Measurements of B(B^0 → Λ_c+^ p) and B(B^- → Λ_c^+ pπ^- and studies of Λ_c^+ π^- resonances

We present an investigation of the decays B^0→Λ_c^+p and B-→Λ_c^+p π^- based on 383×10^6 Υ(4S)→BB decays recorded with the BABAR detector. We measure the branching fractions of these decays; their ratio is B(B^-→Λ_c^+p π^-)/B(B ^0→Λ_c^+p )=15.4±1.8±0.3. The B^-→Λ_c^+p π^- process exhibits an enhancement at the Λ_c^+p threshold and is a laboratory for searches for excited charm baryon states. We observe the resonant decays B^-→Σ_c(2455)^0p and B^-→Σ_c(2800)^0p but see no evidence for B^-→Σ_c(2520)^0p . This is the first observation of the decay B^-→Σ_c(2800)^0p ; however, the mass of the observed excited Σ_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^-→Σ_c(2455)^0p decays and measure the spin of the Σ_c(2455)^0 baryon to be 1/2, as predicted by the quark model.

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