The signal reaches 3.2σ, below the discovery threshold. LHCb also measures the related B⁰s decay at about 3.3 in a billion decays; both rates agree with the Standard Model.
The LHCb collaboration at CERN has found its first single-experiment evidence that a B⁰ meson can decay into a muon and an antimuon. Its combined data put the probability at about 1.1 in 10 billion B⁰ decays. The signal is not yet strong enough to count as a discovery.
A joint LHCb and CMS analysis in 2015 established the related B⁰s decay and also found evidence for the B⁰ decay, but later measurements did not confirm the latter. LHCb’s new paper is available as a preprint on arXiv and has been submitted to Physical Review X.
Mesons are particles made of a quark and an antiquark. B⁰ and B⁰s each contain a beauty antiquark, paired with a down quark and a strange quark, respectively. A muon is a heavier cousin of the electron; an antimuon is its antiparticle.
The team analyzed proton collisions recorded in 2024 and 2025 with LHCb’s upgraded detector, then combined them with data from the experiment’s earlier runs. The new sample alone produced a B⁰ signal with a statistical significance of 3σ; with the earlier data, that rose to 3.2σ. Particle physics uses 5σ as the threshold for a discovery.
To separate the rare decays from unrelated muon pairs, the team trained a machine-learning algorithm to recognize features such as two muons emerging from the same point away from the collision. A plot of the pairs’ combined mass showed a prominent B⁰s peak and a smaller B⁰ contribution at a lower mass.
With earlier data included, LHCb found that around 3.3 in every billion B⁰s mesons decay into muon pairs.
LHCb says its B⁰s decay probability and effective-lifetime measurements are the most precise yet. Its combined decay probabilities agree with the Standard Model, the theory describing known elementary particles and forces: the predicted point lies within the measurements’ joint 68.27% confidence region.
The World Meteorological Organization says the imbalance grew by 0.3 W/m² per decade in 2001–2025, compared with 0.13 W/m² per decade in 1960–2025; these rates measure growth in the imbalance, not temperature increases or its absolute size. It reached a record in 2025 since monitoring began in 1960; preliminary measurements suggest another in January–July 2026.
Analysis
The Standard Model predicts these two-muon decays precisely. They occur through fleeting quantum processes involving heavy particles, and unknown heavy particles could affect those processes too. The model predicts that the B⁰ decay is about 30 times rarer than the B⁰s decay.
LHCb spokesperson Tim Gershon, a physicist at the University of Warwick, told Physics World that the measurements rule out models that would significantly change the decay rates.
The team also measured the B⁰s effective lifetime from the decay-time distribution of those mesons that produce muon pairs, finding 1.6 ± 0.16 ± 0.01 picoseconds, in agreement with the Standard Model. A picosecond is one trillionth of a second. B⁰s mesons change between matter and antimatter and behave as a mixture of two states with different lifetimes. In the Standard Model, only the longer-lived state decays into a muon pair. New physics could change the effective lifetime even if the decay probability stayed the same.
Andrzej Buras of the Technical University of Munich, who was not involved in the study, expects precise B-meson measurements combined with studies of rare kaon decays to strengthen indirect searches for new physics. Kaons are lighter mesons containing a strange quark.
A further LHCb upgrade could increase the rate of data collection by a factor of 10 in the middle of the next decade, improving sensitivity to undiscovered particles. Gershon says the upgrade would provide unprecedented sensitivity to test the Standard Model and search for physics beyond it. But Gershon said UKRI withdrew funding for UK involvement in constructing the upgraded detector, putting the program in jeopardy. The UK reversed an earlier funding commitment in December 2025, and Gershon hopes an agreement can enable full UK participation.
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