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  • April 13, 2021

Exclusive: Particle Mystery Deepens, As Physicists Confirm That the Muon Is More Magnetic Than Predicted

 Exclusive: Particle Mystery Deepens, As Physicists Confirm That the Muon Is More Magnetic Than Predicted

sciencehabit writes: A potential chink in physicists’ understanding of fundamental particles and forces now looks more real. New measurements confirm a fleeting subatomic particle called the muon may be ever so slightly more magnetic than theory predicts, a team of more than 200 physicists reported this week. That small anomaly — just 2.5 parts in 1 billion — is a welcome threat to particle physicists’ prevailing theory, the standard model, which has long explained pretty much everything they’ve seen at atom smashers and left them pining for something new to puzzle over. “Since the 1970s we’ve been looking for a crack in the standard model,” says Alexey Petrov, a theorist at Wayne State University. “This may be it.” But Sally Dawson, a theorist at Brookhaven National Laboratory, notes the result is still not definitive. “It does nothing for our understanding of physics other than to say we have to wait a little longer to see if it is real.”

For decades, physicists have measured the magnetism of the muon, a heavier, unstable cousin of the electron, which behaves like a tiny bar magnet. They put muons in a vertical magnetic field that makes them twirl horizontally like little compass needles. The frequency at which the muons twirl reveals how magnetic they are, which in principle can point to new particles, even ones too massive to be blasted into existence at an atom smasher like Europe’s Large Hadron Collider. That’s because, thanks to quantum uncertainty, the muon sits amid a haze of other particles and antiparticles flitting in and out of existence. These “virtual” particles can’t be observed directly, but they can affect the muon’s properties. Quantum mechanics and Albert Einstein’s theory of special relativity predict the muon should have a certain basic magnetism. Familiar standard model particles flitting about the muon increase that magnetism by about 0.1%. And unknown particles lurking in the vacuum could add another, unpredictable increment of change.

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