Measurement of the B̄ → D(*)πℓν̄ℓ branching fraction

Abstract A study of b semileptonic decays into D , Dπ ± and D ∗ π ± 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 − ν l X)=(7.04±0.34 (stat) ±0.36 (syst.exp) ±0.17 ( BR D ))% , BR (b→D + l − ν l X)=(2.72±0.19 (stat) ±0.16 (syst.exp) ±0.18 ( BR D ))% , BR (b→D ∗+ l − ν l X)=(2.75±0.17 (stat) ±0.13 (syst.exp) ±0.09 ( BR D ))% , where the D 0 and D + results include also contributions from D ∗0 and D ∗+ decays. A fit to the distribution of the π ± impact parameter to the primary interaction vertex provides a measurement of the b semileptonic branching fractions into the D 0 π ± X , D + π ± X and D ∗+ π ± X final states. Assuming that single pion decay modes of B mesons dominate, the partial rates for B →Dπl − ν l and B →D ∗ πl − ν l have been obtained, corresponding to a total branching fraction: BR ( B →Dπl − ν l )+ BR ( B →D ∗ πl − ν 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 →Dl − ν l and D ∗ l − ν l branching fractions.