  The electronic structure of Mott insulators continues to be a major unsolved
problem in physics despite more than half-century of intense research efforts.
Well-developed momentum-resolved spectroscopies such as photoemission or
neutron scattering cannot probe the full Mott gap. We report observation of
dispersive charge excitations across the Mott gap in a high Tc parent cuprate
Ca2CuO2Cl2 using high resolution resonant inelastic x-ray scattering shedding
light on the anisotropy of Mott-gap. The results provide direct support for the
Hubbard model to describe charge excitations across the Mott gap.
