A study of mixing using partial reconstruction

Using the ARGUS detector at the e + e − storage ring DORIS II at DESY, we have studied the decay B 0 → D ∗+ l − v by exploiting a partial D ∗+ reconstruction technique. The branching ratio for this mode was thereby determined to be (4.5 ± 0.3 ± 0.4)%. Using the corresponding sample of tagged B 0 mesons, we measured the B 0 B 0 mixing parameter to be r d = 0.194 ± 0.062 ± 0.054, a result only weakly dependent upon the ratio of semileptonic widths for and production rates of B 0 and B + mesons. We have also determined the branching ratio Br ( B 0 → X l − v to be (9.3 ± 1.1 ± 1.5)%. By comparing the results for full and partial D ∗+ reconstuction we found the absolute branching ratios for D 0 → K − π + and D 0 → K − π + π + π − to be (4.5 ± 0.6 ± 0.4)% and (7.9 ± 1.5 ± 0.9)%, respectively.