Predictions and Tests for Semi-leptonic Decays of Beauty Hadrons,

We compare the features of the semi-leptonic decays B→ D(*) l ν̄ₗ and Λb→ Λc l ν̄ₗ, with l = e, μ and τ. Firstly, we study such processes according to the -s scheme. Then, we write the angular distributions of those decays in the helicity formalism; we focus especially on the case of l = τ and suggest for each decay some tests for distinguishing among the various Dirac operators which may explain the tension with the standard model. In particular, as regards the Λb semi-leptonic decay, we single out five observables, which assume different values for each new-physics interaction; one such observable is related to the longitudinal polarization of the τ-ν̄_τ system, which can be compared with the D^* longitudinal polarization in the B→ D^* l ν̄ₗ decay. Moreover, we perform a phenomenological analysis of the decays, particularly of Λb, starting from an assumption which agrees with previous theoretical calculations and has a predictive power about the total widths of several decays; we give also a theoretical explanation to some results that were found in preceding analyses about the above mentioned B-decays.