The LHCb collaboration has made the first observation of the decay B+ → π+μ+μ. With a branching ratio of about 2 per 100 million decays, this is the rarest decay of a B hadron ever observed, see CERN Courier article.

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The image shows the invariant-mass distribution of selected π+μ+μ combinations, showing the clear peak corresponding to B+ decays (green long-dash). Also shown are the components in the fit from partially reconstructed decays (red dotted) and misidentified K+μ+μ (black dashed) and the total (blue solid line). Candidates for which the μ+μ pair is consistent with coming from a J/ψ or ψ(2S) decay have been excluded.

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