Anatomy of the perturbative QCD approach to the baryonic decays Λb → pπ, pK

We calculate the CP-averaged branching ratios and CP-violating asymmetries for the two-body charmless hadronic decays Lambda(b) -> p pi, pK in the perturbative QCD (pQCD) approach to lowest order in alpha(s). The baryon distribution amplitudes involved in the factorization formulas are considered to the leading twist accuracy, and the distribution amplitudes of the proton are expanded to the next-to-leading conformal spin ( i. e., P'' waves), the moments of which are determined from QCD sum rules. Our work shows that the contributions from the factorizable diagrams in Lambda(b) -> p pi, pK decays are much smaller compared to the nonfactorizable diagrams in the conventional pQCD approach. We argue that this reflects the estimates of the Lambda(b) -> p transition form factors in the k(T) factorization approach, which are found to be typically an order of magnitude smaller than those estimated in the light-cone sum rules and in the nonrelativistic quark model. As an alternative, we adopt a hybrid pQCD approach, in which we compute the factorizable contributions with the Lambda(b) -> p form factors taken from the light-cone QCD sum rules. The nonfactorizable diagrams are evaluated utilizing the conventional pQCD formalism, which is free from the endpoint singularities. The predictions worked out here are confronted with the recently available data from the CDF Collaboration on the branching ratios and the direct CP asymmetries for the decays Lambda(b) -> p pi and Lambda(b) -> pK . The asymmetry parameter alpha relevant for the anisotropic angular distribution of the emitted proton in the polarized Lambda(b) baryon decays is also calculated for the two decay modes.