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 beautymesons 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.