Charmless Exclusive Baryonic B Decays

We present a systematical study of two-body and three-body charmless baryonic B decays. Branching ratios for two-body modes are in general very small, typically less than $10^{-6}$, except for the decays with a $\Delta$ resonance in the final state. For example, the branching ratio of the tree-dominated decay $B^-\to p \bar\Delta^{--}$ can be as large as $1\times 10^{-5}$, and the penguin-dominated decay $B^-\to\Sigma^+\bar\Delta^{--}$ is at the level of $1\times 10^{-6}$. For three-body modes we focus on octet baryon final states. The leading three-dominated modes are $\bar B^0\to p\bar n\pi^-(\rho^-), n\bar p\pi^+(\rho^+)$ with a branching ratio of order $4\times 10^{-6}$ for $\bar B^0\to p\bar n\pi^-$ and $1\times 10^{-5}$ for $\bar B^0\to p\bar n\rho^-$. The first measurement of the penguin-dominated decay $B^-\to p\bar pK^-$ by Belle indicates that the $q^2$ dependence of heavy-to-light baryon form factors is favored to be of the monopole form. While the penguin-dominated decays $B^-\to p\bar p K^{-(*)}$ and $\bar B^0\to p\bar n K^{-(*)}, n\bar n \bar K^{0(*)}$ all have appreciable rates, it is not the case for $\bar B^0\to\Lambda\bar p\pi^+(\rho^+)$; we find $\Gamma(\bar B^0\to\Lambda\bar p\pi^+)\sim ({1\over 2}-{2\over 3}) \Gamma(\bar B^0\to\Lambda\bar p\rho^+)\leq\Gamma(B^-\to\Lambda\bar p)$. We explain why some of charmless three-body final states in which baryon-antibaryon pair production is accompanied by a meson have a larger rate than their two-body counterparts: either the pole diagrams for the former have a $\Lambda_b$ intermediate state, which has a large coupling to the B meson and the nucleon, or they are dominated by the factorizable external W-emission process.