CERN Spots Rare Particle Decay After Four Decades of Searching

Physicists working on the LHCb experiment at CERN have found the first evidence from a single experiment of a rare particle decay that researchers have been hunting for more than four decades. The collaboration announced the result at a seminar at CERN on 3 November 2026, and posted its measurements in a paper on the arXiv preprint server.

The decay involves a B-zero meson, a particle made of a beauty quark and a down antiquark. In the process, the quarks annihilate and the meson turns into a muon and an antimuon, heavier relatives of the electron. According to the Standard Model, the current best theory of fundamental particles and their interactions, this happens in only about 1.03 out of every 10 billion such decays. The LHCb team measured a rate of about 1.14 in ten billion, with substantial uncertainty, a figure that lines up with the prediction.

Physicists describe the result as evidence rather than a discovery. The new data alone reached a significance of 3.0 standard deviations, rising to 3.2 when combined with earlier LHCb measurements. Particle physics reserves the word discovery for results that reach five standard deviations, a threshold that would mean the odds of a statistical fluke are roughly one in several million.

A search that began decades ago

The hunt for this decay stretches back a long way. Theorists first described it in 1984, and the search began in the early 1980s with the CLEO experiment at Cornell University in the United States. The Large Hadron Collider regularly produces B-zero and B-zero-s mesons, but spotting the muon-pair decay among the flood of other particles proved difficult for generations of detectors.

The slightly less rare sibling decay, in which a B-zero-s meson turns into a muon pair, was established in a joint analysis by LHCb and the Compact Muon Solenoid experiment, and confirmed again at LHCb in 2017. Joint evidence for the rarer B-zero decay had been reported before, but later analyses did not confirm it. The new result is the first time any single experiment has produced evidence for it on its own.

Professor Chris Parkes, head of physics and astronomy at the University of Manchester, led the international LHCb collaboration through the construction of its major upgrade and led the collaboration's approval of the new result. Researchers from the University of Manchester played key roles in the experiment and its upgrade, including developing detector technology that helped make the measurement possible. The data analysis was performed by an international team with a strong Italian presence, according to Italy's National Institute for Nuclear Physics.

What the new data show

The evidence turned up in proton collisions recorded at the LHCb detector in 2024 and 2025, after the experiment's upgrade. The upgraded detector can now collect data at a far higher rate than before, and the LHCb experiment records hundreds of millions of proton collisions per second, a data flow of 4 terabytes per second. That larger sample gave the team the statistical power to pick out a signal that earlier datasets had missed.

Alongside the headline result, the collaboration sharpened its measurement of the sibling decay. The team reports a B-zero-s decay rate of about 3.25 per billion decays, now the most precise measurement of that rate in the world. The Standard Model predicts about 3.66 per billion. The researchers also determined that the effective lifetime for B-zero-s decays into muon pairs is 1.60 plus or minus 0.16 picoseconds, with a much smaller systematic uncertainty than before. The effective lifetime is the average time an unstable particle exists before it decays.

All of the results are in line with Standard Model predictions and point to no new physics, the researchers said. That outcome still matters, because these muon-pair decays carry relatively precise theoretical predictions, which makes them useful tests of the theory. A measurement that disagreed with the prediction would have signalled unknown particles or forces. Agreement at this level of precision tightens the limits on what those unknown ingredients could be.

Why the measurement matters

The result gives physicists a sharper tool for testing the Standard Model, and the upgraded experiment is expected to collect a much larger dataset into the early 2030s, allowing the decay to be studied with increasing precision. Future collision data may establish the B-zero decay more firmly and sharpen its measured frequency, and the team says the same techniques can be applied to other rare decays that are sensitive to physics beyond the Standard Model.

The measurement also shows what the upgraded LHCb detector can do. The collaboration spent years rebuilding parts of the experiment so it could handle the higher collision rates of the current LHC run. The payoff is a result that decades of earlier experiments could not reach, and a dataset that will keep producing precision measurements for years.

Read the full report on the phys.org coverage of the LHCb result and the announcement from Italy's National Institute for Nuclear Physics. The underlying paper is posted on arXiv at arxiv.org/abs/2611.00230.