Measurements of $\mathcal{B}({\bar{B}}^0\to {\mathrm{\Lambda }}_c^{+}\bar{p})$ and $\mathcal{B}(B^{-}\to {\mathrm{\Lambda }}_c^{+}\bar{p}{\pi }^{-})$ and Studies of ${\mathrm{\Lambda }}_c^{+}{\pi }^{-}$ Resonances

We present an investigation of the decays $\bar{B}^0 \to \Lambda_{c}^{+} \bar{p}$and$B^- \to \Lambda_{c}^{+} \bar{p} \pi^-$basedon$383\times10^6$ $\mathrm{{\rm Y}}(4S)\to B\bar{B}$ decays recorded with the BABAR detector. We measure the branching fractions of these decays; their ratio is ${\cal B}(B^- \to \Lambda_{c}^{+} \bar{p} \pi^-) / {\cal B}(\bar{B}^0 \to \Lambda_{c}^{+} \bar{p}) = 15.4 \pm 1.8 \pm 0.3$\mathrm{.}The$B^- \to \Lambda_{c}^{+} \bar{p} \pi^-$processexhibitsanenhancementatthe$\Lambda_{c}^{+} \bar{p}$ threshold and is a laboratory for searches for excited charm baryon states. We observe the resonant decays ${\bar{B}}^{-}\to {\mathrm{\Sigma }}_c(2455{)}^0\bar{p}$ and $B^- \to \Sigma_{c}(2800)^{0} \bar{p}$butseenoevidencefor$B^- \to \Sigma_{c}(2520)^{0} \baver{p}$\mathrm{.}Thisisthefirstobservationofthedecay$B^- \to \Sigma_{c}(2800)^{0} \bar{p}$; however, the mass of the observed excited ${\mathrm{\Sigma }}_c^0$ state is $(2846\pm 8\pm 10)MeV\mathrm{/}{c}^2$, which is somewhat inconsistent with previous measurements. Finally, we examine the angular distribution of the $B^{-}\to {\mathrm{\Sigma }}_c(2455{)}^0\bar{p}$ decays and measure the spin of the ${\mathrm{\Sigma }}_c(2455{)}^0$ baryon to be 1/2, as predicted by the quark model.