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Motivated by the calculation of observables in the decays $\Lambda_b \to\Lambda_c\left({1 \over 2}^\pm \right) \ell \overline{\nu}$, as tests of LeptonFlavor Universality, we present a calculation of form factors in the quarkmodel. Our scheme combines a spectroscopic model, providing the internal wavefunctions, and the Bakamjian-Thomas relativistic formalism to deduce the wavefunctions in motion and current matrix elements, that amount in the heavy quarklimit to the Isgur-Wise (IW) function. For baryons we meet difficulties usingstandard spectroscopic models, leading us to propose a simple phenomenologicalmodel : a Q-pointlike-diquark model, non-relativistic with harmonic oscillatorforces, with a reasonable low-lying spectrum and good slope of the IW function.We extract this slope from Lattice data and find $\rho_\Lambda^2 \sim 2$. Weare not able to reproduce the right $\rho_\Lambda^2$ when using standard linear+ Coulomb potential models, both with three quarks $Qqq$ or in aQ-pointlike-diquark picture. These difficulties seem to derive from the highsensitivity of $\rho_\Lambda^2$ to the structure of the light quark subsystem.After fixing the parameters of our interim model to yield correct spectrum and$\rho_\Lambda^2$, we compute observables. Bjorken sum rule shows that theinelastic IW function is large, and therefore $\Lambda_b \to \Lambda_c \left({1\over 2}^-, {3 \over 2}^- \right) \ell \overline{\nu}$ could be studied atLHCb. Some observables in the $\tau$ case present zeroes for specific values of$q^2$ that could be tests of the Standard Model.

physical review. d/physical review. d.

Heavy baryon wave functions, Bakamjian-Thomas approach to form factors,  and observables in ${\Lambda_b \to \Lambda_c\left({1 \over 2}^\pm \right)  \ell \overline{\nu}}$ transitions