Kinetic theory of one-dimensional inhomogeneous long-range interacting $N$-body systems at order $1\mathrm{/}{N}^2$ without collective effects

Long-range interacting systems irreversibly relax as a result of their finite number of particles, $N$. At order $1\mathrm{/}N$, this process is described by the inhomogeneous Balescu–Lenard equation. Yet, this equation exactly vanishes in one-dimensional inhomogeneous systems with a monotonic frequency profile and sustaining only 1:1 resonances. In the limit where collective effects can be neglected, we derive a closed and explicit $1\mathrm{/}{N}^2$ collision operator for such systems. We detail its properties highlighting in particular how it satisfies an $H$-theorem for Boltzmann entropy. We also compare its predictions with direct $N$-body simulations. Finally, we exhibit a generic class of long-range interaction potentials for which this $1\mathrm{/}{N}^2$ collision operator exactly vanishes.