Renewed measurements of muon’s magnetism could open door to new physics

Next week, physicists will pick up an old quest for new physics. A team of 190 researchers at Fermi National Accelerator Laboratory (Fermilab) in Batavia, Illinois, will begin measuring to exquisite precision the magnetism of a fleeting particle called the muon. They hope to firm up tantalizing hints from an earlier incarnation of the experiment, which suggested that the particle is ever so slightly more magnetic than predicted by the prevailing standard model of particle physics. That would give researchers something they have desired for decades: proof of physics beyond the standard model.

“Physics could use a little shot of love from nature right now,” says David Hertzog, a physicist at the University of Washington in Seattle and co-spokesperson for the experiment, which is known as Muon g-2 (pronounced “gee minus two”). Physicists are feeling increasingly stymied because the world’s biggest atom smasher, the Large Hadron Collider (LHC) near Geneva, Switzerland, has yet to blast out particles beyond those in the standard model. However, g-2 could provide indirect evidence of particles too heavy to be produced by the LHC.

The muon is a heavier, unstable cousin of the electron. Because it is charged, it will circle in a magnetic field. Each muon is also magnetized like a miniature bar magnet. Place a muon in a magnetic field perpendicular to the orientation of its magnetization, and its magnetic polarity will turn, or precess, just like a twirling compass needle.

At first glance, theory predicts that in a magnetic field a muon’s magnetism should precess at the same rate as the particle itself circulates, so that if it starts out polarized in the direction it’s flying, it will remain locked that way throughout its orbit. Thanks to quantum uncertainty, however, the muon continually emits and reabsorbs other particles. That haze of particles popping in and out of existence increases the muon’s magnetism and makes it precess slightly faster than it circulates.

Because the muon can emit and reabsorb any particle, its magnetism tallies all possible particles—even new ones too massive for the LHC to make. Other charged particles could also sample this unseen zoo, says Aida El-Khadra, a theorist at the University of Illinois in Urbana. But, she adds, “The muon hits the sweet spot of being light enough to be long-lived and heavy enough to be sensitive to new physics.”

From 1997 to 2001, researchers on the original g-2 experiment at Brookhaven National Laboratory in Upton, New York, tested this promise by shooting the particles by the thousands into a ring-shaped vacuum chamber 45 meters in diameter, sandwiched between superconducting magnets.

Over hundreds of microseconds, the positively charged muons decay into positrons, which tend to be spat out in the direction of the muons’ polarization. Physicists can track the muons’ precession by watching for positrons with detectors lining the edge of the ring.

The g-2 team first reported a slight excess in the muon’s magnetism in 2001. That result quickly faded as theorists found a simple math mistake in the standard model prediction (Science, 21 December 2001, p. 2449). Still, by the time the team reported on the last of its Brookhaven data in 2004, the discrepancy had re-emerged. Since then, the result has grown, as theorists improved their standard model calculations. They had struggled to account for the process in which the muon emits and reabsorbs particles called hadrons, says Michel Davier, a theorist at the University of Paris-South in Orsay, France. By using data from electron-positron colliders, he says, the theorists managed to reduce this largest uncertainty.

Physicists measure the strength of signals in multiples of the experimental uncertainty, σ, and the discrepancy now stands at 3.5 σ—short of the 5 σ needed to claim a discovery, but interesting enough to warrant trying again.

In 2013, the g-2 team lugged the experiment on a 5000-kilometer odyssey from Brookhaven to Fermilab, taking the ring by barge around the U.S. eastern seaboard and up the Mississippi River. Since then, they have made the magnetic field three times more uniform, and at Fermilab, they can generate far purer muon beams. “It’s really a whole new experiment,” says Lee Roberts, a g-2 physicist at Boston University. “Everything is better.”

Over 3 years, the team aims to collect 21 times more data than during its time at Brookhaven, Roberts says. By next year, Hertzog says, the team hopes to have enough data for a first result, which could push the discrepancy above 5 σ.

Will the muon end up being a portal to new physics? JoAnne Hewett, a theorist at SLAC National Accelerator Laboratory in Menlo Park, California, hesitates to wager. “In my physics lifetime, every 3-σ deviation from the standard model has gone away,” she says. “If it weren’t for that baggage, I’d be cautiously optimistic.”

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