On Nov 3, 2026, CERN announced that the LHCb collaboration has found evidence that the B0 meson, made of a beauty quark and a down antiquark, sometimes decays into a pair of muons (a muon is a heavier cousin of the electron). The result is a preprint (arXiv:2611.00230, Oct 30, 2026), submitted to Physical Review X and not yet peer reviewed. The search began with the CLEO experiment in 1984.
The number
About 1 in 10 billion. The Standard Model, the main theory of particles, predicts that about one B0 meson in 10 billion decays into two muons (1.03 ± 0.05 per 10 billion). LHCb measured 1.14 per 10 billion, give or take about 0.4, which agrees with the prediction.
What was found
- The signal reached 3.2 standard deviations (sigma) with 2024 and 2025 data combined with earlier runs; the new data alone give 3.0. Sigma scores how unlikely it is that random background produced the signal.
- CERN calls it the first evidence from a single experiment. A 2015 joint LHCb and CMS claim was not confirmed by data up to 2018.
- The upgraded LHCb detector collects data five times faster, and machine learning (a type of AI) helped suppress the background of look-alike events.
- The related Bs0 decay into two muons happens around three times in every billion; the B0 rate is about thirty times lower.
- All measurements agree with the Standard Model and place limits on possible new particles.
What it does not mean
- It is evidence, not a discovery: evidence needs 3 sigma, a discovery needs 5, and this result has 3.2 sigma.
- It is a preprint, not yet peer reviewed.
- No new particle was found; the result fits the Standard Model.
- "The rarest beauty-particle decay ever observed" holds only if confirmed.
Why it matters
The more unlikely a decay, the more sensitive it is to new particles, including ones too heavy for LHC collisions to produce directly. So far the Standard Model holds up.
Sources: CERN, Nov 3, 2026, home.cern/lhcb-finds-evidence-of-one-in-ten-billion-particle-decay; LHCb collaboration, "Evidence for the decay B0 -> mu+ mu- and improved measurements of the decay Bs0 -> mu+ mu-", preprint arXiv:2611.00230, Oct 30, 2026, arxiv.org/abs/2611.00230; checked Nov 9, 2026.
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