Physicists working with the LHCb experiment at CERN have announced the detection of evidence for one of the rarest known particle decays, a process expected to occur only once in approximately 10 billion beauty particle decays. The discovery, unveiled during a seminar at the facility, represents the first time a single experiment has produced sufficient evidence for this extraordinarily rare event. The decay in question involves a particle called Bd, which contains a beauty quark and a down antiquark, transforming into a pair of muons through a process denoted as Bd → μ+μ-.
Rare particle decays hold particular significance for physicists because quantum effects allow heavy particles—ones too massive to be created directly in the Large Hadron Collider—to potentially leave subtle traces in the rates of these processes. By measuring such decays with high precision, researchers can identify discrepancies between experimental results and theoretical predictions established by the Standard Model. Such differences could provide indirect evidence of previously unknown particles. The Bd decay belongs to a category physicists have long considered valuable for investigating physics beyond the Standard Model. A related process involving a different beauty particle called Bs was previously confirmed in 2015, though subsequent analyses through 2018 could not confirm earlier evidence for the rarer Bd decay.
Significant improvements to the LHCb detector made the breakthrough possible. Following a major upgrade, the experiment began collecting data in 2024 at five times the rate achieved during the previous operational period. The upgraded detector also improved its ability to identify different particle types, while researchers employed machine learning techniques to distinguish the exceptionally rare signals from overwhelming background noise. Using data collected during 2024 and 2025, researchers identified evidence of Bd → μ+μ- at a significance of 3.0 standard deviations, which exceeds the widely accepted threshold for evidence of a new phenomenon.
Although the findings represent a major achievement, physicists have not yet reached the threshold required for a confirmed observation, which demands a significance of five standard deviations. Researchers expect upcoming improvements to the Large Hadron Collider, including the High Luminosity LHC scheduled to begin operations in 2030 and another major upgrade to LHCb planned for the middle of the following decade, will enable even more precise measurements. These enhancements could increase the experiment’s data collection rate by an additional factor of ten, allowing detection of more rare decays and potentially identifying subtle effects associated with undiscovered particles.
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