On the Analytic Structure of the Self-Energy for Massive Gauge Bosons at   Finite Temperature
  We show that the one-loop self-energy at finite temperature has a unique limit as the external momentum $p_\mu\rightarrow 0$ {\it if} the loop involves propagators with distinct masses. This naturally arises in theories involving particles with different masses as is demonstrated for a toy model of two scalars as well as in a $U(1)$ Higgs theory. We show that, in spontaneously broken gauge theories, this observation nonetheless does not affect the difference between the Debye and plasmon masses, which are often thought of as the $(p_0=0, \vec{p}\to 0)$ and $(p_0\to 0,\vec{p}=0)$ limits of the self-energy. 
P. Arnold, P. Bedaque, A. Das, and S. Vokos