Λ_b \to ΛJ/ψdecay in perturbative QCD

Abstract

We calculate the amplitudes involved in the heavy baryon nonleptonic decay $Λ_b \rar ΛJ/ψ$ using perturbative QCD factorization theorem, which are expressed as convolutions of hard $b$ quark decay amplitudes with the $Λ_b$ baryon, $Λ$ baryon and $J/ψ$ meson distribution amplitudes. It is found that nonfactorizable contributions dominate over factorizable ones. Because of soft cancellation in pairs of nonfactorizable diagrams, the $Λ_b \rar ΛJ/ψ$ decay is characterized by a large scale, such that perturbative QCD is applicable. Employing the distribution amplitudes determined in our previous works and from QCD sum rules, we derive the branching ratio $B(Λ_b \rar ΛJ/ψ)=(1.7 \sim 5.3)\times 10^{-4}$ in agreement with data. We predict an asymmetry parameter $α= -0.17 \sim -0.14$ associated with the anisotropic angular distribution of the $Λ$ baryons produced in polarized $Λ_b$ baryon decays.