Analysis of the exclusive semileptonic decay Lambda_b -> Lambda_c + l nu_l within a light-front constituent quark model

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.