Study of decay mechanisms in B- -> Lambda_c+ pbar pi- decay and observation of low mass structure in the (Lambda_c+ pbar) system

Using a sample of 152 million BBbar pairs accumulated with the Belle detector at the KEKB e+e- collider, we perform a Dalitz plot analysis of the three-body charmed decay B- -> Lambda_c+ p_bar pi-. The intermediate two-body decay B- -> Sigma_c(2455)0 p_bar is observed for the first time with a branching fraction of (3.67^{+0.74}_{-0.66}+-0.36+-0.95)x10^(-5) and a statistical significance of 8.4sigma. We also observe a low-mass enhancement in the (Lambda_c+ p_bar) system, which can be parameterized as a Breit-Wigner function with a mass of (3.35^{+0.01}_{-0.02}+-0.02)GeV/c^2 and a width of (0.07^{+0.04}_{-0.03}+-0.04)GeV/c^2. We measure the product of the corresponding decay branching fractions to be (3.87^{+0.77}_{-0.72}+-0.43+-1.01)x10^(-5)$ with a statistical significance of 6.2sigma. The errors are statistical, systematic, and that of the Lambda_c^+ -> pK-pi+ decay branching fraction.