Theory of Radio Frequency Spectroscopy of Polarized Fermi Gases

We present two exact results for singular features in the radio frequency intensity $I(\omega)$ for ultracold Fermi gases. First, in the absence of final state interactions, $I(\omega)$ has a universal high frequency tail $ C\omega^{-3/2}$ for \emph{all} many-body states, where $C$ is Tan's contact. Second, in a \emph{normal} Fermi liquid at T=0, $I(\omega)$ has a jump discontinuity of $Z/(1 - m/m^{*})$, where $Z$ is the quasiparticle weight and $m^*/m$ the mass renormalization. We then describe various approximations for $I(\omega)$ in polarized normal gases. We show why an approximation that is exact in the $n_\dn=0$ limit, fails qualitatively for $n_{\dn} > 0$: there is no universal tail and sum rules are violated. The simple ladder approximation is qualitatively correct for very small $n_{\dn}$, but not quantitatively.