  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 Lepton
Flavor Universality, we present a calculation of form factors in the quark
model. Our scheme combines a spectroscopic model, providing the internal wave
functions, and the Bakamjian-Thomas relativistic formalism to deduce the wave
functions in motion and current matrix elements, that amount in the heavy quark
limit to the Isgur-Wise (IW) function. For baryons we meet difficulties using
standard spectroscopic models, leading us to propose a simple phenomenological
model : a Q-pointlike-diquark model, non-relativistic with harmonic oscillator
forces, with a reasonable low-lying spectrum and good slope of the IW function.
We extract this slope from Lattice data and find $\rho_\Lambda^2 \sim 2$. We
are not able to reproduce the right $\rho_\Lambda^2$ when using standard linear
+ Coulomb potential models, both with three quarks $Qqq$ or in a
Q-pointlike-diquark picture. These difficulties seem to derive from the high
sensitivity 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 the
inelastic 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 at
LHCb. Some observables in the $\tau$ case present zeroes for specific values of
$q^2$ that could be tests of the Standard Model.
