s-wave scattering lengths of the strongly dipolar bosons ${}^{162}$Dy and ${}^{164}$Dy

We report the measurement of the triplet s-partial-wave scattering length $a$ of two bosonic isotopes of the highly magnetic element, dysprosium: $a=122(10)a_0$ for ${}^{162}$Dy and $a=92(8)a_0$ for ${}^{164}$Dy, where $a_0$ is the Bohr radius. The scattering lengths are determined by the cross-dimensional relaxation of ultracold gases of these Dy isotopes at temperatures above quantum degeneracy. In this temperature regime, the measured rethermalization dynamics can be compared to simulations of the Boltzmann equation using a direct-simulation Monte Carlo (DSMC) method employing the anisotropic differential scattering cross section of dipolar particles.