Non-conformal attractor in boost-invariant plasmas

We explore the far-off-equilibrium dynamics of a (0+1)-dimensionally expanding non-conformal system with Bjorken symmetry using kinetic theory and hydrodynamics. It is shown that the breaking of conformal invariance by the introduction of even a small mass (in units of the local temperature) can drastically modify the well-known attractor for the shear Reynolds number previously observed in massless systems. For generic nonzero particle mass, neither shear nor bulk viscous pressure relax quickly to a non-equilibrium attractor; universal hydrodynamic behavior is manifest only at small values of the inverse Reynolds numbers. In kinetic theory, the scaled longitudinal pressure, which is a combination of the scaled shear and bulk viscous pressures, exhibits fast decay to an early-time attractor, driven by the rapid longitudinal expansion of the medium. We demonstrate that second-order dissipative hydrodynamics, based on a gradient expansion around local thermal equilibrium, fails to accurately describe the attractor. These results will be discussed in the light of fixed lines and fixed points of kinetic theory and hydrodynamics. A modified anisotropic hydrodynamic prescription that reproduces the attractor and provides excellent agreement with kinetic theory will be presented.