Observation of missing energy associated with. mu. /sup +/. mu. /sup -/ produced in p-Fe interactions

In a study of interactions of 400-GeV protons in an iron calorimeter, we have observed ..mu../sup +/..mu../sup -/ pairs associated with a significant amount of missing energy indicative of final-state neutrinos. This missing energy is not due to any instrumental effects, nor to any trivial sources like double KK-bar (or ..pi pi..) decay. Interpretation of these data as production of a DD-bar pair followed by a double muonic decay leads to a model-dependent estimate of a total production cross section of the order of 7--20 ..mu..b.