Plasmon in hotφ4theory

We study the two-loop resummed propagator in hot ${\mathit{g}}^2$ ${\mathrm{\phi }}^4$ theory. The propagator has a cut aong the whole real axis in the complex energy plane, but for small g the spectral density is sharply peaked around the plasmon. The dispersion relation and the width of the plasmon are calculated at zero and finite momentum. At large momenta the spectral width vanishes, and the plasmon loses its collectivity and behaves like a noninteracting free particle. © 1996 The American Physical Society.