Physics – High Energy Physics – High Energy Physics - Phenomenology
Scientific paper
2000-09-06
Phys.Rev. D63 (2001) 045026
Physics
High Energy Physics
High Energy Physics - Phenomenology
7 pages, 5 figures. New section added, with the discussion of the case of an unstable heavy particle. Version to appear on Phy
Scientific paper
10.1103/PhysRevD.63.045026
Matsumoto and Yoshimura have recently argued that the number density of heavy particles in a thermal bath is not necessarily Boltzmann-suppressed for T << M, as power law corrections may emerge at higher orders in perturbation theory. This fact might have important implications on the determination of WIMP relic densities. On the other hand, the definition of number densities in a interacting theory is not a straightforward procedure. It usually requires renormalization of composite operators and operator mixing, which obscure the physical interpretation of the computed thermal average. We propose a new definition for the thermal average of a composite operator, which does not require any new renormalization counterterm and is thus free from such ambiguities. Applying this definition to the model of Matsumoto and Yoshimura we find that it gives number densities which are Boltzmann-suppressed at any order in perturbation theory. We discuss also heavy particles which are unstable already at T=0, showing that power law corrections do in general emerge in this case.
Bucci Patrizia
Pietroni Massimo
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