The standard model of particle physics must be incomplete. It doesn’t explain gravity or dark matter, among other phenomena. But the model does an excellent job describing the other basic building blocks and forces of nature, and measurements that violate it are hard to find.
That’s why it was big news last year when the Muon g − 2 collaboration at Fermilab found that the muon’s magnetic moment anomaly differs from the standard-model value by 4.2 standard deviations (see Physics Today, June 2021, page 14). Although a substantial difference, it fell short of the 5 standard deviations that are canonically required to claim a discovery.
In April the Collider Detector at Fermilab (CDF) collaboration published a result that surpasses that threshold and challenges the standard model. Using the now-shut-down Tevatron collider, the 400-person collaboration measured a W-boson mass that is 7 standard deviations higher than predicted and more precise than all previous measurements combined.1 If independently confirmed, the result points to physics beyond the standard model.
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