  We investigate the light-front zero-mode contribution to the weak transition
form factors between pseudoscalar and vector mesons using a covariant fermion
field theory model in $(3+1)$ dimensions. In particular, we discuss the form
factors $a_-(q^2)$ and $f(q^2)$ which have been suspected to have the zero-mode
contribution in the $q^+=0$ frame. While the zero-mode contribution in
principle depends on the form of the vector meson vertex $\Gamma^\mu=\gamma^\mu
- (2k-P_V)^\mu/D$, the form factor $f(q^2)$ is found to be free from the zero
mode if the denominator $D$ contains the term proportional to the light-front
longitudinal momentum fraction factor $(1/x)^n$ of the struck quark with the
power $n>0$. Although the form factor $a_-(q^2)$ is not free from the zero
mode, the zero-mode contribution comes only either from the simple vertex
$\Gamma^\mu=\gamma^\mu$ term or from the other term just with a constant $D$
(i.e. $n=0$), but not with the momentum-dependent denominator (i.e. $D\sim
(1/x)^n$ with $n>0$). We identify the zero-mode contribution to $a_-(q^2)$ and
incorporate it as a convolution of the zero-mode operator with the initial and
final state light-front wave functions. The covariance (i.e. frame
independence) of our model has been checked by performing the light-front
calculations both in the $q^+=0$ and $q^+\neq 0$ frames. We present our
numerical result for the $B\to\rho$ transition for an explicit demonstration of
our findings.
