LHCb observes rare beauty particle decay with 3.2 sigma significance
CERN's LHCb experiment has found the first single-experiment evidence of the extraordinarily rare Bd → μ+μ- decay, a process occurring roughly once in ten billion events, with a significance of 3.2 sigma.
CERN announced on November 9, 2026, that the LHCb experiment has detected evidence of a particle decay occurring once in roughly 10 billion decays, ending a four-decade search. The Bd → μ+μ- decay was observed with 3.2 sigma significance. The decay involves a particle called Bd, which contains a beauty quark and a down antiquark, decaying into a pair of muons. The measurement reached a statistical significance of 3.2 sigma, which constitutes evidence but not a discovery, as the five-sigma threshold is required for a discovery in particle physics.
The evidence reached a statistical significance of 3.0 standard deviations using 2024-2025 data, increasing to 3.2 when combined with 2011-2018 data. The statistical significance of the find is 3.2 sigma, below the 5 sigma threshold for a major discovery. Beauty quarks are heavier than up and down quarks and are produced in huge numbers at the collider but last only around one and a half trillionths of a second before decaying. The results agree with predictions from the Standard Model of particle physics.
Experimental breakthrough enabled by LHCb upgrade
The upgraded LHCb detector, operational since 2024, collected data at five times the previous rate, enabling the rare decay observation. The upgraded detector can collect data at up to five times its previous rate and is expected to gather a much larger dataset into the early 2030s. It improved particle identification and combined with machine learning to distinguish rare signals from background noise. Marco Santimaria, who co-led the analysis team of 19 scientists, stated that physicists had been chasing this decay for more than forty years. He added, "Extreme precision is required to pick the muons out of the overwhelming background."
Decay rarity and significance context
The decay occurs once in approximately 10 billion beauty particle decays, with the 3.2 sigma result falling short of the five sigma discovery threshold. The Bs → μ+μ- decay occurs about three times per billion beauty particle decays, while the Bd decay is predicted to be about 30 times rarer. If confirmed, Bd → μ+μ- would become the rarest beauty particle decay ever observed. A confirmed observation requires a significance of five standard deviations. The decay is considered a 'golden channel' for investigating physics beyond the Standard Model, as it offers indirect evidence of energy scales beyond direct production and could connect to phenomena like dark matter and the asymmetry in normal matter and antimatter.
| Decay Channel | Approximate Frequency per Beauty Particle Decay | Relative Rarity |
|---|---|---|
| Bs → μ+μ- | 3 in 1 billion | Baseline |
| Bd → μ+μ- | 1 in 10 billion | ~30 times rarer than Bs |
Future prospects for physics exploration
Future upgrades, including the HiLumi LHC in 2030 and further LHCb enhancements, will increase data collection by up to tenfold, enhancing sensitivity to new physics. The High Luminosity LHC (HiLumi LHC) is scheduled to begin operating in 2030. Another major LHCb upgrade is planned for the middle of the 2030s. Together, future upgrades could increase the experiment's data collection rate by a factor of ten. Researchers expect future data to allow more precise measurements and greater sensitivity to undiscovered particles.
