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The long awaited baryonic $B$ decay $\bar B{}^0\to p\bar p$ was recentlyobserved by LHCb with a branching fraction of order $10^{-8}$. All the earliermodel predictions are too large compared with experiment. In this work, wepoint out that for a given tree operator $O_i$, the contribution from itsFiertz transformed operator, an effect often missed in the literature, tends tocancel the internal $W$-emission amplitude induced from $O_i$. The wavefunction of low-lying baryons are symmetric in momenta and the quark flavorwith the same chirality, but antisymmetric in color indices. Using thesesymmetry properties and the chiral structure of weak interactions, we find thathalf of the Feynman diagrams responsible for internal $W$-emission cancel.Since this feature holds in the charmless modes but not in the charmful ones,we advocate that the partial cancellation accounts for the smallness of thetree-dominated charmless two-body baryonic $B$ decays. This also explains whymost previous model calculations predicted too large rates as the aboveconsideration was not taken into account. Finally, we emphasize that, contraryto the claim in the literature, the internal $W$-emission tree amplitude shouldbe proportional to the Wilson coefficient $c_1+c_2$ rather than $c_1-c_2$.

On the smallness of Tree-dominated Charmless Two-body Baryonic $B$ Decay Rates