After 40 years, CERN finds evidence of a one-in-10-billion particle decay

Physicists working with the LHCb experiment at CERN have detected evidence of a particle decay predicted to occur only once in approximately 10 billion beauty particle decays. The researchers identified the decay of a B d particle, which contains a beauty quark and a down antiquark, into a pair of muons. This marks the first time a single experiment has produced sufficient evidence for this extraordinarily rare event.

The decay, written as B d → μ+ μ-, reached a statistical significance of 3.0 standard deviations using data from 2024 and 2025. When combined with earlier measurements from 2011 to 2018, the significance increased to 3.2 standard deviations, exceeding the threshold for evidence. A confirmed observation requires five standard deviations.

The results align with predictions from the Standard Model, the leading theory of fundamental particles and their interactions. The decay is valuable because quantum effects allow particles too massive to be created directly in the Large Hadron Collider to leave subtle traces in how rare decays occur. Discrepancies between measurements and theoretical predictions could reveal previously unknown particles.

A major upgrade to the LHCb detector, which began collecting data in 2024 at five times the previous rate, made the discovery possible. The upgraded detector better identifies different particle types. Researchers also applied machine learning techniques to distinguish the rare signal from overwhelming background noise.

Marco Santimaria, who co-led the international analysis team of 19 scientists, stated, "Extreme precision is required to pick the muons out of the overwhelming background."

The team also produced the most precise measurements ever obtained for a related decay, B s → μ+ μ-, which occurs approximately three times per billion beauty particle decays.

Future upgrades to the Large Hadron Collider, including the High Luminosity LHC scheduled to begin in 2030 and another LHCb upgrade planned for the middle of the following decade, could increase data collection rates by another factor of ten. Harry Cliff, LHCb's Rare Decays Convener and Analysis Co-lead, noted that "the future LHCb upgrade will allow us to observe many more of these decays, providing even greater sensitivity to undiscovered particles."