Heavy-to-light form factors: Sum rules on the light cone and beyond

We report the first systematic analysis of the off-light-cone effects in sum rules for heavy-to-light form factors. These effects are investigated in a model based on scalar constituents, which allows a technically rather simple analysis but has the essential features of the analogous QCD calculation. The correlator relevant for the extraction of the heavy-to-light form factor is calculated in two different ways: first, by adopting the full Bethe-Salpeter amplitude of the light meson and, second, by performing the expansion of this amplitude near the light cone x2=0. We demonstrate that the contributions to the correlator from the light-cone term x2=0 and the off-light-cone terms x2≠0 have the same order in the 1/mQ expansion. The light-cone correlator, corresponding to x2=0, is shown to systematically overestimate the full correlator, the difference being ∼ΛQCD/δ, with δ the continuum subtraction parameter of order 1 GeV. Numerically, this difference is found to be 10 divide 20%