Simple fluid with broken time reversal invariance

We characterize a system of hard spheres with a simple collision rule that breaks time reversal symmetry, but conserves energy. The collisions lead to an a-chiral, isotropic, and homogeneous stationary state, whose properties are determined in simulations and compared to an approximate theory originally developed for elastic hard spheres. In the nonequilibrium fluid state, velocities are correlated, a phenomenon known from other nonequilibrium stationary states. The correlations are long-ranged decaying like $1\mathrm{/}{r}^d$ in $d$ dimensions. Such correlations are expected on general grounds far from equilibrium and had previously been observed in driven or non-stationary systems.