A decay forty years in the waiting

Physicists working on the LHCb experiment at CERN's Large Hadron Collider have found evidence for an extremely rare particle decay that scientists have been chasing for more than four decades. The result, announced on 3 November 2026, concerns a neutral B0 meson turning into a pair of muons, the heavier cousins of the electron. The Standard Model predicts this happens only about once in every ten billion B0 decays, making it one of the hardest processes in particle physics to observe.

Evidence, not yet a discovery

The LHCb team reported a signal of 3.0 standard deviations in new data recorded in 2024 and 2025. Combined with earlier LHCb data from 2011 to 2018, the significance rises to 3.2 standard deviations — enough to count as evidence, but below the five-standard-deviation threshold physicists require to claim a discovery. The measured rate came out at about 1.14 in ten billion decays, close to the Standard Model prediction of about 1.03 in ten billion. The findings were submitted to Physical Review X and posted on the arXiv preprint server.

Why the result matters

The team also measured a related, slightly more common decay of the heavier B0s meson into two muons, reporting the most precise measurement of its rate to date and an effective lifetime of 1.60 picoseconds. Because these decays have precise theoretical predictions, they are sensitive tests of the Standard Model and of possible new physics. "This process only happens for one in every ten billion beauty particles, so it's incredibly difficult to find," said Dr Harry Cliff of the University of Cambridge. The researchers stressed that all results agree with the Standard Model and do not point to new physics — but future data may confirm the decay more firmly.

[Source: Phys.org](https://phys.org/news/2026-11-years-physicists-evidence-ultrarare-particle.html) | [Source: Sci.News](https://www.sci.news/physics/cern-ultra-rare-particle-decay-15171.html)