Muons are still behaving oddly, which could break particle physics

The standard model of particle physics is beginning to show cracks. A fundamental particle called the muon has been caught behaving strangely, and new experimental results from Fermilab in Illinois have shown that it is definitely acting differently than the standard model would predict, which could mean that there are strange forces and particles out there beyond our best theoretical model.

What’s strange about the muons’ behaviour?
The discrepancies showed up in the rate at which muons spin when exposed to a magnetic field. This frequency, denoted by a number called the g-factor, is determined by interactions between muons and other particles. If the standard model is correct and accounts for all the particles and forces in existence, the g-factor should be precisely 2. But a series of measurements dating back to 2006 have shown that muons seem to rotate ever-so-slightly faster than expected, giving a g-factor of 2.002.

How is the g-factor measured?
The spin rate of a muon is measured using a physical phenomenon called precession, in which the particle wobbles slightly as it spins. At Fermilab, muons are blasted around a magnetic storage ring at nearly the speed of light, and as they travel they interact with virtual particles that blink in and out of existence due to quantum effects. Then, physicists map the muons’ precession rates on what’s called a wiggle plot, which they use to calculate their g-factors.

How are these new measurements different from the ones taken since 2006?
The new Fermilab measurements are more precise than any that have been taken before, measuring the g-factor to a precision of 0.2 in a million. That is twice as precise as Fermilab’s previous set of measurements, announced in 2021. Crucially, it is precise enough to reach a statistical confidence level of 5 sigma, meaning that there is about a 1 in 3.5 million chance that a pattern of data like this would show up as a statistical fluke if the standard model were actually correct. In particle physics, a 5-sigma measurement is considered a secure discovery, rather than just a hint.

How did they achieve this precision?
For a start,  this new result involved analysing far more data than was possible in 2021. Then, only data collected in 2018 was available to analyse, whereas the new research added data from 2019 and 2020, more than quadrupling the total number of muons observed. The experimental protocol itself has also been improved in a campaign that included stabilising the muon beam and better characterising the magnetic field used to make the muons spin. The researchers are now working to incorporate data from 2021 to 2023 in their final, most precise report on the g-factor of muons, which is expected to be released in 2025.

What does this mean for particle physics?
The broader impact of these measurements is still up in the air, especially as theoretical efforts to understand muons’ g-factors are still ongoing. But if the discrepancy between measurements and observations stays in future calculations, that means that the standard model is most likely missing some sort of particle. That particle could be popping up as a virtual particle, interfering with muons through some as-yet-undetected force, and then disappearing again. But it will take even more precise measurements to tell anything about such a particle, if it exists.