Search for the Flavor-Changing Neutral Current Decay $D^0 \to   \mu^+\mu^-$ with the HERA-B Detector
  We report on a search for the flavor-changing neutral current decay $D^0 \to \mu^+\mu^-$ using $50 \times 10^6$ events recorded with a dimuon trigger in interactions of 920 GeV protons with nuclei by the HERA-B experiment. We find no evidence for such decays and set a 90% confidence level upper limit on the branching fraction $Br(D^0 \to \mu^+\mu^-) <2.0 \times 10^{-6}$. 
HERA-B Collaboration: I. Abt, et al