ARTWORK: Eliza Williams
Cracking Muon g-2 Andy Yin
The date is February 25 of this year. 170 physicists gather in a Zoom call and intently watch two of their number rip open an envelope. Both of them hold up a sheet of paper to the screen. Each has the same number written on it a number that has been kept secret for three years. Someone else types it into a formula in Python, and a single graph appears on the screen. There are just four points on it - it seems like too little to get excited over - but the call erupts into applause in an instant. What these physicists were waiting to see is a quantity called the muon g-2. Their anticipation is well justified - their efforts to measure it have been nine years in the making, and the last time it was measured was 20 years ago. These physicists are members of the muon g-2 collaboration at Fermi National Accelerator Laboratory, better known as Fermilab, in Illinois. On April 8, the world learned what they found, and shared in their excitement: their measurements conflicted with theoretical expectations. It strengthens the possibility of a gap in the Standard Model (SM) of particle physics - our best current model of the fundamental particles
of reality - and hints at the existence of hitherto undescribed particles. The star of the story is the muon - it’s a fundamental particle, meaning it isn’t divided into any smaller objects - sometimes called the heavier cousin of the electron. Muons have the same negative electric charge as the electron, but have about 207 times the mass. The reason that electrons are found in atoms, and muons are not, is that their mass makes them unstable - they quickly decay into lighter subatomic particles. As a consequence, experiments on muons are a lot more challenging, because it’s difficult to get hold of a large number of them before they decay. The quantity measured at Fermilab is the muon’s g-factor. This is a number that describes how a particle behaves in a magnetic field. As an analogy, swap out the magnetic field for a field that we experience constantly: the gravitational field. Swap the muon for a spinning top. If you spin a top, it will spin around its point, but if you tilt the top first, the direction of tilt will also spin. This rotation is called precession. Anything that spins around an axis, if that axis isn’t perfectly vertical, will experience this.
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