On July 24, a large team of researchers gathered in Liverpool to reveal a unique number related to the behavior of the muon, a subatomic particle that could open a doorway to new physics in our universe.
All eyes were on the computer screen when someone wrote a secret code to release the results. The first issue that came out was met with outrage: lots of disturbing shots, oh my god and what did we do wrong. “There was a collective expiration on several continents,” said Kevin Bates, a Virginia Tech physicist who was five hours away and nearly present at the meeting, after making the final calculation. The new measurement was almost identical to the one the physicists had calculated two years earlier – now twice as accurate.
So comes the latest discovery from the Muon g-2 Collaboration, which is conducting an experiment at the Fermi National Accelerator Laboratory, or Fermilab, in Batavia, Illinois, to study muon deflection motion. Measurements, Announced to the Public, and Submitted to Physics Review Letters By Thursday morning, physicists were one step closer to discovering whether there are more types of matter and energy that make up the universe than calculated.
“It all boils down to that single digit,” said Hannah Penney, a physicist at MIT’s Lincoln Laboratory who worked on muon measurements as a graduate student.
Scientists are testing the Standard Model, a grand theory that includes all known particles and forces in nature. Although the Standard Model has successfully predicted the outcome of countless experiments, physicists have long had a hunch that its framework is incomplete. The theory fails to explain gravity, nor can it explain dark matter (the glue that holds our universe together) or dark energy (the force that holds it apart).
One of the many ways researchers look into physics beyond the Standard Model is by studying muons. As the electron’s heavier cousins, muons are unstable, surviving for just two millionths of a second before decaying into lighter particles. They also act like little bar magnets: put the muon in a magnetic field and it wobbles like a top. The speed of this movement depends on a property of the muon called the magnetic moment, which physicists abbreviate with the symbol g.
In theory, g should be exactly equal to 2. But physicists know that this value is undermined by the “quantum foam” of virtual particles that fly out of existence and prevent empty space from being truly empty. These passing particles alter the rate of oscillation of the muon. By taking inventory of all the forces and particles in the Standard Model, physicists can predict how much g will be offset. They call this deviation g-2.
But if there are unknown particles at play, experimental measurements of g will not match this prediction. “And that’s what makes the muon study so interesting,” said Dr. Penney. “It’s sensitive to all the particles out there, even the ones we don’t know about yet.” She added that any difference between theory and experiment means that new physics is on the horizon.
To measure g-2, the Fermilab researchers generated a beam of muons and directed them towards a donut-shaped magnet 15 meters in diameter, whose interior was full of virtual particles that came to life. As the muons swirled around the ring, detectors along its edge recorded how fast they wobbled.
Using 40 billion muons — five times more data than the researchers had in 2021 — the team measured g-2 as 0.00233184110, a deviation of one-tenth of one percent from 2. The result was an accuracy of 0.2 parts per million. Dr. Bates said this is like measuring the distance between New York City and Chicago with an uncertainty of only 10 inches.
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