After a search lasting 42 years, physicists at CERN have found the first evidence of one of the rarest particle decays ever sought — a process so unlikely that it is predicted to occur only once in every 10 billion beauty particles. The finding, announced on November 3 by the international LHCb collaboration at the Large Hadron Collider, could open a window onto the Zeptoverse, an unexplored scale of nature where unknown particles and fundamental forces may be hiding.
The decay at the heart of the discovery is known as Bd to mu mu. It involves a neutral B meson, a particle containing a beauty quark, transforming into two muons — heavier relatives of the electron. Beauty quarks are heavier relatives of the up and down quarks found in ordinary matter, and particles containing them exist for less than two trillionths of a second before decaying.
Only one in every 10 billion of them undergoes this particular transformation, which makes it extraordinarily difficult to detect. Physicists have been hunting for it since 1984.
The new measurement, from the upgraded LHCb experiment at CERN's Large Hadron Collider, reached a statistical significance of 3.2 sigma. That is enough for physicists to describe the result as the first evidence of the decay, although further data will be needed to reach the five-sigma threshold required to claim a discovery in particle physics. The distinction matters: in particle physics, three sigma is evidence, five sigma is a discovery.
The researchers are careful to say that this is the former, not yet the latter. The paper reporting the result was submitted for publication on November 3 and has not yet undergone peer review.
The finding is significant for two reasons. First, it demonstrates the power of the upgraded LHCb detector, which can now collect data at up to five times its previous rate. A key component is the Vertex Locator, a silicon pixel detector built in the United Kingdom that sits just five millimetres from the LHC beams and pinpoints where each particle was produced and where it decayed.
The upgraded experiment is a 15-year design and construction project by a 1,900-member international collaboration, and this result is its first major payoff. The researchers looked at ten years of collider data hunting for an instance of the decay, and found about a dozen candidates.
Second, the measurement offers a new way to test the Standard Model, the leading theory explaining fundamental particles and their interactions. So far, the new result agrees with the Standard Model's predictions. That agreement is itself informative: if the measurement had disagreed with the theory, it would have been evidence of new physics.
But the value of the measurement lies in its precision. Rare decays like this one are sensitive to the effects of particles too heavy to be produced directly, even at the LHC. By measuring them with increasing precision, physicists can probe for the influence of unknown particles and forces that exist at energy scales far beyond the reach of any collider.
That is where the Zeptoverse comes in. The name refers to a scale of nature involving distances as small as a zeptometre, which measures 10 to the power of minus 21 metres — one billionth of a trillionth of a metre, or one sextillionth of a metre. At such distances, physicists believe, new particles and forces may be hiding.
The Zeptoverse is not a place that can be visited or photographed. It is a regime of physics, accessible only indirectly, through measurements so precise that they can reveal the fingerprints of things that exist at unimaginably small scales.
The search for this decay has a long and chequered history. In 2015, a combined analysis from LHCb and the CMS experiment reported evidence for the Bd to mu mu decay alongside the observation of a related process involving a different particle, the Bs meson. Subsequent analyses did not confirm the Bd evidence, and the search went on.
The new result rests on a much larger dataset: measurements from 2011 to 2018, combined with data from the upgraded detector collected in 2024 and 2025. The 2024-2025 sample alone reached three sigma; adding the earlier measurements raised the significance to 3.2 sigma.
Professor Chris Parkes, head of the Department of Physics and Astronomy at the University of Manchester, led the international LHCb collaboration through the construction of the upgrade and led the collaboration's approval of the new result. Researchers from Manchester played key roles in the experiment and its major upgrade, including developing detector technology that helped make the new measurement possible.
Dr Harry Cliff, who led the analysis team at the University of Cambridge's Cavendish Laboratory, has described the observation as proof of the power of the upgraded collider, and as a step towards glimpsing particles that exist at extremely small scales and very high energy levels.
The upgraded LHCb experiment is expected to collect a much larger dataset into the early 2030s, allowing physicists to study the decay with increasing precision. A further planned upgrade could raise the data rate by another factor of ten, sharpening this measurement and others like it. If the decay is confirmed at five sigma, it will become a precision tool for testing the Standard Model.
If future measurements show a deviation from the theory's predictions, it could be the first sign of new physics — the long-sought evidence that the Standard Model, for all its successes, is incomplete.
That incompleteness is one of the great open questions in science. The Standard Model describes the known particles and forces with extraordinary accuracy, but it does not explain dark matter, dark energy, or why there is more matter than antimatter in the universe. Physicists are confident that there is more to discover, but they do not know where it will be found.
Rare decays are one of the most promising places to look, because they are among the few measurements sensitive enough to reveal the effects of particles that cannot be made directly.
The Zeptoverse result is, in that sense, a beginning rather than an end. Forty-two years of searching have produced, for now, evidence — honestly labelled, carefully measured, and pointing at a scale of nature no one has explored before. The physicists behind it are clear about what they have and what they do not: a 3.2-sigma signal, a paper submitted but not yet peer-reviewed, and a result that agrees with the theory it was designed to test.
What they have also produced is a demonstration that the upgraded LHCb detector works as intended, and that the long search for the rarest decays in nature is now entering a new phase.
For CERN, the announcement is a reminder of the institution's role as the place where the smallest scales of nature are explored with the largest machines ever built. The Large Hadron Collider, a ring 27 kilometres in circumference buried beneath the French-Swiss border, accelerates particles to nearly the speed of light and smashes them together, recreating conditions that existed fractions of a second after the Big Bang.
Out of those collisions come the fleeting particles — beauty mesons that live for less than two trillionths of a second — whose decays carry information about the structure of reality itself.
The search will continue. More data is coming, the detector will be upgraded again, and the measurement will get sharper. Whether it confirms the Standard Model or breaks it, the one-in-ten-billion decay that took 42 years to find has already done its job: it has shown physicists a door, and given them a reason to keep knocking.
(Reporting based on Moneycontrol, Phys.org and The Times.)