Form factors of exclusive $b\to u$ transitions

Abstract

We present the form factors of the $B\to\pi,\rho$ transitions induced by the $b\to u$ quark currents at all kinematically accessible $q^2$. Our analysis is based on the spectral representations of the form factors within the constituent quark picture: we fix the soft meson wave functions and the constituent quark masses by fitting $A_1(q^2)$ and $T_2(q^2)$ to the lattice results at small recoils ($17\lesssim q^2\lesssim 20 GeV^2$). We then calculate the $B\to\pi,\rho$ transition form factors down to $q^2=0$. For the $B\to\pi$ case the region $q^2\lesssim 20 GeV^2$ however does not cover the whole kinematically accessible range. Due to the smallness of the pion mass the region of small recoils is close to the nearby $B^*(5234)$ resonance. We develop a parametrization which includes the $B^*$ dominance of the form factors $f_+$ and $f_-$ at small recoils and numerically reproduces the results of calculations at $q^2\lesssim 20 GeV^2$. We find $\Gamma(B\to\pi\ell\nu)=8.0^{+0.8}_{-0.2} |V_{ub}|^2 $ps$^{-1}$ and $\Gamma(B\to\rho\ell\nu)=15.8\pm 2.3 |V_{ub}|^2 $ps$^{-1}$.