CERN Physicists Report First Evidence of One-in-10-Billion Particle Decay

Physicists working on the LHCb experiment at CERN, the European particle physics laboratory outside Geneva, have reported the first evidence of one of the rarest particle decays ever measured: a neutral B meson, a short-lived particle containing a "beauty" quark, transforming directly into two muons. The search for this specific decay began in 1984, and the new result, announced on November 3 and detailed in a paper submitted to Physical Review X, matches the prediction of the Standard Model, the decades-old theory that describes how fundamental particles interact.

The finding does not overturn physics. It confirms it, with a precision that was not possible before CERN's Large Hadron Collider, or LHC, underwent an upgrade that let its detectors record data far faster than in earlier runs.

A decay predicted once in 10 billion

The B meson in question, known to physicists as B⁰, typically survives for about one and a half trillionths of a second before disintegrating into a shower of other particles. The Standard Model predicts that only about one in every 10 billion B⁰ mesons will decay into a muon and its antimatter counterpart, a muon-antimuon pair, rather than following a more common decay path. That predicted rate is roughly 30 times lower than the already-rare decay of a related particle, the B⁰ₛ meson, into the same pair of muons, a process LHCb and the CMS experiment jointly observed a decade ago.

Using data collected in 2024 and 2025, after an upgrade allowed the LHCb detector to record collisions at up to five times its earlier rate, the collaboration measured the B⁰-to-two-muon decay with a statistical significance of 3.0 standard deviations, or "sigma." Combined with data gathered between 2011 and 2018, the significance rose to 3.2 sigma, according to the CERN announcement. In particle physics, three sigma is conventionally labeled "evidence," corresponding to roughly a 1-in-370 chance the signal is a statistical fluke; five sigma, the threshold for a confirmed "discovery," corresponds to odds of about 1 in 3.5 million. The full analysis, which also sharpens earlier measurements of the more common B⁰ₛ decay, is described in a preprint posted to the physics repository arXiv and submitted for peer review at Physical Review X.

Why a 42-year hunt for an unremarkable-looking number matters

The appeal of this decay to physicists has little to do with the particles themselves and everything to do with what might be hiding near them. The Standard Model has withstood nearly every experimental test thrown at it since the 1970s, yet it leaves major questions unanswered, among them the nature of dark matter and why matter so heavily outnumbers antimatter in the universe. Suppressed decays like B⁰-to-muons are sensitive probes for physics beyond that model, because any undiscovered particle or force would nudge the measured rate away from the Standard Model's prediction, even if that new physics is too heavy or too weak to produce directly in a collider.

So far, the nudge has not appeared. The measured rate is consistent, within its uncertainty, with the Standard Model's forecast, which the University of Birmingham, one of the British institutions involved in the analysis, called the rarest decay of a short-lived particle containing a beauty quark ever observed at the LHC. That consistency is itself useful: it rules out, or narrows the room for, certain classes of new particles and forces that would otherwise have pushed the decay rate higher or lower.

Funding for the UK researchers involved came through the Science and Technology Facilities Council, part of the country's research funding agency, which noted in its own release that the hunt for this particular decay has run for more than four decades, since an earlier experiment first set an upper limit on its rate in the 1980s, according to UK Research and Innovation. British groups, including teams at Birmingham, Warwick, Cambridge, Oxford, Imperial College London and the Rutherford Appleton Laboratory, built major components of the upgraded LHCb detector, among them the silicon tracking system closest to the particle beam.

Caution from the physicists involved

Researchers on the analysis were careful to frame the result as a milestone rather than a conclusion. Marco Santimaria, who co-led the roughly 20-person international team behind the analysis, said in the CERN announcement that "physicists have been chasing this decay for more than forty years" and that "extreme precision is required to pick the muons out of the overwhelming background" of other collision debris. Harry Cliff, the LHCb collaboration's rare-decays convener and a co-leader of the analysis, called the result "incredibly exciting" while acknowledging it still falls short of a definitive discovery.

"We're knocking on the door of the Zeptoverse now," said Cliff, using a term physicists have adopted for the realm of distances roughly a billionth of a trillionth of a meter, where new particles or forces, if they exist, are thought most likely to leave a trace.

Tim Gershon, a physics professor at the University of Warwick who leads one strand of the UK's LHCb effort, said the result "showcases the extraordinary capabilities of LHCb to probe nature at the fundamental level," according to a Warwick press release. At Birmingham, Niladri Sahoo, who convenes LHCb's working group on very rare decays, said the measurement "provides a powerful new test of the Standard Model and a window into potential new physics," while Nigel Watson, a Birmingham physics professor and former head of the UK's LHCb program, described it as "the latest of many exciting measurements at LHCb."

What would settle the question

Three sigma is a familiar way station in particle physics: a signal strong enough to publish and discuss, but weak enough that past "evidence" findings have occasionally faded on closer inspection. A 2015 joint analysis by the LHCb and CMS experiments reported an early hint of this same B⁰ decay, a hint that subsequent, more precise measurements failed to confirm, underscoring why physicists reserve the word "discovery" for results that clear five sigma.

Reaching that threshold will take more data and more time. The LHC is scheduled to run at higher luminosity starting around 2030, under an upgrade known as the High-Luminosity LHC, and a further overhaul of the LHCb detector planned for the mid-2030s is designed to increase its data-collection rate again, potentially by another factor of five. Those upgrades, rather than any single new analysis, are what physicists are counting on to determine within the next decade whether this rare decay matches the Standard Model exactly or departs from it in a way that points toward undiscovered physics.

Independent confirmation from other experiments would also help settle the matter. CMS, the LHC's other general-purpose detector, has pursued the same decay with a different detector design, and a future combined analysis of the two experiments' data, as was done for the related B⁰ₛ decay a decade ago, is expected to sharpen the picture further.