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 are numerically as small as indicated from the quark model.