Muon Discovery Moves Physicists One Step Closer to a Theoretical Showdown

On July 24, a big crew of researchers convened in Liverpool to unveil a single quantity associated to the habits of the muon, a subatomic particle that may open a portal to a brand new physics of our universe.

All eyes have been on a pc display screen as somebody typed in a secret code to launch the outcomes. The primary quantity that popped out was met with exasperation: loads of regarding gasps, oh-my-God’s and what-did-we-do-wrong’s. However after a closing calculation, “there was a collective exhale throughout a number of continents,” stated Kevin Pitts, a physicist at Virginia Tech who was 5 hours away, attending the assembly nearly. The brand new measurement matched precisely what the physicists had computed two years prior — now with twice the precision.

So comes the most recent consequence from the Muon g-2 Collaboration, which runs an experiment at Fermi Nationwide Accelerator Laboratory, or Fermilab, in Batavia, Unwell., to check the deviant movement of the muon. The measurement, announced to the public and submitted to the journal Physical Review Letters on Thursday morning, brings physicists one step nearer to determining if there are extra kinds of matter and vitality composing the universe than have been accounted for.

“It actually all comes right down to that single quantity,” stated Hannah Binney, a physicist on the Massachusetts Institute of Know-how’s Lincoln Laboratory who labored on the muon measurement as a graduate scholar.

Scientists are placing to the check the Normal Mannequin, a grand concept that encompasses all of nature’s recognized particles and forces. Though the Normal Mannequin has efficiently predicted the result of numerous experiments, physicists have lengthy had a hunch that its framework is incomplete. The speculation fails to account for gravity, and it can also’t clarify darkish matter (the glue holding our universe collectively), or darkish vitality (the pressure pulling it aside).

Certainly one of many ways in which researchers are on the lookout for physics past the Normal Mannequin is by finding out muons. As heavier cousins of the electron, muons are unstable, surviving simply two-millionths of a second earlier than decaying into lighter particles. In addition they act like tiny bar magnets: Place a muon in a magnetic discipline, and it’ll wobble round like a high. The pace of that movement is dependent upon a property of the muon known as the magnetic second, which physicists abbreviate as g.

In concept, g ought to precisely equal 2. However physicists know that this worth will get ruffled by the “quantum foam” of digital particles that blip out and in of existence and stop empty area from being actually empty. These transient particles change the speed of the muon’s wobble. By taking inventory of all of the forces and particles within the Normal Mannequin, physicists can predict how a lot g will probably be offset. They name this deviation g-2.

But when there are unknown particles at play, experimental measurements of g won’t match this prediction. “And that’s what makes the muon so thrilling to check,” Dr. Binney stated. “It’s delicate to all the particles that exist, even those that we don’t learn about but.” Any distinction between concept and experiment, she added, means new physics is on the horizon.

To measure g-2, researchers at Fermilab generated a beam of muons and steered it right into a 50-foot-diameter, doughnut-shaped magnet, the within brimming with digital particles that have been popping into actuality. Because the muons raced across the ring, detectors alongside its edge recorded how briskly they have been wobbling.

Utilizing 40 billion muons — 5 instances as a lot information because the researchers had in 2021 — the crew measured g-2 to be 0.00233184110, a one-tenth of 1 p.c deviation from 2. The consequence has a precision of 0.2 elements per million. That’s like measuring the gap between New York Metropolis and Chicago with an uncertainty of solely 10 inches, Dr. Pitts stated.

“It’s a tremendous achievement,” stated Alex Keshavarzi, a physicist on the College of Manchester and a member of the Muon g-2 Collaboration. “That is the world’s most exact measurement ever made at a particle accelerator.” The outcomes, when revealed to the general public at a scientific seminar on Thursday morning, have been met with applause.

“The sort of precision that these individuals have managed to realize is simply staggering,” stated Dan Hooper, a theoretical cosmologist on the College of Chicago who was not concerned within the work. “There was loads of skepticism they might get right here, however right here they’re.”

However whether or not the measured g-2 matches the Normal Mannequin’s prediction has but to be decided. That’s as a result of theoretical physicists have two strategies of computing g-2, primarily based on other ways of accounting for the sturdy pressure, which binds collectively protons and neutrons inside a nucleus.

The normal calculation depends on 40 years of strong-force measurements taken by experiments world wide. However with this strategy, the g-2 prediction is just pretty much as good as the info which might be used, stated Aida El-Khadra, a theoretical physicist on the College of Illinois Urbana-Champaign and a chair of the Muon g-2 Principle Initiative. Experimental limitations in that information, she stated, could make this prediction much less exact.

A more recent approach known as a lattice calculation, which makes use of supercomputers to mannequin the universe as a four-dimensional grid of space-time factors, has additionally emerged. This methodology doesn’t make use of knowledge in any respect, Dr. El-Khadra stated. There’s only one drawback: It generates a g-2 prediction that differs from the standard strategy.

“Nobody is aware of why these two are totally different,” Dr. Keshavarzi stated. “They need to be precisely the identical.”

In contrast with the standard prediction, the most recent g-2 measurement has a discrepancy of over 5-sigma, which corresponds to a one in 3.5 million likelihood that the result’s a fluke, Dr. Keshavarzi stated, including that this diploma of certainty was past the extent wanted to assert a discovery. (That’s an enchancment from their 4.2-sigma result in 2021, and a 3.7-sigma measurement completed at Brookhaven Nationwide Laboratory close to the flip of the century.)

However after they in contrast it with the lattice prediction, Dr. Keshavarzi stated, there was no discrepancy in any respect.

Not often in physics does an experiment surpass the idea, however that is a kind of instances, Dr. Pitts stated. “The eye is on the theoretical group,” he added. “The limelight is now on them.”

Dr. Binney stated, “We’re on the sting of our seats to see how this concept dialogue pans out.” Physicists anticipate to higher perceive the g-2 prediction by 2025.

Gordan Krnjaic, a theoretical particle physicist at Fermilab, famous that if the experimental disagreement with concept continued, it will be “the primary smoking-gun laboratory proof of latest physics,” he stated. “And it’d nicely be the primary time that we’ve damaged the Normal Mannequin.”

Whereas the 2 camps of concept hash it out, experimentalists will hone their g-2 measurement additional. They’ve greater than double the quantity of knowledge left to sift by means of, and as soon as that’s included, their precision will enhance by one other issue of two. “The longer term could be very brilliant,” stated Graziano Venanzoni, a physicist on the College of Liverpool and one chief of the Muon g-2 experiment, at a public news briefing in regards to the outcomes.

The most recent consequence strikes physicists one step nearer to a Normal Mannequin showdown. However even when new physics is confirmed to be on the market, extra work will probably be wanted to determine what that truly is. The invention that the recognized legal guidelines of nature are incomplete would lay the muse for a brand new technology of experiments, Dr. Keshavarzi stated, as a result of it will inform physicists the place to look.

“Physicists get actually excited when concept and experiment don’t agree with one another,” stated Elena Pinetti, a theoretical physicist at Fermilab who was not concerned within the work. “That’s once we actually can study one thing new.”

For Dr. Pitts, who has spent practically 30 years pushing the bounds of the Normal Mannequin, proof of latest physics could be each a celebratory milestone and a reminder of all that’s left to do. “On one hand it’s going to be, Have a toast and rejoice successful, an actual breakthrough,” he stated. “However then it’s going to be again to work. What are the subsequent concepts that we will get to work on?”