Physicists on the LHCb experiment at CERN's Large Hadron Collider have reported the first single-experiment evidence for one of the rarest processes the Standard Model predicts: a B-zero meson, made of a beauty quark and a down antiquark, decaying into a pair of muons.
The decay is expected to occur about once in every 10 billion decays of beauty particles. CERN announced the result in a release republished by ScienceDaily.
The analysis was co-led by Marco Santimaria, one of 19 scientists on the international team. "Physicists have been chasing this decay for more than forty years," he said.
The signal reaches 3.0 standard deviations using data collected in 2024 and 2025. Combined with data taken between 2011 and 2018, it rises to 3.2 standard deviations. In particle physics 3 sigma counts as evidence, while a confirmed observation requires 5, which this result does not reach.
"Seeing evidence of this rare decay is incredibly exciting," said Harry Cliff, LHCb's Rare Decays Convener and an analysis co-lead.
The equivalent decay of the B-sub-s meson, which is about 30 times more common at roughly three occurrences per billion, was observed in 2015 in a joint analysis by LHCb and CMS, published in Nature.
LHCb was upgraded and began collecting data again in 2024 at five times the rate of its 2010 to 2018 run. Machine learning was used to separate the signal from the background.
Both decays are stringent tests of the Standard Model, because a measured rate differing from the prediction would be indirect evidence of particles the theory does not contain. CERN says the results agree with predictions from the Standard Model.
The thresholds are where this becomes interesting. At 3.2 sigma the decay has been judged to exist as evidence but not as an observation, which gives it a standing the rest of physics does not have to manage: it happens, provisionally.
The rarity makes that provisional state durable. A process occurring once in ten billion decays cannot be confirmed any faster than the collider can produce ten billion decays, so promotion from evidence to discovery is a question of statistics rather than of nature. Nature settled it long before anybody was looking.
There is a further awkwardness. The measured rate agrees with the Standard Model, which is the theory that predicted the decay in the first place. Forty years of searching have therefore established that the prediction was correct, which is what the prediction said.
Our analysts note the practical implication for the other 9,999,999,999 decays. In every one of them the B-zero meson does something else, and does it reliably, which makes the paper's most dependable finding the one it does not report.
The Department of Unnecessary Research has filed the result under provisional existence and recommends no further action until five sigma. Until then the decay occupies the position of a thing seen often enough to be announced and not often enough to have happened.