Fermilab’s muon g-2 experiment adds further evidence that the standard model is incomplete
Ryan Postel/Fermilab
Cracks are beginning to appear in the standard model of particle physics. A fundamental particle called a muon was caught behaving strangely, new experimental results Researchers at Fermilab, Illinois, have shown that it behaves distinctly differently than the standard model predicts. This could mean that there are strange forces and particles beyond our best theoretical models.
What’s so strange about muon behavior?
This discrepancy manifested itself in the muon’s rotational speed when exposed to a magnetic field. This frequency is indicated by a number called the g-factor and is determined by interactions between muons and other particles. If the standard model is correct and considers all particles and forces present, the g-factor should be exactly 2. However, a series of measurements dating back to 2006 showed that the muons appear to be spinning slightly faster than expected. , with a g-factor of 2.002.
How is the G-factor measured?
A muon’s spin speed is measured using a physical phenomenon called precession, in which the particle wobbles slightly as it rotates. At Fermilab, muons are blasted around a magnetic memory ring at nearly the speed of light, interacting with virtual particles that come and go by quantum effects as they travel. Physicists then map the muon’s precession velocity to something called wiggle. The plot is used to calculate g-factors.
How do these new measurements differ from those measured since 2006?
The new Fermilab measurement is more accurate than any measurement ever made, measuring the g-factor with an accuracy of 0.2 parts per million. This is twice as accurate as Fermilab’s previous series of measurements published in 2021. Importantly, it is accurate enough to reach a statistical confidence level of 5 sigma. This means that the probability that such a pattern of data exists is about 1 in 3.5 million. If the standard model is actually correct, it will show up as a statistical coincidence. In particle physics, the 5-sigma measurement is considered a sure discovery, not just a hint.
How did you achieve this accuracy?
First, the new results involved analyzing far more data than was possible in 2021. After that, only data collected in 2018 was available for analysis, but the new study added data from 2019 and 2020, more than quadrupling the total number of observed muons. . The experimental protocol itself has also been improved with efforts such as stabilizing the muon beam and better characterizing the magnetic field used to rotate the muon. The researchers are now working to incorporate the 2021-2023 data into the most accurate final report on the muon g-factor, due in 2025.
What does this mean for particle physics?
The broad implications of these measurements are still unknown, especially since theoretical efforts to understand the muon g-factor are still underway. However, if discrepancies between measurements and observations persist in future calculations, it means that the standard model is likely missing some particle. That particle could emerge as a virtual particle, interfere with the muon by an as yet undetected force, and then annihilate again. But if such particles exist, we’ll need more precise measurements to know anything about them.
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