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Physicists at Fermilab, European collider find tiniest particles are defying physics’ rulebook

Strange subatomic particles called muons are acting more strangely than the Standard Model predicts. Which means the basic way physicists think the universe works might be wrong.

Nikolai Bondar works on the LHCb Muon system at the European Organization for Nuclear Research Large Hadron Collider facility outside Geneva, Switzerland.
Nikolai Bondar works on the LHCb Muon system at the European Organization for Nuclear Research Large Hadron Collider facility outside Geneva, Switzerland.
CERN

Preliminary results from long-running experiments at Fermilab and in Europe suggest the basic way physicists think the universe works might be wrong.

Tiny particles called muons aren’t doing what the Standard Model — the rulebook that physicists use to describe and understand how the universe works at the subatomic level — predicts.

And that has experts in particle physics baffled — and thrilled.

“We think we might be swimming in a sea of background particles all the time that just haven’t been directly discovered,” Fermilab experiment co-chief scientist Chris Polly said.

Chris Polly.
Chris Polly.
Fermilab

“There might be monsters we haven’t yet imagined that are emerging from the vacuum interacting with our muons,” Polly said. “And this gives us a window into seeing them.”

The Standard Model was developed about 50 years ago. Experiments have repeatedly affirmed its descriptions of the particles and the forces that make up and govern the universe were pretty much on the mark.

Until now.

The U.S. Energy Department’s Fermi National Accelerator Laboratory, known as Fermilab, announced the results Wednesday of 8.2 billion races along a track in Batavia that have physicists astir: The magnetic field around a muon — a strange and fleeting subatomic particle — isn’t what the Standard Model says it should be.

This follows results last month from the Large Hadron Collider in Geneva, Switzerland, operated by CERN, the European Organization for Nuclear Research, that found a surprising proportion of subatomic particles in the aftermath of high-speed collisions.

Theoretical physicist Matthew McCullough of CERN said untangling the mysteries could “take us beyond our current understanding of nature.”

The point of the experiments, as Johns Hopkins University theoretical physicist David Kaplan explains it, is to pull apart particles and find out if there’s “something funny going on” with both the particles and the seemingly empty space between them.

“The secrets don’t just live in matter,” Kaplan said. “They live in something that seems to fill in all of space and time. These are quantum fields. We’re putting energy into the vacuum and seeing what comes out.”

Both sets of results involve the muon — think of it as the heavier cousin of the electron that orbits an atom’s center.

Unlike the electron, the muon isn’t part of the atom. And it usually exists for only two microseconds — less time than it takes to say its name.

After it was discovered in cosmic rays in 1936, it so confounded scientists that the American physicist Isidor Isaac Rabi asked, “Who ordered that?”

“Since the very beginning, it was making physicists scratch their heads,” said Graziano Venanzoni, an experimental physicist at an Italian national lab.

Venanzoni is one of the top scientists on the Fermilab experiment, called Muon g-2, which sends muons around a magnetized, 50-foot track in Kane County that keeps the particles in existence long enough for researchers to get a closer look.

Preliminary results suggest the magnetic “spin” of the muons is 0.1% off what the Standard Model predicts. That’s huge — more than enough to upend current understanding.

Researchers need another year or two to finish analyzing the results. If they hold up, it will count as a major discovery, Venanzoni said.

Separately, at CERN, home to the world’s largest atom-smasher (it took that title from Fermilab), physicists have been crashing protons against each other there to see what happens. One of the experiments measures what happens when particles called beauty or bottom quarks collide.

The Standard Model predicts these beauty quark crashes should result in equal numbers of electrons and muons — like flipping a coin 1,000 times and getting about equal numbers of heads and tails, said Chris Parkes, the beauty quark experiment chief.

But that’s not what happened.

Researchers pored over the data from several years and a few thousand crashes and found a 15% difference, with significantly more electrons than muons, said researcher Sheldon Stone of Syracuse University.

Neither experiment is an official discovery yet. There’s still a tiny chance the results are statistical quirks. Running the experiments more times could, in a year or two, reach the incredibly stringent statistical requirements for physics to hail it as a discovery.

If the results do hold, they would upend “every other calculation made” in particle physics, Kaplan said.

Perhaps, he said, there’s some undiscovered particle — or force — that explains both strange results.

Or these could be mistakes. In 2011, a strange finding that a particle called a neutrino seemed to travel faster than light threatened the model. It turned out to be the result of a loose electrical connection in the experiment.

This time, Stone said: “We checked all our cable connections, and we’ve done what we can to check our data. We’re kind of confident. But you never know.”