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We calculate the CP-averaged branching ratios and CP-violating asymmetriesfor the two-body charmless hadronic decays $\Lambda_b \to p \pi, pK$ in theperturbative QCD (pQCD) approach to lowest order in $\alpha_s$. The baryondistribution amplitudes involved in the factorization formulae are consideredto the leading twist accuracy and the distribution amplitudes of the proton areexpanded to the next-to-leading conformal spin (i.e., "P" -waves), the momentsof which are determined from QCD sum rules. Our work shows that thecontributions from the factorizable diagrams in $\Lambda_b \to p \pi, pK$decays are much smaller compared to the non-factorizable diagrams in theconventional pQCD approach. We argue that this reflects the estimates of the$\Lambda_b \to p$ transition form factors in the $k_T$ factorization approach,which are found typically an order of magnitude smaller than those estimated inthe light-cone sum rules and in the non-relativistic quark model. As analternative, we adopt a hybrid pQCD approach, in which we compute thefactorizable contributions with the $\Lambda_b \to p$ form factors taken fromthe light cone QCD sum rules. The non-factorizable diagrams are evaluatedutilizing the conventional pQCD formalism which is free from the endpointsingularities. The predictions worked out here are confronted with the recentlyavailable data from the CDF collaboration on the branching ratios and thedirect CP asymmetries for the decays $ \Lambda_b \to p \pi$, and $\Lambda_b \top K$. The asymmetry parameter $\alpha$ relevant for the anisotropic angulardistribution of the emitted proton in the polarized $\Lambda_b$ baryon decaysis also calculated for the two decay modes.

Anatomy of the pQCD Approach to the Baryonic Decays $\Lambda_b \to p\pi, p K$