In particle physics, ten years is a long time to sit with a puzzle. Since 2013, measurements of a rare decay—a neutral B meson (B0) transforming into an excited kaon (K*0) and a muon–antimuon pair (µ+µ –)—have stubbornly refused to match the predictions of the standard model, the theory that describes all known particles and forces [1]. Small enough to be dismissed at first as a statistical fluctuation, the pattern of discrepancies has grown with each new dataset into one of the most tantalizing hints of new physics in experimental particle physics. Now the LHCb Collaboration at CERN in Switzerland has published its most comprehensive analysis of the decay to date [2]. The result is clear: The anomaly persists. Encouragingly, the theoretical and experimental tools to understand it have never been sharper.
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