The facts
LHCb announced on 3 November the first evidence for the B⁰→μ⁺μ⁻ decay using data from a single experiment. Run 3 data from 2024–25 gave 3.0-sigma significance; combining Run 1 and Run 2 data raised it to 3.2 sigma. [1][2]
CERN says the decay is predicted to occur about once in every ten billion B⁰ mesons. The new measurement is consistent with theoretical predictions and places limits on possible particles beyond the Standard Model. [1][4]
The combined 3.2-sigma result is below the five-sigma convention for claiming a discovery in particle physics; it is evidence, not a discovery. [2][4][5]
A 2015 joint analysis reported earlier evidence for the same decay, but subsequent analyses using data through 2018 did not confirm it. [1][5]
The LHCb collaboration said it had submitted its paper for publication; the preprint was posted on arXiv on 30 October. Peer review was not complete in the sources checked. [2][3]
Relevant context
Rare B-meson decays can be sensitive to the influence of heavier particles that cannot be produced directly in LHC collisions. This result does not establish such particles and agrees with the Standard Model. [1][3][4]
Still unconfirmed
- Whether additional data will raise the statistical significance to the discovery threshold or change the Standard Model comparison remains unknown.
Sources
References support the numbered facts above. Source checks: 5 Nov 2026, 08:16 UTC.
CERN and the LHCb collaboration provide the primary result, supported by the collaboration’s preprint. UKRI and Der Standard independently explain the statistical threshold and caveat. The article distinguishes the new evidence from a discovery, gives the separate Run 3 and combined significance levels, and preserves the unsuccessful earlier analysis.
- LHCb finds evidence of one-in-ten-billion particle decay
CERN · Primary source · 3 Nov 2026
Opened CERN’s announcement. It describes the first single-experiment evidence, gives the predicted rate and says the result is consistent with theory.
- Evidence for the decay B⁰→μ⁺μ⁻ and improved measurements of Bₛ⁰→μ⁺μ⁻
LHCb Collaboration · Primary source · 3 Nov 2026
Opened the collaboration’s result. It reports 3.0-sigma significance for Run 3 alone, 3.2 sigma with Run 1 and Run 2, and says the paper was submitted for publication.
- Evidence for the decay B⁰→μ⁺μ⁻ and improved measurements of Bₛ⁰→μ⁺μ⁻
LHCb Collaboration · Primary source · 30 Oct 2026
Opened the collaboration’s preprint record; it is the paper submitted for publication and was not yet peer-reviewed in the sources checked.
- Scientists find evidence of rarest particle decay after 42 years
UK Research and Innovation · Primary source · 3 Nov 2026
Opened the funding agency’s account, which states the combined 3.2-sigma result remains below the five-sigma discovery threshold and is consistent with the Standard Model.
- Nach 40 Jahren Suche: Hinweise auf extrem seltenen Teilchenzerfall
Der Standard · Reporting · 4 Nov 2026
Opened independent Austrian science reporting. It describes the result as evidence rather than discovery and says the 2015 indication did not survive later analyses.