Measurement of the B0bar --> D*+ l- nubar_l Decay Rate and |Vcb|

Abstract

We present a measurement of the Cabibbo-Kobayashi-Maskawa matrix element |Vcb| based on a sample of about 53,700 B0bar --> D*+ l- nubar_l decays observed by the BaBar detector. We obtain the branching fraction averaged over l = e, mu, BR(B0bar --> D*+ l- nubar_l) = (4.90 +- 0.07(stat.) +0.36-0.35(syst.))%. We measure the differential decay rate as a function of w, the relativistic boost gamma of the D*+ in the B0bar rest frame. By extrapolating dGamma/dw to the kinematic limit w --> 1, we extract the product of |Vcb| and the axial form factor A_1(w=1). We combine this measurement with a lattice QCD calculation of A_1(w=1) to determine |Vcb| = (38.7 +- 0.3(stat.) +- 1.7(syst.) +1.5-1.3(theory)) * 10^-3.