LHCb Rare Decay Evidence Arrives After a 42-Year Search

The LHCb rare decay search has produced its first evidence for a B0 meson breaking down into two muons, a process predicted to happen only about once in every ten billion decays. In a CERN announcement published after a November 3, 2026 seminar, the LHCb collaboration reported 3.2 sigma significance, enough to call it evidence but short of the 5 sigma needed for a discovery. The measured rate matches the Standard Model.

What did LHCb actually see?

A B0 meson is a short-lived particle made of a beauty (bottom) quark and a down antiquark. Muons are heavier cousins of the electron, roughly 207 times its mass. Using 18 fb-1 of proton collision data collected at 13.6 TeV in 2024 and 2025, LHCb found 3.0 standard deviations from this new Run 3 data alone. Combined with earlier 2011 to 2018 data, the significance rises to 3.2 sigma. The measured branching fraction is 1.26 (+0.51 -0.47) x 10^-10, close to the Standard Model expectation of about 1.03 x 10^-10.

Why does a one in ten billion decay matter?

This decay cannot happen directly. It only proceeds through quantum loops involving heavy particles such as W bosons and top quarks, so any undiscovered heavy particle could nudge the rate up or down. That makes it a sensitive probe for new physics. Because the result lines up with the Standard Model, it does not reveal anything exotic, but it narrows the room where new particles could hide.

The University of Liverpool describes it as the end point of a 42 year search. The hunt began in 1984 with the CLEO experiment, which could only set upper limits.

How did the upgraded detector make this possible?

LHCb rebuilt much of its detector for Run 3. The Upgrade I detector dropped the old hardware first level trigger and now selects collisions entirely in software, which lets it keep far more interesting events. The analysis then used a boosted decision tree, a machine learning classifier, to separate the handful of real signals from background. It is another example of algorithms reshaping hard science problems, much like the DeepMind RSA-1024 work did in cryptography.

What else did LHCb measure?

The collaboration also reported the most precise measurement by a single experiment of the related Bs0 decay into two muons, which LHCb first observed in 2015. Run 3 data puts its branching fraction at 3.31 x 10^-9, with an effective lifetime of 1.42 +/- 0.18 picoseconds. Both values agree with theory. The full paper is available as arXiv:2611.00230 and has been submitted for publication, so it has not yet passed peer review.

What comes next for LHCb?

CERN says the collaboration will keep collecting data to reach the 5 sigma threshold. If it gets there, B0 to two muons would become the rarest beauty particle decay ever observed. Until then, the honest reading is that this is strong evidence, not a confirmed discovery. For readers who like the theory side of hard limits, our piece on the exponential time hypothesis is a good companion.

Item	Result
Decay	B0 to two muons
Data set	18 fb-1, Run 3, 2024 to 2025, 13.6 TeV
Significance	3.0 sigma Run 3 alone, 3.2 sigma combined
Branching fraction	1.26 (+0.51 -0.47) x 10^-10
Bs0 to two muons	3.31 x 10^-9, lifetime 1.42 ps
Status	Evidence, not discovery