Derivation of anisotropic dissipative fluid dynamics from the Boltzmann equation

TL;DR

This paper develops a new anisotropic dissipative fluid dynamics framework derived from the Boltzmann equation, suitable for highly anisotropic systems like early-stage relativistic heavy-ion collisions.

Contribution

It introduces a novel expansion around an anisotropic distribution function, extending traditional fluid dynamics to highly anisotropic conditions.

Findings

Derived equations of motion for anisotropic dissipative fluids.

Implemented a 14-moment approximation for practical equations.

Applicable to early-stage heavy-ion collision modeling.

Abstract

Fluid-dynamical equations of motion can be derived from the Boltzmann equation in terms of an expansion around a single-particle distribution function which is in local thermodynamical equilibrium, i.e., isotropic in momentum space in the rest frame of a fluid element. However, in situations where the single-particle distribution function is highly anisotropic in momentum space, such as the initial stage of heavy-ion collisions at relativistic energies, such an expansion is bound to break down. Nevertheless, one can still derive a fluid-dynamical theory, called anisotropic dissipative fluid dynamics, in terms of an expansion around a single-particle distribution function, $\hat{f}_{0\bf k}$, which incorporates (at least parts of) the momentum anisotropy via a suitable parametrization. We construct such an expansion in terms of polynomials in energy and momentum in the direction of the anisotropy and of irreducible tensors in the two-dimensional momentum subspace orthogonal to both the fluid velocity and the direction of the anisotropy. From the Boltzmann equation we then derive the set of equations of motion for the irreducible moments of the deviation of the single-particle distribution function from $\hat{f}_{0\bf k}$. Truncating this set via the 14-moment approximation, we obtain the equations of motion of anisotropic dissipative fluid dynamics.