Geneva, Switzerland — November 10, 2026: Scientists at CERN have reported a major step forward in a decades-long hunt for one of the rarest particle processes ever investigated. The Large Hadron Collider beauty (LHCb) experiment has found evidence that a subatomic particle known as the neutral B meson can decay into a pair of muons — a process predicted to happen only about once in every 10 billion beauty-particle decays.
The result, announced on November 3, is the first time a single experiment has produced evidence for this particular decay. It marks an important milestone after more than four decades of searching, but researchers are careful to distinguish evidence from a confirmed discovery: the combined result reaches 3.2 standard deviations, below the five-sigma threshold conventionally required in particle physics.
What Exactly Have Scientists Observed?
The process is written as B0 → μ+μ−. It involves a neutral B meson — a short-lived particle containing a beauty quark and a down antiquark — transforming into two muons with opposite electric charges. Muons resemble electrons but are much heavier.
In particle physics, a decay is not simply an object breaking apart in the everyday sense. It is a transformation in which the original particle’s constituent quarks interact and produce other particles, subject to the laws governing fundamental forces and conservation rules. The beauty quark and its antiquark partner annihilate through a highly suppressed process, producing the muon pair.
The probability is extraordinarily small. CERN says the decay is predicted to occur around once in every 10 billion beauty-particle decays. That rarity makes it difficult to separate the sought-after signal from the enormous number of other particle interactions produced when proton beams collide inside the Large Hadron Collider.
Why This Rare Decay Matters
Scientists are interested in rare decays because they can act as indirect tests of the Standard Model, the leading framework describing known fundamental particles and three of the four fundamental forces. Some hypothetical particles may be too massive to produce directly in the LHC, yet their quantum effects could subtly influence how often an extremely rare decay occurs.
If the measured rate were significantly different from the Standard Model prediction, it could point towards physics beyond the established theory — potentially involving new particles or interactions. However, the latest LHCb measurements are consistent with Standard Model expectations. The announcement therefore does not establish the existence of a new particle, force or theory.
The LHCb collaboration says the analysis also improved measurements of a related process, the decay of the strange B meson into two muons, written as Bs0 → μ+μ−. That related decay is more common and was observed previously; the new work provides the most precise measurements yet of its branching fraction and effective lifetime.
How the LHCb Team Found the Signal
The new result was made possible by an upgrade to the LHCb detector, which began collecting data in 2024. CERN says the upgraded experiment can record collision data at up to five times the rate achieved during the earlier operating period. Better particle-identification capabilities and machine-learning methods helped researchers distinguish the two-muon signal from a much larger background of unrelated events.
The analysis used data collected in 2024 and 2025. When those results were combined with earlier LHCb data from 2011 to 2018, the statistical significance reached 3.2 sigma. The new data alone gave a significance of 3.0 sigma. This is the first single-experiment evidence for the decay, rather than a result assembled from different experiments.
The experimental challenge is comparable to finding a very specific, fleeting pattern among an immense number of collisions. The detector records the tracks and properties of particles emerging from collisions; software and statistical analysis then assess whether a small excess of events is consistent with the expected decay or could plausibly arise from background fluctuations.
Why 3.2 Sigma Is Not Yet a Confirmed Discovery
In high-energy physics, researchers express the strength of a signal in standard deviations, commonly called sigma. A result around three sigma is generally described as evidence, while the widely accepted threshold for claiming a discovery is five sigma. The higher threshold is designed to reduce the chance that a statistical fluctuation is mistaken for a genuine physical process.
The LHCb result therefore represents strong progress, not the final confirmation. Further data and analysis will be needed to determine whether the signal grows to five sigma. CERN has emphasised that the next stage is to confirm the decay with additional measurements.
What Happens Next?
Researchers expect future data to sharpen the measurement. CERN points to the planned High-Luminosity LHC, expected to begin operation around 2030, and a further LHCb upgrade anticipated in the middle of the next decade. Together, these developments could increase the experiment’s data-collection rate by another order of magnitude, giving scientists a much larger sample in which to search for exceptionally rare events.
The significance of the result extends beyond one particle decay. Precision studies of rare processes help physicists test the limits of the Standard Model and narrow the possibilities for theories that seek to explain unanswered questions in fundamental physics. The search is gradual: a small deviation, if one is eventually established, could guide future experiments towards phenomena that current instruments cannot observe directly.
For now, the central finding is both striking and carefully qualified: after more than 40 years of searching, the LHCb collaboration has reported the first single-experiment evidence for a decay expected to occur roughly once in 10 billion beauty-particle decays. The result is compatible with the Standard Model, and scientists will need stronger evidence before calling it a confirmed discovery.
Sources And Further Reading
- CERN: LHCb finds evidence of one-in-ten-billion particle decay — official announcement and explanation of the result.
- LHCb Collaboration: Evidence for the decay B⁰ → μ⁺μ⁻ and improved measurements of Bₛ⁰ → μ⁺μ⁻ — technical summary of the measurements.
- UK Research and Innovation: Scientists find evidence of rarest particle decay after 42 years — research context and explanation of the statistical significance.
- University of Cambridge: Physicists knock at the door of the zeptoverse — background on the physics and the international research effort.
- Italian National Institute for Nuclear Physics: LHCb identifies a decay that occurs once in ten billion — details on the analysis team and detector contributions.
Image credits: Science and Technology Facilities Council / Kathleen Yurkewicz; LHCb Collaboration; Maximilien Brice / CERN; and Max Braun. Images are used under the respective Creative Commons licences. They illustrate the facilities and experiments and are not photographs of the individual particle-decay event.