Particle Physicists Are Getting a First Glimpse Into the ‘Zeptoverse’

CERN scientists have found evidence of an exceptionally rare decay in which a neutral B meson transforms into a muon and an antimuon. The result marks the first experimental evidence of a process physicists have sought for more than 40 years, and gives them a precise way to test whether the standard model describes matter at extremely small scales.

The standard model predicts this decay happens about once in every 10 billion neutral B meson decays, making it difficult to distinguish from other collision events. Faster data collection following a 2024 upgrade to the Large Hadron Collider, combined with machine-learning methods to filter background signals, helped researchers identify it. The finding agrees with current predictions; further measurements could point to new physics in the “zeptoverse”, where distances are measured in zeptometers.