Measurements of ${\cal B}(\bar{B}^0 \to \Lambda_{c}^{+} \bar{p})$ and ${\cal B}(B^- \to \Lambda_{c}^{+} \bar{p} \pi^-)$ and Studies of $\Lambda_{c}^{+}\pi^-$ Resonances

20 pages, 11 postscript figures, submitted to PRDWe present an investigation of the decays $\bar{B}^0 \to \Lambda_{c}^{+} \bar{p}$ and $B^- \to \Lambda_{c}^{+} \bar{p} \pi^-$ based on $383\times10^6$ $\Upsilon(4S) \to B\bar{B}$ decays recorded with the BABAR detector. We measure the branching fractions of these decays; their ratio is ${\cal B}(B^- \to \Lambda_{c}^{+} \bar{p} \pi^-) / {\cal B}(\bar{B}^0 \to \Lambda_{c}^{+} \bar{p}) = 15.4 \pm 1.8 \pm 0.3$. The $B^- \to \Lambda_{c}^{+} \bar{p} \pi^-$ process exhibits an enhancement at the $\Lambda_{c}^{+} \bar{p}$ threshold and is a laboratory for searches for excited charm baryon states. We observe the resonant decays $\bar{B}^- \to \Sigma_{c}(2455)^{0} \bar{p}$ and $B^- \to \Sigma_{c}(2800)^{0} \bar{p}$ but see no evidence for $B^- \to \Sigma_{c}(2520)^{0} \baver{p}$. This is the first observation of the decay $B^- \to \Sigma_{c}(2800)^{0} \bar{p}$; however, the mass of the observed excited $\Sigma_{c}^{0}$ state is $(2846\pm8\pm10)MeV/c^2$, which is somewhat inconsistent with previous measurements. Finally, we examine the angular distribution of the $B^- \to \Sigma_{c}(2455)^{0} \bar{p}$ decays and measure the spin of the $\Sigma_{c}(2455)^{0}$ baryon to be 1/2, as predicted by the quark model