A study of b semileptonic decays into D, D pi(+/-) and D*pi(+/-) final states is presented. The D-0, D+ and D*(+) mesons are exclusively reconstructed in Z decay data recorded from 1992 to 1995 in the DELPHI experiment at LEP. The overall branching fractions are measured to be: BR(b --> D(0)l(-)<(upsilon)over bar>X-l) = (7.04 +/- 0.34(stat) +/- 0.36 (syst.exp) +/- 0.17(BRD))%, BR(b --> D(+)l(-)<(upsilon)over bar>X-l) = (2.72 +/- 0.19(stat) +/- 0.16(syst.exp) +/- 0.18(BRD))%, BR(b --> D*(+)l(-)<(upsilon)over bar>X-l) = (2.75 +/- 0.17(stat) +/- 0.13 (syst.exp) +/- 0.09 (BRD))%, where the D-0 and D+ results include also contributions from D*(0) and D*(+) decays. A fit to the distribution of the pi(+/-) impact parameter to the primary interaction vertex provides a measurement of the b semileptonic branching fractions into the D(0)pi(+/-)X, D(+)pi(+/-)X and D*(+)pi(+/-)X final states. Assuming that single pion decay modes of B mesons dominate, the partial rates for (B) over bar --> D pi l(-)<(upsilon)over bar>(l) and (B) over bar --> D*pi l(-)<(upsilon)over bar>(l) have been obtained, corresponding to a total branching fraction: BR((B) over bar --> D pi l(-)<(upsilon)over bar>(l)) + BR((B) over bar --> D*pi l(-)<(upsilon)over bar>(l)) = (3.40 +/- 0.52 (stat) +/- 0.32 (syst))%. This result agrees well with the observed difference between the total B semileptonic branching fraction and the sum of the (B) over bar --> Dl(-) upsilon(l) and D* l(-)<(upsilon)over bar>(l) branching fractions. (C) 2000 Elsevier Science B.V. All rights reserved.
