CERN's LHCb reports evidence of ultra-rare beauty particle decay
The LHCb experiment at CERN observed the decay of a B⁰ meson into two muons, a process predicted to occur once in every 10 billion decays, reaching a statistical significance of 3.2 sigma.
Physicists at CERN's LHCb experiment have observed the ultra-rare decay of a B⁰ meson into a pair of muons (B⁰ → μ⁺μ⁻). The decay is expected to happen about once in every 10 billion beauty-particle decays, making it roughly 30 times rarer than the related Bₛ⁰ → μ⁺μ⁻ process, which occurs three times per billion decays.
The observation combines data collected in 2024 and 2025 with earlier runs from 2011 to 2018, yielding a combined statistical significance of 3.2 standard deviations – a 0.3 % probability that the result is a statistical fluctuation. While this does not meet the five-sigma threshold required for a formal discovery, it aligns with Standard Model predictions.
The 2024 LHCb upgrade increased data-taking speed to five times that of the 2010-2018 period, and machine-learning techniques were employed to isolate the muon signals from background noise. The analysis involved a team of about 19 researchers from the United Kingdom, Germany, Italy, Spain and the Netherlands.
The measured branching fraction for the decay is approximately 1.26 × 10⁻⁹, with a statistical uncertainty of about 0.5 × 10⁻⁹. The findings have not yet undergone peer review.