Chapman-Enskog expansion for chirally colliding hard disks

We study a two-dimensional fluid of hard disks undergoing chiral, energy- and momentum-conserving collisions. Despite the microscopic breaking of time-reversal symmetry, the system obeys the H-theorem and relaxes to equilibrium in the absence of external forces. In the dilute limit, a Chapman–Enskog expansion yields analytical expressions for the even and odd shear viscosities and the even and odd thermal conductivity. Theoretical predictions are confirmed by nonequilibrium molecular dynamics simulations.