Measurements of [script B]([overline B]^0 → Λ_c^+[overline p]) and [script B](B^- → Λ_c^+[overline p]π^-) and studies of Λ_c^+π^- resonances

We present an investigation of the decays [overline B]^0 → Λ_c^+[overline p] and B^- → Λ_c^+[overline p]π^- based on 383×10^6 γ(4S) → B[overline B] decays recorded with the BABAR detector. We measure the branching fractions of these decays; their ratio is [script B](B^- → Λ_c^+[overline p]π^-)/[script B]([overline B]^0 → Λ_c^+[overline p])=15.4 ± 1.8 ± 0.3. The B^- → Λ_c^+[overline p]π^- process exhibits an enhancement at the Λ_c^+[overline p] threshold and is a laboratory for searches for excited charm baryon states. We observe the resonant decays B^- → ∑_c(2455)^0[overline p] and B^- → ∑_c(2800)^0[overline p] but see no evidence for B^- → ∑_c(2520)^0[overline p]. This is the first observation of the decay B^- → ∑_c(2800)^0[overline p]; however, the mass of the observed excited ∑_c^0 state is (2846 ± 8 ± 10) MeV/c^2, which is somewhat inconsistent with previous measurements. Finally, we examine the angular distribution of the B^- → ∑_c(2455)^0[overline p] decays and measure the spin of the ∑_c(2455)^0 baryon to be 1/2, as predicted by the quark model.