Asymptotics of Heavy-Meson Form Factors

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Using methods developed for hard exclusive QCD processes, we calculate the asymptotic behaviour of heavy-meson form factors at large recoil. It is determined by the leading- and subleading-twist meson wave functions. For 1≪ |v· v'|≪ mQ/Λ, the form factors are dominated by the Isgur--Wise function, which is determined by the interference between the wave functions of leading and subleading twist. At |v· v'|≫ mQ/Λ, they are dominated by two functions arising at order 1/mQ in the heavy-quark expansion, which are determined by the leading-twist wave function alone. The sum of these contributions describes the form factors in the whole region |v· v'|≫ 1. As a consequence, there is an exact zero in the form factor for the scattering of longitudinally polarized B* mesons at some value v· v' mb/Λ, and an approximate zero in the form factor of B mesons in the timelike region (v· v' -mb/Λ). We obtain the evolution equations and sum rules for the wave functions of leading and subleading twist as well as for their moments. We briefly discuss applications to heavy-meson pair production in e+ e- collisions.