Physics at work: Physicists move closer to a theoretical showdown
“It really all comes down to that single number,” said Hannah Binney, a physicist at the Massachusetts Institute of Technology’s Lincoln Laboratory who worked on the muon measurement as a graduate student
Representative Image
KATRINA MILLER
On July 24, a large team of researchers convened in Liverpool to unveil a single number related to the behavior of the muon, a subatomic particle that might open a portal to a new physics of our universe. All eyes were on a computer screen as someone typed in a secret code to release the results. The first number that popped out was met with exasperation: a lot of concerning gasps, oh-my-God’s and what-did-we-do-wrong’s. But after a final calculation, “there was a collective exhale across multiple continents,” said Kevin Pitts, a physicist at Virginia Tech who was five hours away, attending the meeting virtually. The new measurement matched exactly what the physicists had computed two years prior — now with twice the precision.
So comes the latest result from the Muon g-2 Collaboration, which runs an experiment at Fermi National Accelerator Laboratory, or Fermilab, in Batavia, Ill., to study the deviant motion of the muon. The measurement, announced to the public and submitted to the journal Physical Review Letters on Thursday morning, brings physicists one step closer to figuring out if there are more types of matter and energy composing the universe than have been accounted for.
“It really all comes down to that single number,” said Hannah Binney, a physicist at the Massachusetts Institute of Technology’s Lincoln Laboratory who worked on the muon measurement as a graduate student.
Scientists are putting to the test the Standard Model, a grand theory that encompasses all of nature’s known particles and forces. 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 account for gravity, and it also can’t explain dark matter (the glue holding our universe together), or dark energy (the force pulling it apart).
One of many ways that researchers are looking for physics beyond the Standard Model is by studying muons. As heavier cousins of the electron, muons are unstable, surviving just two-millionths of a second before decaying into lighter particles. They also act like tiny bar magnets: Place a muon in a magnetic field, and it will wobble around like a top. The speed of that motion depends on a property of the muon called the magnetic moment, which physicists abbreviate as g.
In theory, g should exactly equal 2. But physicists know that this value gets ruffled by the “quantum foam” of virtual particles that blip in and out of existence and prevent empty space from being truly empty. These transient particles change the rate of the muon’s wobble. By taking stock 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 so exciting to study,” Dr. Binney said. “It’s sensitive to all of the particles that exist, even the ones that we don’t know about yet.” Any difference between theory and experiment, she added, means new physics is on the horizon.