Measurement of B0→Ds(*)+D*- branching fractions and B0→Ds*+D*- polarization with a partial reconstruction technique

We present a study of the decays B0→Ds*+D*-, using 20.8 fb-1 of e+e- annihilation data recorded with the BABAR detector. The analysis is conducted with a partial reconstruction technique, in which only the D(*)+ and the soft pion from the D*- decay are reconstructed. We measure the branching fractions γ(B0 →Ds+D*-)= (1.03±0.14±0.13±0.26)% and ß(B0→Ds *+D*-)= (1.97±0.15±0.30±0.49)%, where the first error is statistical, the second is systematic, and the third is the error due to the Ds+→φπ+ branching fraction uncertainty. From the B0→Ds*+D*- angular distributions, we measure the fraction of longitudinal polarization ΓL/Γ=(51.9±5.0±2.8)%, which is consistent with theoretical predictions based on factorization.