Bulk viscosity of two-flavor quark matter from the Kubo formalism

We study the bulk viscosity of quark matter in the strong coupling regime within the two-flavor Nambu-Jona-Lasinio model. The dispersive effects that lead to nonzero bulk viscosity arise from quark-meson fluctuations above the Mott transition temperature, where meson decay into two quarks is kinematically allowed. We adopt the Kubo-Zubarev formalism and compute the equilibrium imaginary-time correlation function for pressure in the O(1/Nc()) power counting scheme. The bulk viscosity of matter is expressed in terms of the Lorentz components of the quark spectral function and includes multiloop contributions which arise via resummation of infinite geometrical series of loop diagrams. We show that the multiloop contributions dominate the single-loop contribution close to the Mott line, whereas at high temperatures the one-loop contribution is dominant. The multiloop bulk viscosity dominates the shear viscosity close to the Mott temperature by factors 5 to 20, but, with increasing temperature, the shear viscosity becomes the dominant dissipation mechanism of stresses as the one-loop contribution becomes the main source of bulk viscosity.