What is Initial State of Scattering at Finite Temperature? *

This letter addresses a problem concerning temperature corrections to the scattering processes. Although former studies had given the temperature-dependent phase space factors in the corrections, how to derive them from the first principles was not clear. In this letter, we consider this question within the formalism of thermal field dynamics. We find that if the initial and final states in scattering processes at finite temperature are constructed from the expanded vacuum states, the scattering rate in medium can be derived with the desired temperature-dependent phase space factors.