  We report the electronic structure of YbB6, a recently predicted moderately
correlated topological insulator, measured by angle-resolved photoemission
spectroscopy. We directly observed linearly dispersive bands around the
time-reversal invariant momenta {\Gamma} and X with negligible kz dependence,
consistent with odd number of surface states crossing the Fermi level in a Z2
topological insulator. Circular dichroism photoemission spectra suggest that
these in-gap states possess chirality of orbital angular momentum, which is
related to the chiral spin texture, further indicative of their topological
nature. The observed insulating gap of YbB6 is about 100 meV, larger than that
reported by theoretical calculations. Our results present strong evidence that
YbB6 is a correlated topological insulator and provide a foundation for further
studies of this promising material.
