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The pseudoscalar quarkonia exclusive decays to light mesons still poses achallenge to the theoretical understanding of quarkonium properties in decay.In this work, we evaluate the processes of pseudoscalar heavy quarkonium decaysinto vector meson pairs, especially the helicity suppressed processes of$\eta_b\rightarrow J/\psi J/\psi$ and $\eta_c\rightarrow VV$. In the frame ofNRQCD, the branching fraction of $Br[\eta_b\rightarrow J/\psi J/\psi]$ areevaluated at the next-to-leading order of perturbative QCD; and within thelight-cone distribution formalism, we calculate also the higher twist effectsin these processes. Numerical results show that the higher twist termscontribute more than what from the NLO QCD corrections in the process of$\eta_b\rightarrow J/\psi J/\psi$. It is found that the experimental results on$\eta_c\rightarrow VV$ are hard to be understood by merely the quark model andperturbative QCD calculation.

Pseudoscalar Quarkonium Exclusive Decays to Vector Meson Pair