Study of the B-0 semileptonic decay spectrum at the Y(4S) resonance

We have made a first measurement of the lepton momentum spectrum in a sample of events enriched in neutral B's through a partial reconstruction of B-0 --> D*(-)l(+)<(nu)over bar>. This spectrum, measured with 2.38 fb(-1) of data collected at the Y(4S) resonance by the CLEO II detector, is compared directly to the inclusive lepton spectrum from all Y(4S) events in the same data set. These two spectra are consistent with having the same shape above 1.5 GeV/c. From the two spectra and two other CLEO measurements, we obtain the B-0 and B+ semileptonic branching fractions, b(0) and b(+), their ratio, and the production ratio f(+-)/f(00) of B+ and B-0 pairs at the Y(4S). We report b(+)/b(0) = 0.950(-0.080) (+0.117) +/- 0.091, b(0) = (10.78 +/- 0.60 +/- 0.69)%, and b(+) = (10.25 +/- 0.57 +/- 0.65)%. b(+)/b(0) is equivalent to the ratio of charged to neutral B lifetimes, tau(+)/tau(0). (C) 1997 Published by Elsevier Science B.V.