Physicists at CERN find first evidence of a decay predicted four decades ago

Physicists working on the LHCb experiment at CERN's Large Hadron Collider have found the first evidence of a particle decay so rare that it was predicted theoretically more than 40 years ago but had never been glimpsed by a single experiment. The collaboration announced the result this week, reporting that a neutral B meson, known as B⁰, occasionally decays into a muon and its antimatter counterpart, an antimuon, at almost exactly the rate the Standard Model of particle physics says it should.

The finding, described in a paper submitted to arXiv on October 30 and not yet peer-reviewed, carries a statistical significance of 3.2 standard deviations once new data are combined with earlier runs. In particle physics, that threshold is classified as "evidence," a notch below the 5-sigma standard required to declare a formal discovery. Even so, it is the first time any single experiment has seen a credible signal of this exact decay, which the Standard Model predicts should occur in roughly one of every ten billion B⁰ meson decays, making it one of the rarest subatomic processes ever measured.

A prediction four decades in the waiting

The decay of a B⁰ meson into two muons was theorized in 1984, not long after the top quark and other features of the Standard Model were being worked out. It belongs to a family of so-called flavor-changing neutral current processes, which the Standard Model forbids at the simplest level of interaction and allows only through indirect, higher-order quantum effects. That suppression is precisely what makes the decay so useful: because theorists can calculate its expected rate with unusual precision, any sizable deviation from that prediction would be a strong hint of particles or forces not accounted for in the Standard Model.

A closely related decay, in which a slightly heavier cousin particle called the B⁰ₛ meson decays into the same muon pair, has a longer and better-documented history at CERN:

• 2012 — LHCb reported the first evidence for the B⁰ₛ decay, at 3.5 sigma.
• 2015 — A joint analysis by LHCb and the CMS experiment confirmed it as a full observation, at 6.2 sigma.
• 2017 — LHCb alone observed the B⁰ₛ decay independently, at 7.8 sigma.
• 2026 — LHCb reports the first single-experiment evidence for the rarer B⁰ decay, at 3.2 sigma, while also sharpening its B⁰ₛ measurements.

Because the Standard Model predicts the B⁰ decay occurs roughly 30 times less often than the B⁰ₛ one, and B⁰ mesons themselves are produced about four times more abundantly in LHC collisions, researchers expect to see B⁰ decays into muon pairs at only a fraction of the rate of the B⁰ₛ channel. The new result is consistent with that expectation.

A detector built to catch a needle in a haystack

The measurement draws on proton-proton collisions recorded in 2024 and 2025 with LHCb's newly upgraded detector, which began operating at the start of the LHC's current data-taking run, combined with earlier data from the collider's first two runs. The LHCb experiment is built specifically to study particles containing a "beauty" or bottom quark, like the B⁰ meson, and to pick out the handful of genuine signal events from the vastly more common background produced in roughly a billion proton collisions per second inside the Large Hadron Collider. The LHCb collaboration includes roughly 1,200 physicists and engineers from institutions across Europe, the Americas and Asia.

Alongside the B⁰ evidence, the team also produced the world's most precise measurement yet of how often the B⁰ₛ meson decays this way and of how long it survives before doing so, a quantity known as its effective lifetime. Both values again line up with Standard Model expectations, with no sign of the kind of deviation that would point unambiguously to new physics.

Why a null result still matters

Agreement with the Standard Model might sound like an anticlimax, but it is exactly the kind of measurement theorists have waited decades for. Many proposed extensions of the Standard Model — including theories with extra Higgs bosons, a hypothetical new force carrier called a Z′ boson, or particles known as leptoquarks that would let quarks and leptons interact directly — predict that they would alter the rate of exactly this decay, or the ratio between the B⁰ and B⁰ₛ channels, even while leaving other measurements untouched. By pinning that ratio down for the first time with a single experiment, physicists have closed off some of the room such theories had to hide in, even as the broader search for the particles or forces that would explain phenomena the Standard Model cannot — including the nature of dark matter and the imbalance between matter and antimatter — continues elsewhere at CERN and beyond.

The result also stands on firmer footing than a similar claim a decade ago. A joint LHCb-CMS analysis once reported hints of the B⁰ decay as well, but subsequent independent analyses failed to confirm them, underscoring how easily faint signals in this kind of data can fluctuate. This week's result, by contrast, comes from a single, upgraded detector with a cleaner data sample, which collaboration members say gives them more confidence that the signal is real rather than a statistical mirage — while being careful to note that 3.2 sigma results have occasionally evaporated before.

What comes next

Reaching the 5-sigma threshold needed to call the B⁰ decay a confirmed discovery will require considerably more data. The LHC is expected to keep colliding protons through the end of this decade, steadily adding to the sample LHCb can analyze, and a further detector overhaul, LHCb Upgrade II, is planned for the mid-2030s and would increase the rate of usable collision data by roughly a factor of ten. In the meantime, the current paper is under internal collaboration review ahead of formal submission to a peer-reviewed journal. For now, physicists have a new, sharper ruler with which to measure the Standard Model — and, so far, the universe keeps measuring up to it.