  We determine the effective field theory in (2+1)-dimensional space and time
that captures the long wave length and low-energy limit of fermions hopping on
a honeycomb lattice at half-filling when both a dominant intrinsic and
subdominant Rashba spin-orbit couplings are present. This effective field
theory for a Z_2 topological insulator (the Kane-Mele model at vanishing
uniform and staggered chemical potentials) is a perturbation around a double
Chern-Simons theory, with the U(1) gauge invariance associated to spin
conservation explicitly broken due to the Rashba spin orbit coupling.
Nonetheless, we find that the effective field theory has a BRST symmetry that
allows us to construct the bulk-edge correspondence.
