Search for the decay D0 → μ+μ−

Abstract We have searched for the decay D 0 → μ + μ − among 1.25 × 10 5 μ + μ − pairs produced by 350 GeV/ c π − particles interacting in copper and tungsten targets. Using a high-resolution silicon-microstrip detector followed by a large-acceptance magnetic spectrometer and a muon filter we are able to discriminate between prompt and non-prompt muons and to measure dimuon masses. No candidate compatible with a D 0 → μ + μ − decay has been found, allowing us to set an upper limit on the branching fraction B( D 0 → μ + μ − ) of 7.6 × 10 −6 at the 90% confidence level.