Summary
The LHCb Collaboration reports 3.2-standard-deviation evidence for the rare decay of a beauty particle into two muons. The result is evidence, not yet a confirmed observation; further data will be needed to reach the 5-standard-deviation threshold.
The LHCb Collaboration at CERN reports evidence for a beauty particle decaying into a pair of muons, a process predicted to occur about once in every 10 billion beauty-particle decays. The result has a statistical significance of 3.2 standard deviations when new data are combined with earlier measurements. It is evidence for the decay, not yet a confirmed observation.
The process, written as Bd → μ⁺μ⁻, involves a particle containing a beauty quark and a down antiquark. LHCb found 3.0-standard-deviation evidence in data collected from 2024 to 2025; combining those data with measurements from 2011 to 2018 raised the significance to 3.2 standard deviations. CERN announced the result on 3 November 2026.
Why this rare decay matters
In this decay, the beauty quark and antiquark annihilate, producing a muon and its antiparticle. Muons resemble electrons but are heavier. The decay is exceptionally rare: its predicted rate is about 30 times lower than that of the related Bs → μ⁺μ⁻ decay, which occurs around three times per billion beauty-particle decays.
Rare decays can be sensitive to particles too heavy to produce directly in collisions. In quantum physics, other particles can influence how a decay happens, so careful measurements of an unlikely process can help test predictions of the Standard Model and constrain possible new particles. CERN says the new result is consistent with theoretical predictions and places limits on possible particles beyond the Standard Model.
The Bd decay has been sought for decades. A 2015 joint analysis by LHCb and CMS reported its first evidence, but later updates using data collected through 2018 did not confirm that evidence. The new result is the first evidence for the decay from a single experiment, according to CERN.
How LHCb improved the search
An upgrade in 2024 enabled LHCb to collect data at five times the rate achieved between 2010 and 2018. The upgraded detector also improved its ability to distinguish between particle types. The collaboration used these improvements alongside machine-learning techniques to suppress background processes that could obscure the rare signal.
In particle physics, a result at three standard deviations is generally described as evidence; five standard deviations is the threshold the collaboration says it will seek for confirmation. Further research will test whether the signal reaches that level. If confirmed, CERN says this would be the rarest beauty-particle decay observed to date.
CERN points to further gains ahead: the HiLumi LHC is due to start in 2030, and a planned LHCb upgrade in the middle of the next decade could increase the data-collection rate by another order of magnitude. More data would give researchers greater sensitivity to this and other rare decays.