Study of decay mechanisms in B-→Λc+p̄π- decays and observation of low-mass structure in the Λc+p̄ system

Using a sample of 152×106 BB̄ pairs accumulated with the Belle detector at the KEKB e+e- collider, we study the decay mechanism of three-body charmed decay B-→Λc+p̄π-. The intermediate two-body decay B-→Σc(2455)0p̄ is observed for the first time with a branching fraction of (3.7±0.7±0.4±1.0)×10-5 and a statistical significance of 8.4σ. We also observe a low-mass enhancement in the (Λc+p̄) system, which can be parametrized as a Breit-Wigner function with a mass of (3.35-0.02+0.01±0.02)GeV/c2 and a width of (0.07-0.03+0.04±0.04)GeV/c2. We measure its branching fraction to be (3.9-0.7+0.8±0.4±1.0)×10-5 with a statistical significance of 6.2σ. The errors are statistical, systematic, and that of the Λc+→pK-π+ decay branching fraction. © 2006 The American Physical Society.