Muon magnetic moment differs slightly from theoretical predictions, potentially indicating new particles or forces. Confirmation would revolutionize physics.
Reidar Hahn, CC BY-SA 4.0, via Wikimedia Commons
A hint of new physics
In the Muon g-2 experiment is a fundamental particle and a puzzle. The muon is a heavier cousin of the electron, about 200 times more massive. Like a tiny spinning top, it has a property called spin and an internal magnet. When placed in a powerful magnetic field, the muon's spin axis doesn't stay fixed; it wobbles, or "precesses." The speed of this wobble is determined by a number called the g-factor.
The Standard Model of particle physics, our current best description of the subatomic world, predicts the muon's g-factor with incredible precision. Basic theory says the g-factor should be almost exactly 2. The "g-2" in the experiment's name refers to the tiny, anomalous deviation from 2, which is caused by the muon interacting with a sea of "virtual" particles that pop in and out of existence in the quantum vacuum. The experiment's goal is to measure this deviation with a precision of 0.14 parts per million.
The puzzle arises because the measured value of this wobble does not quite match the theoretical prediction. The first hints of this discrepancy appeared in an earlier experiment at Brookhaven National Laboratory in the late 1990s. The results from Fermilab, first announced on April 7, 2021, confirmed Brookhaven's findings. The combined results showed a difference with theory at a significance of 4.2 sigma, corresponding to a 1-in-40,000 chance of being a statistical fluke. This persistent anomaly suggests that there might be undiscovered particles or forces influencing the muon's behavior.
A 3,200-mile journey for a magnet
The centerpiece of the experiment is a massive superconducting electromagnet, 50 feet (about 14 meters) in diameter. This machine creates an exceptionally uniform magnetic field of 1.45 Tesla, which is about 30,000 times stronger than the Earth's magnetic field. To achieve this, its superconducting coils must be kept at extremely low temperatures.
This complex instrument was originally built and used for the experiment at Brookhaven National Laboratory on Long Island, New York. Building a new one at Fermilab would have been ten times more expensive than moving the original. So, in the summer of 2013, the magnet began a remarkable 35-day, 3,200-mile trip. It was transported on a specially built truck, loaded onto a barge, and floated down the East Coast, around Florida, and up the Mississippi and Illinois rivers. For the final leg of the trip, it was driven over three nights to its new home in Illinois. A critical requirement of the move was that the ring could not be tilted or twisted more than a few degrees to protect the delicate superconducting wiring inside. Data collection at Fermilab began in 2017 and concluded on July 9, 2023, after six years.
💡Fun Facts
The electromagnet's 3,200-mile trip from New York to Illinois took 35 days by land and sea.
During its entire trip, the 50-foot-wide magnet could not be tilted more than a few degrees to avoid damaging its internal superconducting coils.
The magnetic field inside the ring is about 30,000 times stronger than the Earth's magnetic field.
The experiment aims to measure the muon's properties with a precision equivalent to measuring the length of a football field to within one-tenth the thickness of a human hair.
The outdoor areas of the Fermilab site are open to the public daily from dawn to dusk. The Lederman Science Center and parts of Wilson Hall have more limited weekday hours.
Admission
Free. All visitors 18 and older must present a REAL ID-compliant photo ID to enter the site.
Accessibility
The main buildings like the Lederman Science Center and Wilson Hall are accessible. The grounds include paved roads and natural trails over flat terrain.
