Physicists at CERN report evidence for an ultrarare particle decay that theorists predicted 40 years ago. The decay involves a B⁰ meson's beauty quark annihilating with a down-type quark, producing a muon and antimuon pair.
The LHCb detector at the Large Hadron Collider recorded proton collisions in 2024-2025 that revealed the B⁰→ µ⁺µ⁻ decay. According to Standard Model predictions, this decay occurs in only about 1.03 in 10 billion events, making it extremely difficult to detect. Previous joint analyses reported some evidence for this decay, but it was not confirmed in later studies.
The new analysis showed 3.0 standard deviations of significance in the fresh data alone. When combined with earlier LHCb measurements, the significance rose to 3.2 standard deviations. The combined data indicated a B⁰ decay rate of around 1.14 in 10 billion, though with substantial uncertainty.
The team also made improved measurements of the slightly less rare B⁰ₛ decay, which produces the same muon-antimuon pair. The combined data yielded a B⁰ₛ decay rate of about 3.25 per billion decays, described as the world's most precise measurement of this rate. The effective lifetime for B⁰ₛ decays into muon pairs measured 1.60 ± 0.16 picoseconds.
All results align with Standard Model predictions and show no signs of new physics. The ideal threshold for such measurements is five standard deviations. Future collision data may establish the B⁰ decay more firmly.