Critical dynamics of helium belowTc

The frequency- and wave-number-dependent density correlation function has been calculated below $T_c$ for the symmetric planar-spin model of helium to first order in $4-d$. In the hydrodynamic limit, the ratio of the damping of second sound to its velocity is an order of magnitude smaller than was found by Tyson. The theoretical results seem consistent with the light-scattering measurements however.