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

TL;DR

This paper analyzes zero-mode contributions to weak transition form factors in pseudoscalar to vector meson decays using a covariant light-front model, identifying conditions under which zero modes affect specific form factors and confirming frame independence.

Contribution

It clarifies the conditions under which zero-mode contributions appear in light-front calculations of transition form factors, especially for the form factor $f(q^2)$ and $a_-(q^2)$, and demonstrates frame independence.

Findings

Zero-mode contribution to $f(q^2)$ is absent if the denominator $D$ includes a term proportional to $(1/x)^n$ with $n>0$.

Zero-mode contribution to $a_-(q^2)$ arises only from specific vertex terms or constant denominators, not from momentum-dependent ones.

Frame independence of the model is confirmed by calculations in both $q^+=0$ and $q^+ eq 0$ frames.

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