Investigation of the decays\(\bar B^0 \to D^{* + } \ell ^ - \bar v\) and\(\bar B \to D^{**} \ell ^ - \bar v\)

Exclusive semileptonicB decays with aD*+ meson in the final state have been studied using the ARGUS detector at the DORIS II storage ring. The branching ratio for the decay\(\bar B^0 \to D^{* + } \ell ^ - \bar v\), where l- is eithere− or μ-, has been measured to be (5.2±0.5±0.6)%. A significant rate for the decay\(\bar B \to D^{**} \ell ^ - \bar v\) has been observed. From an angular analysis of the cascade\(\bar B^0 \to D^{* + } ( \to D^0 \pi ^ + )\ell ^ - \bar v\) the forward-backward asymmetryAFB and theD*+ polarization parameter α have been determined to beAFB=0.20±0.08±0.06 and α=1.1±0.4±0.2.