Ferromagnetism in repulsive Fermi gases: upper branch of Feshbach resonance versus hard spheres

We use quantum Monte Carlo, including backflow corrections, to investigate a two-component Fermi gas on the upper branch of a Feshbach resonance and contrast it with the hard sphere gas. We find that, in both cases, the Fermi liquid becomes unstable to ferromagnetism at a $k_F a$ smaller than the mean field result, where $k_F$ is the Fermi wavevector and $a$ the scattering length. Even though the total energies $E(k_F a)$ are similar in the two cases, their pair correlations and kinetic energies are completely different, reflecting the underlying potentials. We discuss the extent to which our calculations shed light on recent experiments.