A Cambridge-led team at CERN’s LHCb experiment has reported the first single-experiment evidence for an exceptionally rare beauty-particle decay, B⁰ → μ⁺μ⁻. The University of Cambridge says the result agrees with the Standard Model, the framework describing known particles and how they interact. Source Source
The signal reached a statistical significance of 3.2 sigma, above the three-sigma threshold Cambridge identifies for evidence. This is evidence for the decay, not a definitive discovery. Cambridge’s announcement also described the results as not yet peer-reviewed. Source
A search dating back to 1984
The decay, also called “Bd to mu mu,” produces a muon and an antimuon. Harry Cliff of Cambridge’s Cavendish Laboratory, who led the analysis team, explained its rarity this way: “This process only happens for one in every ten billion beauty particles, so it’s incredibly difficult to find.” Source
Cliff said physicists have been searching for this type of decay since 1984. Particle physics does not offer much in the way of instant gratification. Source
To extract the signal, researchers used machine-learning techniques to suppress background noise in beauty-decay data from 2024 and 2025. They combined those observations with earlier data collected between 2011 and 2018 to reach the reported 3.2-sigma significance, Cambridge says. Source
What the “zeptoverse” means here
The term refers to extremely short distance scales; a zeptometre is a trillionth of a billionth of a metre. Source
Cambridge explains that hypothetical particles beyond the LHC’s direct reach could provide an additional route for a decay to occur, changing how often it happens. Measuring these rare processes precisely could help physicists test for effects beyond the Standard Model. Source
A further LHCb upgrade planned for the middle of the 2030s could increase the data-collection rate tenfold, according to Cambridge, helping turn rare decays into more precise tests for undiscovered particles. The route deeper into the “zeptoverse” involves considerably more data and considerably fewer portal-opening ceremonies. Source
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