Representation of spectral functions and thermodynamics

Understand

In this paper we study the question of effective field assignment to measured or nonperturbatively calculated spectral functions.

• The straightforward procedure is to approximate it by a sum of independent Breit-Wigner resonances, and assign an independent field to each of these resonances.
• The problem with this idea is that it introduces new conserved quantities in the free model (the new particle numbers), therefore it changes the symmetry of the system.
• We avoid this inconsistency by representing each quantum channel with a single effective field, no matter how complicated the spectral function is.