Time-dependent Feshbach resonance scattering and anomalous decay of a Na Bose-Einstein condensate

We study Feshbach resonance scattering in a time-dependent magnetic field. We explain the extremely rapid decay observed in a recent experiment invest igating Feshbach resonances in a Na Bose-Einstein condensate. In our pictur e, the decay is stimulated by the formation of a molecular condensate of qu asibound atom pairs. Another essential element is the concept of a global a nd a local resonance lifetime. The predicted decay rates are large, about 5 orders of magnitude larger than typical dipole decay rates, and 1 order la rger than typical exchange decay rates. We paint out the possible role of a Josephson-like oscillation between the atomic condensate and a long-range molecular condensate.