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

Abstract

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 $q^{+}\ne 0$ frames. We present our numerical result for the $B\to \rho$ transition for an explicit demonstration of our findings.

Comments: 23 pages, 2 figures, version to appear in Nuclear Physics A