Using a 281pb-1 data sample collected at the ψ(3770) with the CLEO-c detector, we present the first absolute branching fraction measurement of the decay D0→K-π+π-e+νe at a statistical significance of about 4.0 standard deviations. We find 10 candidates consistent with the decay D0→K-π+π-e+νe. The probability that a background fluctuation accounts for this signal is less than 4.1×10-5. We find B(D0→K-π+π-e+νe)=[2.8-1.1+1.4(stat)±0.3(syst)]×10-4. By restricting the invariant mass of the hadronic system to be consistent with K1(1270), we obtain the product of branching fractions B(D0→K1-(1270) e+νe)×B(K1-(1270)→K-π+π-)=[2.5-1.0+1.3(stat)±0. 2(syst)]×10-4. Using B(K1-(1270)→K-π+π-)=(33±3)%, we obtain B(D0→K1-(1270)e+νe)=[7.6-3.0+4.1(stat)±0.6(syst)±0. 7]×10-4. The last error accounts for the uncertainties in the measured K1-(1270)→K-π+π- branching fractions. © 2007 The American Physical Society.
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