New Measurements of Muons Might Rewrite Particle Physics

The gap between theoretical predictions and the experimental measurements isn’t a full-blown discovery yet

A 50-foot-wide blue ring used at Fermilab to study particle physics
The 50-foot-wide racetrack used to study muons traveled by barge around Florida and up the Mississippi, and then by truck across Illinois. Reidar Hahn, Fermilab

About 50 years ago, physicists came up with a rulebook to describe the ways fundamental particles interact to create the world as we know it. Since then, researchers have pushed that theoretical framework, called the Standard Model, to its limits in order to study its imperfections.

Now, results from two particle physics experiments have come tantalizingly close to discovering a gap in the Standard Model.

The experiments focused on muons, which are similar to electrons. Both have an electric charge and spin, which makes them wobble in a magnetic field. But muons are over 200 times larger than electrons, and they split apart into electrons and another particle, neutrinos, in 2.2 millionths of a second. Luckily, that’s just enough time to gather precise measurements, given the right equipment, like a 50-foot-wide magnet racetrack.

Physicist Chris Polly of the Fermi National Accelerator Laboratory presented a graph during a seminar and news conference last week that showed a gap between theoretical calculation and the actual measurements of muons moving in the racetrack.

“We can say with fairly high confidence, there must be something contributing to this white space,” said Polly during the news conference, per Dennis Overbye at the New York Times. “What monsters might be lurking there?”

The Standard Model aims to describe everything in the universe based on its fundamental particles, like electrons and muons, and its fundamental forces. The model predicted the existence of the Higgs boson particle, which was discovered in 2012. But physicists know that the model is incomplete—it takes into account three fundamental forces, but not gravity, for example.

A mismatch between theory and experimental results could help researchers uncover the hidden physics and expand the Standard Model so that it explains the universe more fully.

“New particles, new physics might be just beyond our research,” says Wayne State University particle physicist Alexey Petrov to the Associated Press’ Seth Borenstein. “It’s tantalizing.”

The Standard Model requires such complex calculations that it took a team of 132 theoretical physicists, led by Aida El-Khadra, to find its prediction for the muon-wobble in the Fermilab experiment. The calculations predicted a lower wobble than the Fermilab experiment measured.

This week’s results closely follow new findings from the Large Hadron Collider. Last month, researchers at LHC showed a surprising ratio of particles leftover after smashing muons at high speeds.

“The LHC, if you like, is almost like smashing two Swiss watches into each other at high speed. The debris comes out, and you try to piece together what’s inside,” says University of Manchester physicist Mark Lancaster, who worked on the Fermilab experiments, to Michael Greshko at National Geographic. At Fermilab, “we’ve got a Swiss watch, and we watch it tick very, very, very, very painstakingly and precisely, to see whether it’s doing what we expect it to do.”

The Fermilab group used the same 50-foot-wide ring that was first used in the 2001 muon experiments. The researchers shoot a beam of particles into the ring, where the particles are exposed to superconducting magnets. The particles in the beam decay into several other particles, including muons. Then those muons whirl around the racetrack several times before they decay, giving physicists a chance to measure how they interact with the magnetic field, writes Daniel Garisto for Scientific American.

To avoid bias, the instruments that the researchers used to measure the muons gave encrypted results. The key—a number written on a piece of paper and hidden in two offices in Fermilab and the University of Washington—remained secret until a virtual meeting in late February. When the key entered the spreadsheet, the results became clear: the experiment did not match the theory.

“We were all really ecstatic, excited, but also shocked—because deep down, I think we’re all a little bit pessimistic,” says Fermilab physicist Jessica Esquivel to National Geographic.

If the results hold up as more data from the experiment emerges, then they would upend “every other calculation made” in the field of particle physics, says David Kaplan, a theoretical physicist at Johns Hopkins University, to the Associated Press.

Free University of Brussels physicist Freya Blekman, who wasn’t involved with the work, tells National Geographic that the work “is Nobel Prize-worthy, without question,” if it holds up.

The results so far are expected to publish in the journals Physical Review Letters, Physical Review A&B, Physical Review A and Physical Review D. These results have come from just six percent of the data that the Fermilab experiment expects to collect. Between that six percent, and the 2001 experimental results, there is a one-in-40,000 chance that the difference between theory and experiment is a mistake.

“This is strong evidence that the muon is sensitive to something that is not in our best theory,” says University of Kentucky physicist Renee Fatemi to the New York Times.

But particle physics demands that the researchers bring that down to a one-in-3.5 million chance. The research team may have the final results by late 2023.