Mini-Throughput: A Crack in the Standard Model?

The Fermilab g-2 experiment. Credit: Reidar Hahn, Fermilab

The universe may have just gotten a lot more interesting:

Twenty years after an apparent anomaly in the behavior of elementary particles raised hopes of a major physics breakthrough, a new measurement has solidified them: Physicists at Fermi National Accelerator Laboratory near Chicago announced today that muons — elementary particles similar to electrons — wobbled more than expected while whipping around a magnetized ring.

The widely anticipated new measurement confirms the decades-old result, which made headlines around the world. Both measurements of the muon’s wobbliness, or magnetic moment, significantly overshoot the theoretical prediction, as calculated last year by an international consortium of 132 theoretical physicists. The Fermilab researchers estimate that the difference has grown to a level quantified as “4.2 sigma,” well on the way to the stringent five-sigma level that physicists need to claim a discovery.

Taken at face value, the discrepancy strongly suggests that unknown particles of nature are giving muons an extra push. Such a discovery would at long last herald the breakdown of the 50-year-old Standard Model of particle physics — the set of equations describing the known elementary particles and interactions.

This is a bit beyond my field, but I’ll try to explain it the way I understand it. The Standard Model of particle physics is a set of equations that describes the behavior of subatomic particles and the interactions of three of the four fundamental forces (the strong and weak forces inside atoms and the electromagnetic force that is interacting with your eyeballs as you read this). It has, so far, withstood every experimental test. You may remember a decade ago when the Higgs Boson was detected, confirming large parts of the theory.

But particle physicists are always trying to push the boundaries. Because while the Standard Model works, it is not complete. It does not explain gravity or dark energy, for example. We think there is more to the theory than what we have but we can’t see it because our experiments aren’t powerful enough. It’s like our picture of the universe is still slightly out of focus. To go beyond our current limits — to turn the focus knob up — you’d need higher energy collisions in particle accelerators. In fact, to probe the deepest mysteries of the particle physics, you might need as much energy as there was in the Big Bang.

(Aside: One of the most beautiful things in science is how our understanding of the Universe on the most massive scales — cosmology — depends critically on our understanding of the Universe on the smallest scales — particle physics. When I teach cosmology, I have to talk about Grand Unifying Theories and subatomic particles because the Universe once was the size of a subatomic particle with energies and forces we can only recreate inside a particular accelerator. Astronomy shows us the echoes of the Big Bang. Our understanding of those echoes and our ability to trace the history of the Universe down to the tiniest fraction of a second depends on our understanding of particle physics. All of science is connected. Scientists may work in different wings, but it’s one building. And the Big Bang is where two of those wings make a bridge.)

But … maybe you don’t need to recreate the Big Bang to understand the deep magic. One of the windows into the depths of existence may come from the muon — a kind of heavy electron. The muon has a very precisely measured g-factor — a ratio between its magnetic field and its spin. Early theory predicted this number would be exactly 2. But the Standard Model predicts it is very slightly more than two because the muon doesn’t just chug along in space minding its own business. It interacts with a sea of virtual particles popping in and out of existence and affecting its spin. The Standard Model predicts this ratio to very high precision — better than one part in a billion. Twenty years ago, however, experiments hinted that ratio was a bit higher than expected, which could indicate muons interact with hitherto unknown tiny particles — something beyond the standard model. But the result wasn’t quite statistically significant.

The new experiment whips muons around a magnetic field and looks at the positrons they spray out to measure their g-factor. And what they got was slightly higher than expected — 2.0023318412. They can be compared to the 2.00233183620 predicted by the standard theory. This confirms the old result but raises the significance to 4.2 sigma, just shy of the 5 sigma we like and indicating 1-in-40,000 odds that this is just chance result.

Particle physicist Dr. Claire Lee has a great Twitter thread explaining what this all means, with animated gifs and everything. I strongly recommend it as she understand this stuff far better than I do. Another great thread can be found here. And the article I link above goes into it in great depth. But to sum it all in one sentence: we may be entering a new era of physics because a tiny spinny thing is spinning a tiny bit differently than we expect the tiny spinny thing to spin.

This is all heady stuff. But knowing that the Standard Model doesn’t explain everything … is everything: gravity, dark energy and ultimately the fate of the Universe itself. And with Fermilab still taking data on their magnificent machine, we may just be getting the first taste of a brave new world.