CERN scientists confirm rare B meson decay after 40 years of research

CERN scientists working on the LHCb experiment announced today that they have observed the ultra‑rare decay of a neutral B meson into two muons, a process that happens only once in roughly ten billion decays. The finding, based on data collected between 2015 and 2022, marks the end of a search that has lasted more than four decades.

The decay is a key test of the Standard Model of particle physics, which predicts exactly how often it should occur. Any deviation could signal hidden particles or forces that the model does not account for. Physicists have chased this signal for years because it offers a clean window into possible new physics, such as supersymmetry or exotic Higgs‑like particles, without needing higher‑energy collisions. Earlier runs only set upper limits, leaving the question open.

LHCb analysed a data set equivalent to 9 inverse femtobarns of proton‑proton collisions, corresponding to billions of B mesons. The team reports a statistical significance of 5.2 sigma, the standard threshold for a discovery. “Seeing this decay at the predicted rate is a triumph for the Standard Model,” said Dr. Elena Graverini, spokesperson for LHCb. “It also tightens the constraints on many theories that predict enhancements to this process.” Independent experts praised the result, noting that it sharpens the picture of flavour physics and narrows the space for new particles.

The observation paves the way for more precise measurements in the coming years. LHCb will collect additional data during the LHC’s Run 3, and the detector is being upgraded to improve sensitivity. Physicists hope to measure the decay’s rate to a few percent and to search for related rare processes that could reveal subtle cracks in the current theory. If any discrepancy emerges, it could point to the hidden sector that lies beyond today’s most powerful accelerators.