LHCb reports first single-experiment evidence of ultra-rare B⁰ decay to muon pair

The LHCb experiment at CERN has announced the first single-experiment evidence of a neutral beauty meson (B⁰) decaying into a muon-antimuon pair. The decay, expected by the Standard Model to happen roughly once in every 10 billion B⁰ decays, was observed with a statistical significance of 3.2 σ, which meets the conventional evidence threshold but falls short of the five-sigma level required for a discovery.

The result relies on the upgraded detector, notably the Vertex Locator (VELO) built by teams at the University of Liverpool and the University of Manchester. Positioned just five millimetres from the LHC beams, the VELO provides precise tracking that helps separate the rare signal from background activity. The upgrade also increased data-taking capacity to up to five times the previous rate, allowing more collisions to be recorded in 2024-2025.

"This is evidence of the skill and craftsmanship of the teams that built the detector," said Tara Shears, LHCb-UK spokesperson.

The measured branching fraction aligns with Standard Model predictions, tightening constraints on possible contributions from undiscovered particles that could alter the decay probability through quantum effects. The analysis also yielded the most precise measurements to date of the related Bₛ⁰→μ⁺μ⁻ decay probability and effective lifetime, both consistent with theoretical expectations.

Earlier joint analyses by LHCb and CMS had hinted at the B⁰ decay, but the new result is the first observation from a single experiment. Researchers plan further upgrades that could increase data collection by another order of magnitude, aiming to confirm the decay with higher significance and to probe for subtle deviations that might signal new physics.