Covariant Light-Front Approach for $B_c$ transition form factors

In the covariant light-front quark model, we investigate the form factors of $B_c$ decays into $D,D^{\ast },D_s,D_s^{\ast },{\eta }_c,J\mathrm{/}\psi ,B,B^{\ast },B_s,B_s^{\ast }$ mesons. The form factors in the spacelike region $q^2<0$ are directly evaluated. To extrapolate the form factors to the full kinematic region, we fit the form factors by adopting a three-parameter form from the spacelike region. $b\to u,d,s$ transition form factors at maximally recoiling point ($q^2=0$) are smaller than $b\to c$ and $c\to d,s$ transition form factors, while the $b\to d,s,c$ form factors at zero recoiling point are close to each other. In the fitting procedure, we find the parameters for the form factors $A_2(B_c\to B^)$and$A_2(B_c\to B^_s)$stronglydependonthedecayconstantsof$B^$and$B_s^$mesons\mathrm{.}Fortunately,thesemileptonicandnonleptonic$B_c$ decays are not sensitive to these form factors. With the form factors, we also investigate the branching fractions, polarizations of the semileptonic $B_c\to Ml\nu$decays\mathrm{.}Semileptonic$B_c\to (\eta_c,J/\psi)l\nu$and$B_c\to (B_s,B_s^)l\nu$decayshavemuchlargerbranchingfractionsthan$B_c\to (D,D^,B,B^*)l\nu$\mathrm{.}Inthethreekindsof$B_c\to Vl\nu$ decays, contributions from the longitudinal polarized vector is comparable with those from the transversely polarized vector. These predictions will be tested at the ongoing and forthcoming hadron colliders.