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New Measurements of Muons Might Rewrite Particle Physics

The gap between theoretical predictions and the experimental measurements isn’t a full-blown discovery yet

The 50-foot-wide racetrack used to study muons traveled by barge around Florida and up the Mississippi, and then by truck across Illinois. (Reidar Hahn, Fermilab)

About 50 years ago, physicists came up with a rulebook to describe the ways fundamental particles interact to create the world as we know it. Since then, researchers have pushed that theoretical framework, called the Standard Model, to its limits in order to study its imperfections.

Now, results from two particle physics experiments have come tantalizingly close to discovering a gap in the Standard Model.

The experiments focused on muons, which are similar to electrons. Both have an electric charge and spin, which makes them wobble in a magnetic field. But muons are over 200 times larger than electrons, and they split apart into electrons and another particle, neutrinos, in 2.2 millionths of a second. Luckily, that’s just enough time to gather precise measurements, given the right equipment, like a 50-foot-wide magnet racetrack.

Physicist Chris Polly of the Fermi National Accelerator Laboratory presented a graph during a seminar and news conference last week that showed a gap between theoretical calculation and the actual measurements of muons moving in the racetrack.

“We can say with fairly high confidence, there must be something contributing to this white space,” said Polly during the news conference, per Dennis Overbye at the New York Times. “What monsters might be lurking there?”

The Standard Model aims to describe everything in the universe based on its fundamental particles, like electrons and muons, and its fundamental forces. The model predicted the existence of the Higgs boson particle, which was discovered in 2012. But physicists know that the model is incomplete—it takes into account three fundamental forces, but not gravity, for example.

A mismatch between theory and experimental results could help researchers uncover the hidden physics and expand the Standard Model so that it explains the universe more fully.

“New particles, new physics might be just beyond our research,” says Wayne State University particle physicist Alexey Petrov to the Associated Press’ Seth Borenstein. “It’s tantalizing.”


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