LHCb physicists report evidence of rare Bd meson decay after 42-year search

Evidence of a rare particle decay

Physicists working with the LHCb Collaboration at CERN announced on November 3, 2026, that they have captured the first single-experiment evidence of an extremely rare particle decay. The phenomenon involves a short-lived "beauty" particle known as a Bd meson—composed of a beauty quark and a down antiquark—transforming into a muon and an anti-muon. The search for this decay began in the early 1980s.

The findings were presented in a seminar and detailed in a preprint paper on arXiv that has not yet been peer-reviewed. The specific decay is estimated to occur only once in every ten billion events.

Statistical significance and data

The research combines data gathered between 2011 and 2018 with fresh observations recorded by the Large Hadron Collider Beauty experiment across 2024 and 2025. This combined dataset achieved a statistical significance level of 3.2 sigma, surpassing the standard 3-sigma evidence threshold. Claiming evidence for a phenomenon like this requires a significance level of three standard deviations, corresponding to a 0.3 percent chance that the result is a fluke.

Attaining the "gold standard" 5-sigma significance value will require further research to confirm this decay. Future upgrades in the 2030s will boost the number of LHC collisions and increase the data collection rate by an order of magnitude.

Technology and context

Capturing such an elusive signal required major technological upgrades. The LHCb detector underwent a significant overhaul that became operational in 2024, boosting its particle collision detection rate to five times its previous speed. Researchers also implemented advanced machine-learning techniques to increase the signal-to-noise ratio.

Instruments like the VELO sub-detector, positioned less than a finger's width from the particle beam, allowed scientists to closely monitor the debris of near-light-speed proton collisions. In these collisions, beauty quarks and down antiquarks are produced and bind via the strong nuclear force to create ephemeral mesons that decay in 1.5 trillionths of a second.

The results align with the Standard Model of particle physics and constrain new physics contributions at energy scales beyond those accessible through direct production at the LHC. Niladri Sahoo, a particle physicist at the University of Birmingham and convener of the LHCb Very Rare Decays working group, said the measurements provide "a powerful new test of the Standard Model and a window into potential new physics."

Harry Cliff, a particle physicist at the University of Cambridge and leader of the analysis team, said: "Seeing clear evidence for this rarest decay for the first time was thrilling." He added: "We're knocking on the door of the zeptoverse now. I can't wait to see what we find as we explore deeper."

Why it matters

The Standard Model explains much of the universe but does not illuminate dark matter or the slight asymmetry between normal matter and antimatter. Researchers refer to the subatomic scale being explored as the "zeptoverse," where undiscovered particles or forces may exist at zeptometer scales. The discovery suggests that novel physics may be detected through indirect evidence in rare decay rates rather than solely through the direct production of new particles at the LHC.

Sources

• sciencealert.com: After 42 Years of Searching, Physicists Find Evidence of a One-in-10-Billion Particle Decay : ScienceAlert
• yahoo.com: After 42 Years of Searching, Physicists Find Evidence of a One-in-10-Billion Particle Decay
• azat.tv: CERN Physicists Capture First Evidence of Rare Particle Decay After Decades – Azat TV

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