B → ρ helicity form factors within the QCD light-cone sum rules

We study the B -> rho helicity form factors (HFFs) by applying the light-cone sum rules (LCSR) up to twist-4 accuracy. The HFF has some advantages in comparison to the conventionally calculated transition form factors; e.g., the HFF parametrization can be achieved via diagonalizable unitarity relations, etc. At the large recoil point, only the rho-meson longitudinal component contributes to the HFFs, and we have H-rho,H-0(0) = 0.435(-0.045)(+0.055) and H-rho,H-{1,H-2}(0) = 0. We extrapolate the HFFs to the physically allowable q(2) region and apply them to the B -> rho semileptonic decay. We observe that the rho-meson longitudinal component dominates its differential decay width in the low-q(2) region, and its transverse component dominates the high-q(2) region. Two ratios, R-low and R-high, are used to characterize those properties, and our LCSR calculation gives R-low = 0.967(-0.285)(+0.308) and R-high = 0.219(-0.070)(+0.058), which agree with the BABAR measurements within errors.