Zero-Recoil Sum Rules for ${\mathrm{\Lambda }}_b\to {\mathrm{\Lambda }}_c$ Form Factors

We set up a zero recoil sum rule to constrain the form factors of the ${\mathrm{\Lambda }}_b\to {\mathrm{\Lambda }}_c$ transition. Our results are compared with the recent lattice calculation for these transitions. We find the same situation as in the case for $B\to D^{\ast }$: The lattice results practically saturate the sum rules, leaving basically no room for excited states.