Measurement of and determination of |Vcb|

We present a measurement of the branching fraction for the semileptonic B decay , where l− can be either an electron or a muon. We find , and the resulting branching fraction , where the first error is statistical and the second systematic. We also investigate the form factor and the implications of the result for |Vcb|. From a fit to the differential decay distribution we obtain the rate normalization |Vcb|FD(1)=(4.11±0.44±0.52)×10−2. Using a theoretical calculation of FD(1), the Cabibbo–Kobayashi–Maskawa matrix element |Vcb|=(4.19±0.45±0.53±0.30)×10−2 is obtained, where the last error comes from the theoretical uncertainty of FD(1). The results are based on a data sample of 10.2fb−1 recorded at the ϒ(4S) resonance with the Belle detector at the KEKB e+e− collider.