Dispersive Bounds on The Shape Of Lambda_b to Lambda_c l {\bar \nu_l} Formfactors

We derive a theoretically allowed domain for the charge radius $\rho$ and curvature $c$ of the Isgur-Wise function describing the decay $\Lambda_b \to \Lambda_c l {\bar \nu_l}$. Our method uses crossing symmetry, dispersion relations and analyticity in the context of the Heavy Quark Effective Theory but is independent of the specifics of any given model. The experimentally determined values of the $\Upsilon$ masses have been used as input information. The results are of interest for testing different models employed to calculate the heavy baryon formfactors which are used for the extraction of $| V_{cb} |$ from the experimental data.