After a search lasting more than forty years, CERN's LHCb Collaboration has found evidence of one of the rarest particle decays ever seen. A particle made of a beauty quark and a down antiquark, called the B-d meson, can decay into a pair of muons, but only about once in ten billion decays.
Data from 2024 and 2025 showed the decay at a significance of 3.0 standard deviations; combined with data from 2011 to 2018, this rose to 3.2, above the usual threshold for "evidence." A 2024 upgrade that lets LHCb collect data at five times its earlier rate, plus machine-learning tools, helped pick the signal out of a huge background.
The result agrees with Standard Model predictions and limits possible new particles. Physicists now aim for 5 standard deviations, which would make it the rarest beauty-particle decay ever observed. The result was announced at a CERN seminar on November 3.
Source: CERN, November 3, 2026 — https://home.cern/lhcb-finds-evidence-of-one-in-ten-billion-particle-decay
