  The rare baryonic decays $\Lambda_b \to \Lambda \mu^+ \mu^-$ and $\Lambda_b
\to \Lambda \gamma$ can complement rare $B$ meson decays in constraining models
of new physics. In this work, we calculate the relevant $\Lambda_b \to \Lambda$
transition form factors at leading order in heavy-quark effective theory using
lattice QCD. Our analysis is based on RBC/UKQCD gauge field ensembles with 2+1
flavors of domain-wall fermions, and with lattice spacings of $a\approx 0.11$
fm and $a\approx 0.08$ fm. We compute appropriate ratios of three-point and
two-point correlation functions for a wide range of source-sink separations,
and extrapolate to infinite separation in order to eliminate excited-state
contamination. We then extrapolate the form factors to the continuum limit and
to the physical values of the light-quark masses.
