From moments of the distribution function to hydrodynamics: The nonconformal case

We study the one-dimensional boost-invariant Boltzmann equation in relaxation-time approximation using special moments of distribution function for a system with finite particle mass. The infinite hierarchy can be truncated by keeping only three lowest that correspond to independent components energy-momentum tensor. show such three-moment truncation reproduces accurately exact solution kinetic after simple renormalization takes into account effects neglected higher moments. derive second-order Israel-Stewart hydrodynamic equations from equations, and that, most physically relevant initial conditions, these yield results comparable those truncation, albeit less accurate. attribute this feature fact structure is similar truncation. In particular, presence relaxation term yields an early-time regime mimics approximately collisionless regime. A detailed comparison non-conformal hydrodynamics reveals ambiguities definition transport coefficients. These affect ability reproduce theory.