  The exclusive semileptonic decay Lambda_b -> Lambda_c + l nu_l is
investigated including both radiative and first-order power corrections in the
inverse heavy-quark mass, while the Isgur-Wise function is calculated within a
light-front constituent quark model. It turns out that the dependence on the
model parameters can be effectively constrained using recent lattice QCD
results at low values of the recoil. Our final predictions for the exclusive
semileptonic branching ratio, the longitudinal and transverse asymmetries, and
the longitudinal to transverse decay ratio are: Br(Lambda_b -> Lambda_c + l
nu_l) = (6.3 \pm 1.6) % |V_{bc} / 0.040|**2 \tau(\Lambda_b) / (1.24 ps), a_L =
-0.945 \pm 0.014, a_T = -0.62 \pm 0.09 and R_L/T = 1.57 \pm 0.15, respectively.
Moreover, the theoretical uncertainties both on a_L and the (partially
integrated) R_L/T are found to be quite small and, therefore, the experimental
determination of these quantities is a very interesting tool for testing the
Standard Model and for investigating possible New Physics. In this respect the
sensitivity to extract unique information both on the strength and phase of
possible hadronic right-handed currents is illustrated.
