Improved QCD form factor constraints and Λ_b → Λ_cℓν̄

We construct model-independent parametrizations of the individual QCD form factors relevant to Λ_b → Λ_cℓν̄ 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 B̄ → Dℓν̄ and B̄ → D* ℓν̄ form factor is well described by a single-variable parametrization. B̄ → πℓν requires a total of only three to five parameters, depending on the desired accuracy.