\Lambda_b \to \Lambda J/\psi decay in perturbative QCD

We calculate the amplitudes involved in the heavy baryon nonleptonic decay ${\mathrm{\Lambda }}_b\text{rar}\mathrm{\Lambda }J\mathrm{/}\psi$ using perturbative QCD factorization theorem, which are expressed as convolutions of hard $b$ quark decay amplitudes with the ${\mathrm{\Lambda }}_b$ baryon, $\mathrm{\Lambda }$ baryon and $J\mathrm{/}\psi$ meson distribution amplitudes. It is found that nonfactorizable contributions dominate over factorizable ones. Because of soft cancellation in pairs of nonfactorizable diagrams, the ${\mathrm{\Lambda }}_b\text{rar}\mathrm{\Lambda }J\mathrm{/}\psi$ 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 $B(\Lambda_b \rar \Lambda J/\psi)=(1.7 \sim 5.3)\times 10^{-4}$inagreementwithdata\mathrm{.}Wepredictanasymmetryparameter$\alpha= -0.17 \sim -0.14$associatedwiththeanisotropicangulardistributionofthe$\Lambda$baryonsproducedinpolarized$\Lambda_b$ baryon decays.