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Particle Physics Could Be Disrupted by Odd Muon Behavior

Fermilab’s Muon g-2 experiment is adding to evidence that the standard model is incomplete

Ryan Postel/Fermilab

The standard model of particle physics is starting to show flaws. An elementary particle known as the muon has exhibited peculiar behavior, and recent experimental findings from Fermilab in Illinois have solidified the fact that it behaves differently than what the standard model predicts. This suggests the existence of unknown forces and particles that go beyond our current understanding.

What is peculiar about the behavior of muons?
The anomalies are observed in the rate at which muons rotate when subjected to a magnetic field. This rotation frequency, represented by the g-factor, is influenced by the interactions between muons and other particles. According to the standard model, which accounts for all known particles and forces, the g-factor should be precisely 2. However, measurements dating back to 2006 indicate that muons spin slightly faster than expected, resulting in a g-factor of 2.002.

How is the g-factor measured?
The spin rate of a muon is measured using a phenomenon called precession, which causes the particle to wobble slightly as it spins. At Fermilab, muons are propelled around a magnetic storage ring at nearly the speed of light. During this journey, they interact with virtual particles that emerge and vanish due to quantum effects. Physicists then analyze the muons’ precession rates using a wiggle plot, which enables them to calculate the g-factors.

How do these new measurements differ from previous ones since 2006?
The recent measurements conducted at Fermilab are more precise than any previously taken, offering a precision of 0.2 in a million for the g-factor. This level of precision is twice that of Fermilab’s previous measurements announced in 2021. Importantly, these measurements have achieved a statistical confidence level of 5 sigma, indicating a 1 in 3.5 million chance that this data pattern is a statistical fluke if the standard model were accurate. In particle physics, a 5-sigma measurement is considered a reliable discovery, rather than a mere suggestion.

How was this precision achieved?
To begin with, the new results involved analyzing a significantly larger amount of data than was available in 2021. In the previous research, only data from 2018 was utilized, whereas the recent findings incorporated data from 2019 and 2020, resulting in more than four times the number of observed muons. Additionally, the experimental procedures were enhanced, including the stabilization of the muon beam and better characterization of the magnetic field that induces muon spin. The researchers are currently working on integrating data from 2021 to 2023 into their final and most precise report on muon g-factors, planned to be released in 2025.

What implications does this have for particle physics?
The broader impact of these measurements remains uncertain, especially as theoretical efforts to comprehend muon g-factors are still ongoing. However, if the discrepancy between measurements and predictions persists in future calculations, it suggests that the standard model likely overlooks some type of particle. This particle could manifest as a virtual particle that interacts with muons through an undetected force before vanishing. Yet, more precise measurements are necessary to gain any insights into the existence of such a particle.

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