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 fraction 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.