  Universal relations that hold for any state provide powerful constraints on
systems consisting of fermions with two spin states interacting with a large
scattering length. In radio-frequency (rf) spectroscopy, the mean shift in the
rf frequency and the large frequency tail of the rf transition rate are
proportional to the contact, which measures the density of pairs with small
separations. We show that these universal relations can be derived and extended
by using the short-time operator product expansion of quantum field theory.
This is a general method for identifying aspects of many-body physics that are
controlled by few-body physics.
