Measurements of B(B¯0→Λc+p¯) and B(B-→Λc+p¯π-) and studies of Λc+π- resonances

We present an investigation of the decays B̅ 0→Λc+p̅ and B-→Λc+p̅ π- based on 383×106 Υ(4S)→BB̅ decays recorded with the BABAR detector. We measure the branching fractions of these decays; their ratio is B(B-→Λc+p̅ π-)/B(B̅ 0→Λc+p̅ )=15.4±1.8±0.3. The B-→Λc+p̅ π- process exhibits an enhancement at the Λc+p̅ threshold and is a laboratory for searches for excited charm baryon states. We observe the resonant decays B-→Σc(2455)0p̅ and B-→Σc(2800)0p̅ but see no evidence for B-→Σc(2520)0p̅ . This is the first observation of the decay B-→Σc(2800)0p̅ ; however, the mass of the observed excited Σc0 state is (2846±8±10)  MeV/c2, which is somewhat inconsistent with previous measurements. Finally, we examine the angular distribution of the B-→Σc(2455)0p̅ decays and measure the spin of the Σc(2455)0 baryon to be 1/2, as predicted by the quark model.