CERN’s LHCb experiment finds evidence of a decay that occurs once in roughly 10 billion cases
After more than 40 years of searching, physicists at CERN’s LHCb experiment have reported evidence of one of the rarest known particle decays: a B_d meson turning into a muon and an antimuon, a process expected only about once in 10 billion beauty-particle decays. The result was announced at a CERN seminar and is the first time a single experiment has gathered enough evidence for this channel. B_d contains a beauty quark and a down antiquark; the muons are heavier cousins of electrons. The measured rate matches Standard Model predictions and thereby tightens the room left for unknown particles and forces.
Such decays are prized as “golden channels” because quantum effects let even particles too heavy to be produced in the Large Hadron Collider leave traces in the rate. A beauty quark and an oppositely charged antiquark annihilate and their energy becomes a muon pair. A related process, B_s decaying to two muons, is more common — about three times per billion beauty decays, roughly 30 times more frequent than B_d. In 2015 a joint LHCb and CMS analysis confirmed B_s → μ⁺μ⁻ and offered only initial hints of B_d; later work including data through 2018 did not confirm those hints.
The breakthrough rests on an upgraded LHCb detector that in 2024 began taking data at five times the 2010–2018 rate and identifies particles more cleanly, plus machine-learning methods to pick rare muon pairs from overwhelming background. Marco Santimaria, who co-led a 19-scientist analysis, stressed the precision needed. Using 2024 and 2025 data, LHCb reached 3.0 standard deviations — the conventional threshold for “evidence,” corresponding to about a 0.3% chance of a fluctuation at least this extreme — and then combined those measurements with earlier 2011–2018 data.
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