CERN LHCb Finds Evidence of an Ultra-Rare Particle Decay
British scientists working on CERN’s LHCb experiment have found evidence of a particle decay expected to occur once in every ten billion decay events. The result, based on data collected from 2011 to 2018 and in 2024 and 2025, has a statistical significance of 3.2 sigma and awaits peer review.
British scientists working on CERN’s Large Hadron Collider Beauty experiment found evidence of an exceptionally rare particle decay, a science breakthrough that could help test current physics theories. The result, announced at a seminar on November 3, describes a Bd meson transforming into a muon and an anti-muon.
The decay is expected to occur once in every ten billion decay events. The findings appear in a preprint paper on arXiv and have not yet undergone peer review.
The analysis combined data gathered from 2011 to 2018 with observations from 2024 and 2025. Researchers reported a statistical significance of 3.2 sigma, above the three-sigma threshold used to claim evidence of a phenomenon, but short of the five-sigma standard for definitive confirmation.
The particle, called a Bd meson, contains a beauty quark and a down antiquark. Beauty quarks do not occur naturally today, according to the account of the research; scientists produce them by colliding protons at speeds approaching the speed of light.
The collisions create short-lived particles. The resulting mesons exist for about one and a half trillionths of a second before decaying, while the muons from this decay last approximately two millionths of a second before breaking down into electrons and positrons.
Recent upgrades to the LHCb detector improved its ability to detect collisions, while machine-learning algorithms helped distinguish signals from background noise. The VELO sub-detector sits within a finger’s width of the particle beam, allowing it to track particles produced in the collisions.
Niladri Sahoo, a particle physicist at the University of Birmingham and convenor of the LHCb Very Rare Decays working group, said the result offers “a powerful new test of the Standard Model and a window into potential new physics.”
The findings are consistent with predictions from the Standard Model, which describes many physical phenomena but does not explain dark matter or the imbalance between matter and antimatter. Researchers say rare decay patterns may offer indirect signs of physics beyond the model.
Planned upgrades in the 2030s are expected to increase the number of collisions by an order of magnitude, potentially giving researchers more data to pursue the five-sigma threshold.
