Rare decays of B-> J/\psi D^(*) and B->\eta_c D^(*)in pQCD Approach

Motivated by the recent measurement of the upper limit of ${B}^0 \to J/\psi D$branchingratio,whichisimportantinaccountingforthesoft$J/\psi$productioninBdecays,weinvestigate${B}^0 \to J/\psi D^{(\star)}$and$\eta_c D^{(\star)}$decaysinperturbativeQCDapproachbasedon$k_T$ factorization. Being pure annihilation (W-exchange) decays, these branching ratios are estimated to be at the order of ${10}^{-5}\sim {10}^{-7}$, which are just at the corner of being observable at the $B$ factories. The measurements of these decay channels may help us to understand the QCD dynamics in the corresponding energy scale, especially the reliability of pQCD approach to these processes.