Nonperturbative calculation of the shear viscosity in hot phi**4 theory in real time

Abstract

Starting from the Kubo formula we calculate the shear viscosity in hot phi**4 theory nonperturbatively by resumming ladders with a real-time version of the Bethe-Salpeter equation at finite temperature. In the weak coupling limit, the generalized Fluctuation-Dissipation Theorem is shown to decouple the Bethe-Salpeter equations for the different real-time components of the 4-point function. The resulting scalar integral equation is identical with the one obtained by Jeon using diagrammatic ``cutting rules'' in the Imaginary Time Formalism.

4 pages LateX. The 2 postscript figures are now incorporated into the text, facilitating download. No other changes. Accepted by Physics Letters B