 
Summary: 18 October 2001
Physics Letters B 518 (2001) 315322
www.elsevier.com/locate/npe
Spectral function at high temperature in the
classical approximation
Gert Aarts
Institut für theoretische Physik, Universität Heidelberg, Philosophenweg 16, 69120 Heidelberg, Germany
Received 16 August 2001; received in revised form 29 August 2001; accepted 31 August 2001
Editor: P.V. Landshoff
Abstract
At high temperature the infrared modes of a weakly coupled quantum field theory can be treated nonperturbatively in real
time using the classical field approximation. We use this to introduce a nonperturbative approach to the calculation of finite
temperature spectral functions, employing the classical KMS condition in real time. The method is illustrated for the oneparticle
spectral function in a scalar field theory in 2 + 1 dimensions. The result is compared with resummed twoloop perturbation
theory and both the plasmon mass and width are found to agree with the analytical prediction. 2001 Published by Elsevier
Science B.V.
PACS: 11.10.Wx; 11.15.Kc
1. Introduction
Finitetemperature field theory has received consid
erable attention during recent years (see [1] for a com
