Thermal (or "KMS") states as well as ground states are characterized by analyticity properties in the (complexified) time variable. Such a characterization is applied to the quantum field theoretical systems on Minkowski, de Sitter and anti-de Sitter spacetimes. Privileged theories (or "vacua") can be defined on the basis of general principles which ensure "maximal" analyticity properties of the correlation functions. In such theories, there exists an observer-dependent thermal interpretation of the "vacuum" which is due to the (complex) geometry. In Minkowski spacetime, the (non-privileged) thermal quantum field theories at arbitrary temperature are investigated for their particle aspect at asymptotic times. This aspect is encoded in the corresponding two-point functions through a certain "damping factor", which is shown to depend on the dynamics of the interacting fields and suggests a possible substitute to the usual pole-particle concept in the thermal case.
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