  In a recent paper we studied the effect of new-physics operators with
different Lorentz structures on the semileptonic $\Lambda_b \to \Lambda_c \tau
\bar{\nu}_{\tau}$ decay. This decay is of interest in light of the
$R({D^{(*)}})$ puzzle in the semileptonic $\bar{B} \to D^{(*)} \tau
{\bar\nu}_\tau$ decays. In this work we add tensor operators to extend our
previous results and consider both model-independent new physics (NP) and
specific classes of models proposed to address the $R({D^{(*)}})$ puzzle. We
show that a measurement of $R(\Lambda_c) = {\cal B}[\Lambda_b \to \Lambda_c
\tau \bar{\nu}_{\tau}] / {\cal B}[\Lambda_b \to \Lambda_c \ell
\bar{\nu}_{\ell}]$ can strongly constrain the NP parameters of models discussed
for the $R({D^{(*)}})$ puzzle. We use form factors from lattice QCD to
calculate all $\Lambda_b \to \Lambda_c \tau \bar{\nu}_{\tau}$ observables. The
$\Lambda_b \to \Lambda_c$ tensor form factors had not previously been
determined in lattice QCD, and we present new lattice results for these form
factors here.
