  Using inelastic neutron scattering, we map a 14 meV coherent resonant mode in
the topological Kondo insulator SmB6 and describe its relation to the low
energy insulating band structure. The resonant intensity is confined to the X
and R high symmetry points, repeating outside the first Brillouin zone and
dispersing less than 2 meV, with a 5d-like magnetic form factor. We present a
slave-boson treatment of the Anderson Hamiltonian with a third neighbor
dominated hybridized band structure. This approach produces a spin exciton
below the charge gap with features that are consistent with the observed
neutron scattering. We find that maxima in the wave vector dependence of the
inelastic neutron scattering indicate band inversion.
