At the Large Hadron Collider, physicists of the LHCb experiment have for the first time obtained convincing evidence of an ultra-rare decay of the neutral B meson. A particle consisting of a b quark and a d antiquark turned into a pair of muons — an event that, according to the predictions of the Standard Model, occurs only once in ten billion decays of such mesons.
The search lasted more than forty years. The first upper limits on the probability of the process were set back in 1984 at the CLEO experiment. Since then, scientists from various collaborations have tried to catch the signal, but it remained hidden in the enormous background of proton collisions. Only after the upgrade of the LHCb detector in 2024, which made it possible to collect data five times faster, and the combination of new data from 2024–2025 with previous runs did the significance of the signal reach 3,2 standard deviations.
This is no longer merely a hint, but the first evidence obtained within a single experiment. A full-fledged discovery requires five sigma, but 3,2 sigma is enough to state that the process has been seen. The result is fully consistent with the calculations of the Standard Model and at the same time narrows the space for hypotheses about new particles beyond it.
In parallel, the collaboration refined the parameters of the more frequent but still rare decay of the Bs meson into a pair of muons. The measurement precision has become record-breaking, and the measured lifetime of the particle matched the theoretical prediction. Such processes serve as a kind of magnifying glass: any deviation from the predictions could point to the existence of new forces or particles at scales inaccessible to direct production at the collider.
For the ordinary person, these figures mean that the fundamental laws of nature are being tested with unprecedented precision. Each confirmed rare decay strengthens confidence in the Standard Model and at the same time prepares the ground for the next step — the search for those very deviations that will open up the “zeptouniverse” of new phenomena.
