Measurement of the decay B0→D−ℓ+ν

Using the ARGUS detector at the e + e − storage ring DORIS II, we have investigated the decay B 0 →D − l + ν, where l + is e + or μ + . The B 0 mesons were produced in 150 000 ϒ(4S) decays. Assuming electron-muon universality we obtain a branching ratio BR(B 0 →D − e + ν )=BR(B 0 →D − μ + ν )=(1.8±0.6±0.5)%.