We present a new empirical model for the mass assembly of dark matter halos. We approximate the growth of individual halos as a simple power-law function of time, where the power-law index smoothly decreases as the halo transitions from the fast-accretion regime at early times, to the slow-accretion regime at late times. Using large samples of halo merger trees taken from high-resolution cosmological simulations, we demonstrate that our 3-parameter model can approximate halo growth with a

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... l accuracy of 0.1 dex for t 1 Gyr for all halos of present-day mass M halo 10 11 M , including subhalos and host halos in gravity-only simulations, as well as in the IllustrisTNG hydrodynamical simulation. We additionally present a new model for the assembly of halo populations, which not only reproduces average mass growth across time, but also faithfully captures the diversity with which halos assemble their mass. Our python implementation is based on the autodiff library JAX, and so our model selfconsistently captures the mean and variance of halo mass accretion rate across cosmic time. We show that the connection between halo assembly and the large-scale density field, known as halo assembly bias, is accurately captured by our model, and that residual errors in our approximations to halo assembly history exhibit a negligible residual correlation with the density field. Our publicly available source code can be used to generate Monte Carlo realizations of cosmologically representative halo histories; our differentiable implementation facilitates the incorporation of our model into existing analytical halo model frameworks. Subject headings: Cosmology: large-scale structure of Universe; methods: N-body simulations arXiv:2105.05859v2 [astro-ph.CO] 28 Jul 2021