Heavy-to-light baryonic form factors at large recoil

Abstract

We analyze heavy-to-light baryonic form factors at large recoil and derive the scaling behavior of these form factors in the heavy quark limit. It is shown that only one universal form factor is needed to parameterize Λ _b  → p and Λ _b  → Λ matrix elements in the large recoil limit of light baryons, while hadronic matrix elements of Λ _b  → Σ transition vanish in the large energy limit of Σ baryon due to the space-time parity symmetry. The scaling law of the soft form factor η(P ^′ ·v), P ^′ and v being the momentum of nucleon and the velocity of Λ _b baryon, responsible for Λ _b  → p transitions is also derived using the nucleon distribution amplitudes in leading conformal spin. In particular, we verify that this scaling behavior is in full agreement with that from light-cone sum rule approach in the heavy-quark limit. With these form factors, we further investigate the Λ baryon polarization asymmetry α in Λ _b  → Λγ and the forward-backward asymmetry A _{F B} in Λ _b  → Λl ⁺ l ⁻. Both two observables (α and A _{F B} ) are independent of hadronic form factors in leading power of 1/m _b and in leading order of α_s. We also extend the analysis of hadronic matrix elements for Ω _b  → Ω transitions to rare Ω _b  → Ω γ and Ω _b  → Ω l ⁺ l ⁻ decays and find that radiative Ω _b  → Ω γ decay is probably the most promising FCNC b → s radiative baryonic decay channel. In addition, it is interesting to notice that the zero-point of forward-backward asymmetry of Ω _b  → Ω l ⁺ l ⁻ is the same as the one for Λ _b  → Λl ⁺ l ⁻ to leading order accuracy provided that the form factors ${\overline \zeta_{\text{i}}}\left( {i = { 3},{4},{5 }} \right)$ are numerically as small as indicated from the quark model.