  We present an investigation of the Isgur-Wise form factor relevant for the
semileptonic decay Lambda_b -> \Lambda_c + l nu_l performed within a
light-front constituent quark model. Adopting different baryon wave functions
it is found that the Isgur-Wise form factor depends sensitively on the baryon
structure. It is shown however that the shape of the Isgur-Wise function in the
full recoil range relevant for the Lambda_b -> \Lambda_c + l nu_l decay can be
effectively constrained using recent lattice QCD results at low recoil. Then,
the Lambda_b -> \Lambda_c + l nu_l decay is investigated including both
radiative effects and first-order power corrections in the inverse heavy-quark
mass. 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 +/- 1.6) % |V_bc /
0.040|**2 ~ tau(Lambda_b) / (1.24 ps), a_L = -0.945 +/- 0.014, a_T = -0.62 +/-
0.09 and R_L/T = 1.57 +/- 0.15, respectively. Moreover, both the longitudinal
asymmetry and the (partially integrated) longitudinal to transverse decay ratio
are found to be only marginally affected by the model dependence of the
Isgur-Wise form factor as well as by first-order power corrections; therefore,
their experimental determination might be a very interesting tool for testing
the SM and for investigating possible New Physics.
