  We investigate fermion pairing in the unitary regime for a mass ratio
corresponding to a 6Li-40K mixture using Quantum Monte Carlo methods. The
ground-state energy and the average light and heavy particle excitation
spectrum for the unpolarized superfluid state are nearly independent of the
mass ratio. In the majority light system, the polarized superfluid is close to
the energy of a phase separated mixture of nearly fully polarized normal and
unpolarized superfluid. For a majority of heavy particles, we find an energy
minimum for a normal state with a ratio of ~ 3:1 heavy to light particles. A
slight increase in attraction to kF*a ~ 2.5 yields a ground state energy of
nearly zero for this ratio. A cold unpolarized system in a harmonic trap at
unitarity should phase separate into three regions, with a shell of unpolarized
superfluid in the middle.
