Power-suppressed thermal effects from heavy particles - art. no. 096009

In quantum field theory, heavy particles with mass M much greater than the temperature T give not only effects suppressed by the Boltzmann factor c(-M /T), but also effects suppressed by powers of T/M. We show that power-suppr essed terms in equilibrium observables arise from effective interactions am ong light particles due to virtual heavy particles. We study a model introd uced by Matsumoto and Yoshimura in which heavy bosons interact only through a term that allows pair annihilation into light particles. We construct an effective Lagrangian fur the light held by integrating out the heavy field , and use it to calculate the leading power-suppressed terms in the energy density. The thermal average of the Hamiltonian density for the heavy field includes a term proportional to T-6/M-2, but we show that this term can he eliminated by a field redefinition and therefore cannot have any physical significance.