2000/09/30 by Patrizia Bucci, Massimo Pietroni · 1 citation
Physics and Astronomy · #Dark Matter and Cosmic Phenomena #High-Energy Particle Collisions Research #Quantum Mechanics and Applications #astro-ph #hep-ph #hep-th
paper · pdf · doi:10.1103/physrevd.63.045026
published as Phys.Rev. D63 (2001) 045026 · 7 pages, 5 figures. New section added, with the discussion of the case of an unstable heavy particle. Version to appear on Phys. Rev. D
arxiv created 2000/11/23 · openalex publication_date 2001/01/30 · arxiv updated 2009/11/30 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/28
Matsumoto and Yoshimura have recently argued that the number density of heavy particles in a thermal bath is not necessarily Boltzmann suppressed for T\ensuremath≪M, as power-law corrections may emerge at higher orders in perturbation theory. This fact might have important implications on the determination of weakly interacting massive particle relic densities. On the other hand, the definition of number densities in an 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 annihilation model of Matsumoto and Yoshimura we find that it gives number densities which are Boltzmann suppressed at any order in perturbation theory. We also discuss heavy particles which are unstable already at T=0, showing that power-law corrections do in general emerge in this case.