LHCb Announces First Single-Experiment Evidence for the Rarest Particle Decay Ever Observed
After a 42-year search, LHCb reports 3.2σ evidence for the Bd → μ⁺μ⁻ decay, a process the Standard Model predicts at roughly one in ten billion.
On 3 November 2026, the LHCb collaboration at CERN announced the first single-experiment evidence for the rare decay of a neutral B meson into a pair of muons, reaching a significance of 3.2 sigma — a result the collaboration describes as a milestone in a search that has run, in one form or another, since the early 1980s. The decay, often written as Bd → μ⁺μ⁻, is the rarest particle decay ever observed at the LHC, and its confirmation by a single experiment rather than a joint claim carries a different weight in the particle-physics record.
The process is rare in the most literal sense. The Standard Model predicts that when a B meson decays, this particular channel — a quark-antiquark pair annihilating into two muons — occurs in roughly one event for every ten billion such decays, as STFC, the experiment’s UK funding body, framed the announcement. That is not a matter of the decay being difficult to trigger; it is a matter of the underlying quantum-mechanical amplitude being suppressed so severely that the vast majority of B mesons decay through far more common channels involving hadrons. Finding the muon pair in the noise of ten billion other things the meson could do is, in practice, an exercise in statistical patience at the scale of tens of millions of recorded events.
The 3.2 sigma figure comes from combining data from two distinct eras of running: the upgraded LHCb detector’s 2024–2025 dataset and the earlier 2011–2018 run. The 2024–2025 data alone yield 3.0 sigma, which means the newer detector configuration is doing most of the heavy lifting, but the combination with the older dataset pushes the result past the 3-sigma threshold that the field conventionally treats as the boundary between “hint” and “evidence.” Neither number reaches the 5-sigma bar for a formal discovery, and the collaboration is explicit that this is an evidence-level result, not a confirmed observation.
That framing matters because of what happened in 2015. LHCb and CMS jointly reported evidence for this same decay channel at that time, a result that drew significant attention as a potential portal to new physics. Neither experiment has confirmed it in subsequent data updates, and the CERN release notes that the 2015 joint claim was not carried forward. The 2026 result is therefore not a confirmation of that earlier hint; it is a fresh, independent, single-experiment measurement that happens to point in the same direction. The distinction is not nitpicking. In a field where a 2015 result that later evaporated is a well-known cautionary tale, the collaboration appears to be deliberately separating the new measurement from the old one.
The physics content of the result, for what it is worth, is that the measured rate is consistent with the Standard Model prediction. There is no significant deviation to claim as evidence of new particles or forces. What the measurement does open, however, is what CERN’s release calls a new “golden channel” probe of new physics at the so-called Zeptoverse scale — the range of energy distances and masses where hypothetical new particles beyond the Standard Model would be expected to leave a measurable imprint on decay rates. A “golden channel” in this context is a decay mode that is theoretically clean enough to calculate precisely, experimentally accessible enough to measure, and sensitive enough to new physics that even a small deviation from the predicted rate would be significant. Getting a solid, single-experiment evidence-level number for such a channel is the prerequisite for making it a genuine constraint, and that prerequisite has now been met for the first time.
The 42-year timeline is not merely a figure of emphasis. The theoretical motivation for looking at this specific decay goes back to the 1980s, when the Standard Model’s predictions for rare B meson decays were being worked out and experimental programs at CERN’s SPS had not yet reached the luminosity needed to see a one-in-ten-billion process. The fact that the search finally produces a 3-sigma signal in 2026, and that the signal is consistent with the theory it was designed to test, is a quieter kind of milestone than a discovery would be, but it is the kind that lets the next generation of measurements — the ones that might actually find a deviation — start from a well-characterized baseline rather than from uncertainty about whether the decay happens at all.