Transport coefficients in non-quasiparticle systems

Abstract: Transport coefficeints, in particular the shear viscosity to entropy density ratio is studied in systems where the small-width quasiparticle assumption is not valid. It is found that $\eta/s$ has no unversal lower bound, the minimal value depends on the system and the temperature, and can be even zero. We construct models where the $1/4\pi$ conjectured bound is violated.