Optimal decay rates for linear kinetic equations in the half-space

We prove that solutions to linear kinetic equations in a half-space with absorbing boundary conditions decay for large times like $t^{-\frac{1}{2}-\frac{d}{4}}$ in a weighted $\sfL^{2}$ space and like $t^{-1-\frac{d}{2}}$ in a weighted $\sfL^{\infty}$ space, i.e., faster than in the whole space and in agreement with the decay of solutions to the heat equation in the half-space with Dirichlet conditions.

The class of linear kinetic equations considered includes the linear relaxation equation, the kinetic Fokker-Planck equation and the Kolmogorov equation with spherical velocities associated with the kinetic Brownian motion.