Observation of the semileptonic decays of BS and Λb hadrons at LEP

In 450000 hadronic Z decays recorded with the ALEPH detector at LEP, the yields of D(s)-l+ and LAMBDA(c)+l- combinations have been measured. 16.0 +/- 4.3 D(s)-l+ combinations were observed in the D(s)- --> phipi- channel and 17.0 +/- 4.5 combinations were observed in the D(s)- --> K*0K- channel. 21.0 +/- 5.0 LAMBDA(c)+l- combinations were observed, with the LAMBDA(c)+ reconstructed in the decay mode LAMBDA(c)+ --> pK-pi+. These events provide evidence for the decays B(s) --> D(s)-Xl+nu and LAMBDA(b) --> LAMBDA(c)+Xl-nuBAR. Assuming that the B(s) and LAMBDA(b) semileptonic decays are dominantly three-body, these observed yields, after background subtraction, translate into the following product branching ratios: Br(bBAR --> B(s))Br(B(s) --> D(s)-Xl+nu) = 0.040 +/- 0.011 +0.010/-0.012, Br(b --> LAMBDA(b))Br(LAMBDA(b) --> LAMBDA(c)+Xl-nuBAR) = 0.030 +/- 0.007 +/- 0.009.