The muon “g minus 2” magnet has a radius of 7m and operates at negative 450 degrees Fahrenheit. The rotational spin of muons is measured as they travel through the ring’s magnetic field. The ring was transported 3,200 miles from New York to Illinois over three days and travelled by barge and along toll roads to reach its destination. (Reidar Hahn/FERMILAB)
By Poonam Narotam and Fiona Skeggs
Particle physics research takes time. Scientists at Fermi National Accelerator Laboratory (Fermilab) know this all too well as they accelerated particles to near light speed, collided streams of them into each other and helped find several of the 17 building blocks of matter in the debris for decades.
“When you decide to go into particle physics, you’re lucky if you get to do an experiment every 10 years,” says physicist Chris Polly.
After 10 years of logistical planning and a 200-strong collaboration of physicists, Fermilab recently confirmed that previous experimentation on subatomic particles called muons may suggest the existence of a new, undiscovered 18th particle.
“We sort of had the equivalent of our Mars Rover landing moment where for the first time ever we unblinded our results,” said Chris Polly, senior scientist at Fermilab working on the muon “g minus 2” experiment. The g-factor in this experiment is related to angular momentum and minus 2 suggests the muon doesn’t react as scientists expected.
Located just west of Chicago, Fermilab is home to scientists and engineers studying the fundamental particles of the universe. Million-dollar equipment, such as a particle accelerator, facilitates experimentation of the smallest particles of the universe, including the neutrino and the muon.
FERMILAB’S QUEST FOR THE 18TH PARTICLE
The previous theoretical model predicted that “g = 2”, but results from Fermilab indicate that “g” is actually greater than two for muons. The results matched a previous experiment carried out by the Brookhaven National Laboratory in New York 20 years earlier, and it’s only the second time that this type of quantum physics experiment has been run.
“It was really gratifying to see that those results were in good agreement,” said Polly. “Meaning that the experimental result from 20 years ago is fairly robust.” The vastly increased capacity of the Fermilab to run the experiment using superconducting magnets that operate near absolute zero and eliminate friction enabled Fermilab to definitively confirm the results.
So, what exactly is a muon?
According to Fermilab’s explainer video, a muon is an electrically charged particle with similar properties to an electron, though much heavier and more unstable. The defining characteristic of a muon is that it spins, like a spinning top, and the particles essentially generate their own magnetic field.
The strength of this internal magnet is referred to as the “g-factor,” and it is determined by placing the muon in a magnetic field and measuring its rotation rate.
The muon “g minus 2” experiment measures the “g-actor” of muons in a large magnetic ring. The goal of the experiment is to compare the measurements from the experiment to theoretical predictions. If the two results are not equal, it indicates something is present in nature that is not present in theory.
“Particles in the universe are never really alone,” Polly said. “They’re constantly surrounded by an entourage of other particles that hop in and out of existence.”
Polly said these extra particles interfere with the muon’s magnetic field, and that could be the reason for the difference between the predicted value of “g” and the results from the Brookhaven and Fermilab experiments.
The muon experimental team at Fermilab hope to carry out five experimental runs by 2022 to increase precision and further confirm the difference between the theoretical “g-factor” value and the experimental value.
“The reason the results from this experiment are so exciting,” said Polly. “Is because the results from the muon 'g minus 2' experiment strongly suggest there must be yet an 18th particle out there.”
“Either that or there’s something incorrect about our understanding of the forces that are governing the 17 particles that we know are the fundamental building blocks of nature,” he said.
A QUBIT’S NEED FOR SPEED
Another exciting initiative at Fermilab focuses on the fundamental building blocks of quantum computers, called quantum bits or “qubits,” whose computational power is significantly faster than regular computers.
“The power of quantum computation is not just that it can be applied to a couple of fields, but rather that it can be transformational for pretty much any field where procuring large datasets is critical in order to draw meaningful trends,” said Akshay Murthy, a postdoctoral research associate who studies qubits at Fermilab, in an email.
Fermilab is one of five Department of Energy National Quantum Centers supported by the National Quantum Initiative, created by Congress in 2018 to invest up to $625 million over five years to develop quantum computers.
Google and IBM released research in 2019 showing that a qubit-powered computer can perform complex calculations within three minutes that a normal computer would take 10,000 years to complete, according to a New York Times article.
The limitation of the devices that Google and IBM have built so far is that they “can still only address problems that can be addressed by classical computers,” Murthy said.
Murthy studies ways to improve coherence time, which is the amount of time it takes for a qubit to process information. He said improving coherence times enables quantum computers to solve problems in ways that classical computers cannot.
“What’s really neat about quantum computers is they rely on these very odd quantum mechanical phenomena,” Murthy said.
Murthy said due to “very strange phenomena that tend to persist when you have features that are around the size of an electron,” qubits give computers the ability to rapidly assess numerous possibilities at once to decide on the correct solution to a query at top speed.
“The fact that this technology is truly transformational if we’re able to deliver on its promises,” is something Murthy said he is most excited about.
“It’s starting to feel like in the next five to 10 years, this will actually be realizable,” he said.
“If we were able to make these sorts of advances… we could build a supercomputer and that would really enable transformation across a variety of different fields.”