Measurement of the semileptonic branching fraction of b baryons.

We present the first measurement of the ratio R_{Λ ℓ}=BR(Λ _b → Λ ℓX)/BR(Λ _b → Λ X) where Λ _b denotes generic b baryons. In the spectator model, this ratio is equivalent to the average b baryon semileptonic branching ratio. We measure R_{Λ ℓ} = (6.8 ± 1.3 ± 1.0)% as well as the products ƒ(b → Λ _b) · BR(Λ _b → ΛX) = (3.88 ± 0.55 ± 0.48)% and ƒ(b → B) · BR(B → ΛX) = (2.31 ± 0.35 ± 0.51)%. In all cases, the uncertainties shown are statistical and systematic, respectively. All these results are preliminary.