CERN's LHCb Finds Hint of One-in-10-Billion Rare Particle Decay

Physicists at CERN's LHCb experiment report evidence of an extremely rare particle decay after a search lasting 42 years. The Bd meson decaying into a muon and an anti-muon is expected in only about one in ten billion decays of beauty-containing particles, making it among the most elusive processes in particle physics.

The team combined data from 2024 and 2025 with collisions recorded between 2011 and 2018, reaching a significance of 3.2 sigma. Confirmation as a discovery requires five sigma. The analysis, led by Harry Cliff of the University of Cambridge, was presented at a CERN seminar on November 3 and posted on arXiv.

All results agree with Standard Model expectations, but they tighten constraints on possible new physics. The findings are not yet peer reviewed. Detector upgrades planned for the 2030s should raise collision rates and data volumes roughly tenfold, giving physicists a much better chance of confirming the decay or revealing surprising deviations.