The new measurement of muons baffles scientists

Modern physics is in an alarming state. The Standard Model is the name of the best theory ever devised to explain subatomic physics, and it is largely successful, with many measurements agreeing very well with the predictions. However, some very big mysteries remain. For example, the current theory cannot explain why antimatter is not observed in nature, nor can it provide an explanation for dark matter or dark energy. Therefore, the Standard Model is clearly incomplete.

Despite decades of experimentation with large particle accelerators, researchers have found no paradox that points them in a promising direction. However, particle accelerators are not the only way to study the laws of nature. Other scientists use tabletop experiments to measure fundamental constants very precisely, hoping to find differences between predictions and measurements that will allow scientists to develop better theories.

Measurement of the magnetic moment of an electron

Now, a new measurement of the humble electron’s magnetic properties has achieved amazing accuracy and agreed well with the prediction, while simultaneously baffling the world’s physics research community.

Like many subatomic particles, the electron has an electrical charge and acts like a tiny magnet. A quantum mechanics theory developed in the 1920s predicted the magnetic strength of a single electron (known as the Attractive moment) for decent accuracy. However, in 1947, measurements and calculations found that early predictions were somewhat inaccurate. Improved calculations that included the effects of all known subatomic particles changed the value of the electron’s magnetic properties by 0.1%.

Although this is a small effect, it gives researchers a way to search for the presence of new particles — that is, particles that are not currently accounted for in the Standard Model. If there are more particles, the calculation will again change slightly.

Accordingly, the researchers embarked on a decades-long program to achieve a more accurate measurement of the electron’s magnetic properties. In the fall of 2022, the researchers announced a result in which the measurement and prediction were consistent with an astonishing twelve-digit accuracy. The new measurement claims to be correct to a factor of 1.3 out of 10 trillion.

The fact that prediction and measurement agree so incredibly well is a triumph of both experimental and theoretical ingenuity and provides a strong argument that this measurement is not sensitive to influences beyond the Standard Model. In other words, there is no “new physics” to be seen here.

The muon puzzle

But this is not the whole story. The electron is not the only subatomic particle that acts as a small magnet, and the strength of the magnet depends on every subatomic particle known to scientists.

The muon is the cousin of the electron. Like the electron, it has the same charge and acts like a magnet. But the muon is about 200 times heavier than the electron and is unstable, decaying in 2.2 microseconds. As with the electron, the muon has magnetic properties 0.1% greater than what quantum mechanics predicted in the 1920s.

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Scientists can measure and calculate the magnetic moment of a muon, though with less precision than an electron: The reported uncertainty is about 4.6 parts per ten million. (Full disclosure: The measurement of the muon’s magnetic moment was done at the Fermi National Accelerator Laboratory where I’m a senior scientist.)

For the muon, the experimentally measured and theoretically calculated value of its magnetic properties do not agree completely. When two numbers differ, the reason may be that one or both of them are not accurate. Or it could be a statistical fluke (like flipping heads ten times in a row with a fair coin). Most excitingly, it could refer to a little-known phenomenon – “new physics”.

A proper statistical analysis shows that we would need to run the experiment about 40,000 times to see the difference observed just by chance. Seeing this as highly unlikely, scientists have seriously begun to consider the possibility that the discrepancy observed in the muon’s measurements may be a hint of undiscovered physics.

New physics?

It is noteworthy that both the measurement and prediction of the muon magnetic moment are still in a state of flux, and updates are expected soon. But there’s a reason to get excited (at least a little).

The new measurement of the electron’s magnetic moment is a bit puzzling. It is 3,100 times more accurate than the same measurement for the muon, and the electron measurement is in good agreement with the Standard Model. Why is the muon measurement less accurate and not consistent with the prediction of the Standard Model? It’s as if the electron and muon tell different stories.

Perhaps further investigations into the fundamental nature of electrons and muons will provide important clues to undiscovered laws of nature.

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