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Lawbreaking Particles May Point to a Previously Unknown Force in the Universe

For decades physicists have sought signs of misbehaving particles—evidence of subtle cracks in the “Standard Model” of particle physics, the dominant theory describing the most fundamental building blocks of our universe. Although the Standard Model has proved strikingly accurate, scientists have long known some adjustments will be needed. Now, as a recent review paper in Nature documents, experimenters have started seeing suggestions of particles flouting the theory—but they’re not quite the violations theorists were looking for.

The evidence comes from electrons and their more massive cousins, muons and tau leptons. According to the Standard Model, these three particles should behave like differently sized but otherwise identical triplets. But three experiments have produced growing evidence—including results announced in just the last few months—that the particles react differently to some as-yet mysterious influence. The findings are not yet conclusive, but if they hold up, “it would be a complete revolution,” says California Institute of Technology theorist Mark Wise.

Tantalizing Signs

A shake-up in the Standard Model would be huge. This theory has formed the bedrock of particle physics research since it was fleshed out in the late 20th century. It carves the universe into twelve elementary particles that make up all matter, plus ‘force-carrier’ particles that transmit the fundamental forces of nature. (For instance, particles exert electrical or magnetic forces by exchanging transient photons.) Despite its successes, however, the Standard Model predicts nothing that would explain gravity or the dark matter thought to invisibly inhabit space. To marry particle physics with these larger-scale observations, theorists have proposed all manner of “new physics”—matter or forces beyond the Standard Model’s menagerie. But most experiments have stubbornly upheld the theory with impressive fidelity, finding no evidence of the…

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