British scientists make ‘thrilling’ one-in-10-billion discovery after 40 years of searching

CERN’s Large Hadron Collider Beauty experiment has found evidence of an exceptionally rare particle decay, in which a Bd meson turns into a muon and an anti-muon. The result matters because this decay offers a precise way to test the Standard Model and could help researchers look for signs of physics it does not explain.

The process is expected to occur about once in every 10 billion Bd meson decays, and scientists have searched for it for around 40 years. Combining data from 2011 to 2018 with observations from 2024 and 2025, the team reports a significance of 3.2 sigma, above the threshold for evidence but below the five sigma standard for confirmation. The findings are in a preprint awaiting peer review and agree with current Standard Model predictions; planned detector upgrades in the 2030s could help test the result further.