Spectral function at high temperature ∗

For the calculation of static quantities, such as free energies, phase diagrams and screening masses, the euclidean lattice formulation of finitetemperature quantum fields offers a first-principle approach. For intrinsically real-time correlators, such as the spectral function, no first-principle nonperturbative formulation exists. In the last O(10) years the classical approximation at high temperature and weak coupling has provided much insight into real-time dynamics, see [1,2] for recent reviews. In this talk I discuss how spectral functions can be computed in the classical approximation and what can be learned from them. Details can be found in [3]. 2. The spectral function for a bosonic operator O can be defined as the thermal expectation value