  Using $(2+1)$-flavor lattice QCD, we compute the 10 form factors describing
the $\Lambda_b \to \Lambda$ matrix elements of the $b \to s$ vector, axial
vector, and tensor currents. The calculation is based on gauge field ensembles
generated by the RBC and UKQCD Collaborations with a domain-wall action for the
$u$, $d$, and $s$ quarks and the Iwasaki gauge action. The $b$ quark is
implemented using an anisotropic clover action, tuned nonperturbatively to the
physical point, and the currents are renormalized with a mostly nonperturbative
method. We perform simultaneous chiral, continuum, and kinematic extrapolations
of the form factors through modified $z$ expansions. Using our form factor
results, we obtain precise predictions for the $\Lambda_b \to \Lambda(\to p^+
\pi^-) \mu^+ \mu^-$ differential branching fraction and angular observables in
the Standard Model.
