A resummed method of moments for the relativistic hydrodynamic expansion

Abstract: The relativistic method of moments is one of the most successful approaches to extract second order viscous hydrodynamics from a kinetic underlying background. The equations can be systematically improved to higher order, and they have already shown a fast convergence to the kinetic results. In order to generalize that method, we introduced long range effects in the form of effective (medium dependent) masses and gauge (coherent) fields. The most straightforward generalization of the hydrodynamic expansion is problematic, or simply ill-defined, at higher order. Instead of introducing an additional set of approximations, we propose to rewrite the series in terms of moments resumming the contributions of infinite non-hydrodynamics modes. The resulting equations are consistent with hydrodynamics and well defined at all order. We tested the new approximation against the exact solutions of the Boltzmann-Maxwell-Vlasov equations in $(0+1)$-dimensions, finding a fast and stable convergence to the exact results.