Analysis of the Lambda_b -> Lambda_c + l nu_l decay within a light-front   constituent quark model
  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. 
F. Cardarelli and S. Simula