Search for the decayD0→μ+μ−

Using a silicon-microstrip detector array to identify secondary vertices occurring downstream of a short platinum target, we have searched for the decay ${\mathit{D}}^0$→${\mathrm{\mu }}^{+}$ ${\mathrm{\mu }}^{\mathrm{-}}$. Normalized relative to the J/ψ→${\mathrm{\mu }}^{+}$ ${\mathrm{\mu }}^{\mathrm{-}}$ signal observed in the same data sample, for a 3.25-mm minimum decay distance our branching-ratio sensitivity is (4.8±1.4)×${10}^{\mathrm{-}6}$ per event, and after background subtraction we observe -4.1±4.8 events. Using the statistical approach advocated by the Particle Data Group, we obtain a limit B(${\mathit{D}}^0$→${\mathrm{\mu }}^{+}$ ${\mathrm{\mu }}^{\mathrm{-}}$)${10}^{\mathrm{-}5}$ at 90% confidence, confirming with a different technique the limit previously obtained by Louis et al. The interpretation of the upper limit involves complex statistical issues; we present another approach which is more suitable for combining the results of different experiments.