Beyond linear response spectroscopy of ultracold Fermi gases
  We study RF-spectroscopy of ultracold Fermi gas by going beyond the linear response in the field-matter interaction. Higher order perturbation theory allows virtual processes and energy conservation beyond the single particle level. We formulate an effective higher order theory which agrees quantitatively with experiments on the pairing gap, and is consistent with the absence of the mean-field shift in the spin-flip experiment. 
J. Kinnunen and P. Torma