LHCb finds first evidence of B⁰ meson decaying into two muons

The signal significance was 3.0 standard deviations based on the new data and 3.2 after combining it with previous LHCb measurements. This level allows researchers to claim evidence, but not a discovery: a discovery requires 5σ, and for now a random fluctuation cannot be ruled out with the necessary confidence.

According to calculations within the Standard Model, this decay should occur approximately 1.03 times per 10 billion decays. As iXBT writes, the new analysis yielded a value of 1.26 per 10 billion with a large statistical uncertainty (according to Naked Science, 1.14). Prior to this work, the strictest experimental limit was 1.6 per 10 billion. The search began in 1984, when the upper limit of this probability was first determined in the CLEO experiment.

The analysis used data from the collider's third run (Run 3) for 2024–2025, collected at an energy of 13.6 TeV with an integrated luminosity of 18 inverse femtobarns. The upgraded LHCb Upgrade I detector was used for the first time to study decays of B mesons into two muons, and rare events were isolated using a machine-learning algorithm based on decision trees.

In parallel, the decay of the strange Bₛ⁰ meson into two muons was measured: 3.31 cases per 1 billion according to the new data and 3.25 taking previous runs into account, compared to the predicted 3.66. The effective lifetime of the Bₛ⁰ was 1.42 picoseconds according to the new data and 1.60 when combined with earlier measurements. According to the researchers, the results are consistent with the Standard Model and show no signs of new physics.