LHCb Collaboration reports evidence of ultra-rare Bd to muon pair decay
The LHCb experiment at CERN has observed evidence for the Bd → μ⁺μ⁻ decay with a significance of 3.2 sigma, marking the first single-experiment indication of this one-in-ten-billion process.
The LHCb Collaboration announced at a CERN seminar that data collected between 2024 and 2025 provide evidence for the decay of a B⁰_d meson into a pair of muons (μ⁺μ⁻). The result reaches a statistical significance of 3.0 standard deviations on its own and 3.2 standard deviations when combined with earlier data from 2011-2018, surpassing the conventional threshold for evidence of a new phenomenon.
The decay, predicted to occur in roughly one out of ten billion beauty-quark decays, is part of a class of processes considered highly sensitive to physics beyond the Standard Model. Precise measurement of such rare decays can reveal indirect effects of heavy particles that are not directly produced in LHC collisions.
The 2024 upgrade of the LHCb detector increased data-taking rates fivefold and improved particle-identification capabilities. Combined with machine-learning techniques, these enhancements allowed the collaboration to reduce background noise and isolate the rare signal.
"Physicists have been chasing this decay for more than forty years," said Marco Santimaria, co-leader of the 19-person analysis team. "Extreme precision is required to pick the muons out of the overwhelming background."
In the same analysis, LHCb also reported the most precise measurement to date of the previously observed Bs → μ⁺μ⁻ decay. The collaboration plans further data collection to achieve the 5-sigma significance required for a definitive discovery, which would make Bd → μ⁺μ⁻ the rarest beauty-particle decay observed.