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 by 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 by Pietroni. 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. (C) 2000 Elsevier Science B.V. All rights reserved.