Improved bounds for the slope and curvature of form factors

We derive a theoretically allwed domain for the slope and curvature of the physical form factor appearing in the decay . Using heavy-quark symmetry, we relate this function to a particular form factor free of ground-state B_c poles below the threshold for BD production, for which almost model-independent constraints are derived from QCD using unitarity and analyticity. Our results are of interest for the extraction of |V_cb| from the recoil spectrum in exclusive semileptonic B decays. Making conservative estimates of the theoretical uncertainties, we find (up to 1/m_Q corrections) and . We also derive the corresponding bounds for the form factor in the decay .