Measurement of the B -> D^* l nu Branching Fractions and |Vcb|
  We study the exclusive semileptonic B meson decays B- -> D*0 l- nu and B0 -> D*+ l- nu using data collected with the CLEO II detector at CESR. We present measurements of the branching fractions B(B0 -> D*+ l-nu) = 0.5/f00* [4.49+/-0.32+/-0.39]% and B(B- -> D*0 l-nu) = 0.5/f+-*[5.13+/-0.54+/-0.64]%, where f00 and f+- are the neutral and charged B meson production fractions at the Upsilon(4s) resonance. Assuming isospion invariance and taking the charged to neutral B meson lifetimes measured at higher energy machines, we determine the ratio f+-/f00=1.04+/-0.14+/-0.13+-/-0.10; further assuming f+- + f00 = 1 we also determine the partial width G(B->D* l nu) = 29.9+/-1.9+/-2.7+/-2.0 ns-1 (independent of f+-/f00). From this partial width we calculate B -> D* l nu branching fractions that do not depend on f+-/f00, nor the individual B lifetimes, but only on the charged to neutral lifetime ratio. The product of the CKM matrix element |Vcb| times the normalization of the decay form factor at the point of zero recoil of the D* meson, F(y=1), is determined from a linear fit to the combined differential decay rate of the exclusive B->D* l nu decays: |Vcb|F(y) = 0.0351 +/- 0.0019 +/- 0.0018 +/- 0.0008. Using theoretical calculations of the form factor normalization we extract a value for |Vcb|. LATEX (REVTEX style) file with uuencoded figures attached (uses PSBOX). Available on WWW http://w4.lns.cornell.edu/public/CLNS/ 
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