  We investigate the ten independent local form-factors relevant to the
$b$-baryon decay $\Lambda_b \to \Lambda \ell^+\ell^-$, combining information of
lattice QCD and dispersive bounds. We propose a novel parametrization of the
form factors in terms of orthonormal polynomials that diagonalizes the form
factor contributions to the dispersive bounds. This is a generalization of the
unitarity bounds developed for meson-to-meson form-factors. In contrast to
ad-hoc parametrizations of these form factors, our parametrization provides a
degree of control of the form-factor uncertainties at large hadronic recoil.
This is of phenomenological interest for theoretical predictions of, e.g.,
$\Lambda_b\to \Lambda \gamma$ and $\Lambda_b\to\Lambda \ell^+\ell^-$ decay
processes.
