particle’s spin axis.
By millionths of a second later, or rather after a few hundred cycles within the ring, the wobbling muon decays into an electron. The detectors along the track’s inner wall catch the flying electron and reads the emitted energy. The speed of the parent muons is solved by discerning the different energies at various times.
So, what’s all this hype about an invisible particle wobbling a little bit faster than expected? The thing is physicists around the world have long been trying to push the boundaries of the Standard Model in order to explain the holes in the theory.
However, test after test, the theory stubbornly sticks. The deviations from the model observed by muon behaviour could be a gateway into a new physics!
What’s even more exciting is that although this result appears promising, a recently published Nature paper suggest that the muon’s wobbliness is exactly expected by the Standard Model according to different mode of calculation by BMW.
While the BMW and Fermilab calculations are being contested, the fact that both could simultaneously be correct is brewing tension within the physics community. Whether or not the Femilab team is chasing ghosts, this makes for a fascinating puzzle.
As theoretical physicists, Nima Arkani-Hamed (who wasn’t involved in the research) puts it:
“The attitude to take is sort of cautious optimism.”
Although the team of over 200 scientists across seven countries at Femilab have been actively experimenting since 2018, they are keeping in mind that the results aren’t a “discovery” yet.
By strict standards, researchers need to ensure that results achieve a statistical certainty of ‘5 sigma’, or rather show that this anomaly only occurs randomly once every 3.5 million times the experiment is run.
Fermilab researchers are well on track with their current estimate of 4.2 sigma! If Fermilab’s results remain consistent it could take a couple of years to draw firm conclusions from this Nobel-Prize worthy effort.
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