A new analysis at CERN finds evidence that a neutral B meson can turn into two muons, an exceptionally rare process predicted by the Standard Model. Combining recent and earlier data gives a significance of 3.2 sigma, below the discovery threshold. The accompanying research remains a preprint submitted for journal publication.
The LHCb experiment, which studies particles containing heavy quarks at CERN’s Large Hadron Collider near Geneva, reported evidence on November 3 for a neutral B meson turning into a muon pair. Muons are heavier relatives of electrons. This particular transformation is predicted roughly once in ten billion decays by the Standard Model, the theory describing known elementary particles and their interactions.[1], [2]
The latest sample came from collisions collected in 2024 and 2025. Its signal reached approximately 3 sigma, a measure of how far the excess stands above the background expectation. Adding measurements from earlier running periods raised that figure to 3.2 sigma. Particle physics normally requires 5 sigma for a discovery. Within the reported uncertainties, the measurements agree with the Standard Model’s expectations.[1]
The same analysis refined measurements of another neutral beauty meson, the Bₛ, also decaying into two muons. Comparing these channels helps physicists examine the behavior of heavy quarks. A hint of the rarer channel from a joint analysis in 2015 had not persisted in subsequent studies. The new research is available on arXiv as a preprint and was submitted to Physical Review X; peer-reviewed publication is still pending.[1]
