Search for D∗(2007)0 → μ+μ− in B− → π−μ+μ− decays

The very rare D∗(2007)0 → μ+μ− decay is searched for by analysing B− → π−μ+μ− decays. The analysis uses a sample of beauty mesons produced in proton–proton collisions collected with the LHCb detector between 2011 and 2018, corresponding to an integrated luminosity of 9 fb−1 . The signal signature corresponds to simultaneous peaks in the μ+μ− and π−μ+μ− invariant masses. No evidence for an excess of events over background is observed and an upper limit is set on the branching fraction of the decay at B(D∗(2007)0 → μ+μ−) < 2.6 × 10−8 at 90% confidence level. This is the first limit on the branching frac-
tion of D∗(2007)0 → μ+μ− decays and the most stringent limit on D∗(2007)0 decays to leptonic final states. The analysis is the first search for a rare charm-meson decay exploiting production via beauty decays.