Measurements of B(B0→Λc+p) and B(B-→Λc+pπ-) and studies of Λc+π- resonances

We present an investigation of the decays B0→Λc+p and B-→Λc+pπ- based on 383x106 Υ(4S)→BB decays recorded with the BABAR detector. We measure the branching fractions of these decays; their ratio is B(B-→Λc+pπ-)/B(B0→Λc+p)=15.4±1.8±0.3. The B-→Λc+pπ- process exhibits an enhancement at the Λc+p threshold and is a laboratory for searches for excited charm baryon states. We observe the resonant decays B-→Σc(2455)0p and B-→Σc(2800)0p but see no evidence for B-→Σc(2520)0p. This is the first observation of the decay B-→Σc(2800)0p; 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)0p decays and measure the spin of the Σc(2455)0 baryon to be 1/2, as predicted by the quark model.