PARTICLE PHYSICS • CERN • EVIDENCE
LHCb Finds Evidence for an Ultra-Rare Beauty-Particle Decay
A one-in-ten-billion decay reaches 3.2-sigma evidence, offering a precise test of the Standard Model.
ADDED NOVEMBER 3, 2026 • RESEARCH REPORTED NOVEMBER 3, 2026
A rare signal at CERN
On November 3, 2026, the LHCb collaboration reported its first single-experiment evidence for the neutral B meson decay B0 → μ+μ−. A short-lived particle containing beauty, also called bottom, flavor decays into a positively charged muon and a negatively charged muon.
The Standard Model predicts this channel at roughly one decay per ten billion B0 decays. That makes “one in ten billion” the appropriate scale, rather than the screenshot’s one-in-a-billion heading.
Evidence, with confirmation still ahead
The 2024–2025 data reach 3.0-sigma significance. Combining them with LHCb’s 2011–2018 measurements gives 3.2 sigma. Particle physicists generally call a result near three sigma evidence; the conventional threshold for observation is five sigma.
This is evidence for a rare decay, not the discovery of a new particle. The measurement is consistent with the Standard Model. Significance describes how unusual a signal would be under a background-only statistical model; it is not the probability that the theory is true.
Why physicists care
Rare decays offer sensitive tests of proposed physics beyond the Standard Model. A measured rate that reliably disagreed with prediction could point toward additional processes. Agreement also matters: it restricts which alternatives remain plausible.
LHCb’s upgraded detector, improved particle identification and background rejection make the search more powerful. Further data will test whether the signal strengthens and sharpen the measured decay rate.
A long search
UK Research and Innovation describes the milestone as the culmination of a search beginning in 1984. The related Bs0 decay into two muons was established earlier; the much rarer B0 channel is a separate experimental challenge.