Hydrodynamic transport coefficients in relativistic scalar field theory

Hydrodynamic transport coefficients may be evaluated from first principals in a weakly coupled scalar field theory at an arbitrary temperature. In a theory with cubic and quartic interactions, the infinite class of diagrams which contributes to the leading weak coupling behavior is identified and summed. The resulting expression may be reduced to a single linear integral equation, which is shown to be identical to the corresponding result obtained from a linearized Boltzmann equation describing effective thermal excitations with temperature-dependent masses and scattering amplitudes. The effective Boltzmann equation is valid even at very high temperature where the thermal lifetime and mean free path are short compared to the Compton wavelength of the fundamental particles. Numerical results for the shear and the bulk viscosities are presented.