Improved QCD Form Factor Constraints and Lambda_b --> Lambda_c l nu

Abstract

We construct model-independent parametrizations of the individual QCD form factors relevant to Lambda_b --> Lambda_c l nu decays. These results follow from dispersion relations and analyticity, and incorporate an improvement in the technique that reduces the number of necessary parameters. To describe most form factors with 5%--10% accuracy over the entire kinematic range, three parameters are necessary, one of which is its normalization at zero recoil. We also apply the improvement to meson decays, and find, using the heavy quark form factor normalization, that almost every B --> D l nu and B --> D^* l nu form factor is well-described by a single-variable parametrization. B --> pi l nu requires a total of only 3 to 5 parameters, depending on the desired accuracy.