Search for D^∗(2007) → 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 fraction of D∗(2007)0→μ+μ- decays and the most stringent limit on D^∗(2007) decays to leptonic final states. The analysis is the first search for a rare charm-meson decay exploiting production via beauty decays.