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We investigate the light-front zero-mode contribution to the weak transitionform factors between pseudoscalar and vector mesons using a covariant fermionfield theory model in $(3+1)$ dimensions. In particular, we discuss the formfactors $a_-(q^2)$ and $f(q^2)$ which have been suspected to have the zero-modecontribution in the $q^+=0$ frame. While the zero-mode contribution inprinciple 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 zeromode if the denominator $D$ contains the term proportional to the light-frontlongitudinal momentum fraction factor $(1/x)^n$ of the struck quark with thepower $n>0$. Although the form factor $a_-(q^2)$ is not free from the zeromode, 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 $a_-(q^2)$ andincorporate it as a convolution of the zero-mode operator with the initial andfinal state light-front wave functions. The covariance (i.e. frameindependence) of our model has been checked by performing the light-frontcalculations both in the $q^+=0$ and $q^+\neq 0$ frames. We present ournumerical result for the $B\to\rho$ transition for an explicit demonstration ofour findings.

Light-front dynamic analysis of transition form factors in the process of $P\to V\ell\nu_{\ell}$