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CERN Finds Evidence of Rare Particle Decay After Decades-Long Search

The LHCb result passes the threshold for evidence but remains below the standard for a discovery.

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A signal after a long search

November 5, 2026 · 2:45 a.m. Central

Physicists at CERN say they have found evidence of an exceptionally rare particle decay: a neutral beauty meson transforming into a muon and its antiparticle.

The process is predicted to occur about once in 10 billion such decays. The result from CERN’s Large Hadron Collider beauty experiment, or LHCb, is evidence from a single experiment—not yet a confirmed discovery, CERN said November 3.

The finding is consistent with the Standard Model, physicists’ established framework for fundamental particles. It does not establish that scientists have found a new particle or new physics.

What 3.2 sigma means

The team analyzed data collected in 2024 and 2025, then combined it with observations from 2011 through 2018. The combined result reached 3.2 standard deviations, or sigma, according to the LHCb collaboration.

In particle physics, roughly three sigma is conventionally treated as evidence, while five sigma is the customary threshold for claiming discovery. The number describes how unexpected the observed signal would be under a background-only statistical model; it is not the probability that the result is true. More data will be needed to test it.

Why an unlikely decay matters

A muon resembles an electron but is heavier. Rare decays can be sensitive to the influence of particles too heavy to produce directly in the collider. If a precise measurement eventually departs from the Standard Model’s prediction, it could point researchers toward a better theory. This result agrees with that prediction, CERN said.

A detector upgrade allowed LHCb to collect data more quickly and better distinguish particle types; machine-learning methods helped suppress background events. The analysis also improved measurements of a related, previously observed decay. UK Research and Innovation, which supported participating researchers, described the same evidence and discovery thresholds.

How the search developed

  1. An earlier joint LHCb and CMS analysis reported evidence for this decay, but later analyses did not confirm it, according to CERN.
  2. The upgraded LHCb detector collected data used in the latest analysis.
  3. LHCb presented its result at a CERN seminar and submitted a paper for publication.

Sources and visual credits

CERN: announcement · LHCb: analysis details · UK Research and Innovation · Lumacta News: independent report · CERNpress on X.

Visual: generated conceptual illustration; not a detector image or record of an observed event.