Spectral function at high temperature in the classical approximation

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 one-particle spectral function in a scalar field theory in 2+1 dimensions. The result is compared with resummed two-loop perturbation theory and both the plasmon mass and width are found to agree with the analytical prediction.