CERN researchers report first evidence of ultra-rare Bd meson decay
Data from the upgraded LHCb detector show a 3.0-sigma signal for the Bd → μ⁺μ⁻ decay, confirming a prediction of the Standard Model and opening a new avenue for indirect searches for physics beyond it.
Physicists at CERN have announced the first evidence of the extremely rare decay of a Bd meson into a pair of muons, a process predicted to occur only once in about 10 billion decays. The result, presented at a CERN seminar, reaches a statistical significance of 3.0 standard deviations for data collected in 2024-2025 and 3.2 standard deviations when combined with earlier measurements, surpassing the threshold used to claim evidence.
The Bd particle, composed of a beauty quark and a down antiquark, decays via the channel Bd → μ⁺μ⁻, a so-called “golden channel” for probing physics beyond the Standard Model because quantum effects from heavy, unseen particles can alter its rate. The observed decay rate matches Standard Model predictions, narrowing the space for new theories.
The breakthrough follows a major upgrade of the LHCb experiment that began data taking in 2024, increasing the collision data rate fivefold and enhancing particle identification. Machine-learning algorithms were employed to separate the rare muon signal from overwhelming background processes.
"Physicists have been chasing this decay for more than forty years," said Marco Santimaria, co-leader of the international analysis team.
"Seeing evidence of this rare decay is incredibly exciting, opening up new possibilities to probe beyond the direct reach of the LHC," said Harry Cliff, LHCb’s Rare Decays Convener.
While the evidence does not yet meet the five-sigma standard required for a discovery claim, the result provides the most precise measurement to date of the related Bs → μ⁺μ⁻ decay. Future upgrades, including the High-Luminosity LHC slated for 2030 and a further LHCb enhancement in the mid-2030s, are expected to increase data collection by another order of magnitude, improving sensitivity to potential new particles.
The findings mark a significant milestone after more than four decades of searching for this elusive signal and offer a new tool for investigating whether the known laws of particle physics are part of a broader framework.