A consistent nonperturbative approach to thermal damping-rates
  We propose a nonperturbative scheme for the calculation of thermal damping-rates using exact renormalization group (RG)-equations. Special emphasis is put on the thermal RG where first results for the rate were given in M. Pietroni, Phys. Rev. Lett. 81 (1998), 2424. We point out that in order to obtain a complete result that also reproduces the known perturbative behaviour one has to take into account effects that were neglected in the work cited above. We propose a well-defined way of doing the calculations that reproduces perturbation theory in lowest order but goes considerably beyond perturbative results and should be applicable also at second order phase-transitions. 
Bastian Bergerhoff and Juergen Reingruber