Reproductive Freeze-In of Self-Interacting Dark Matter
  We present a mechanism for dark matter (DM) production involving a
self-interacting sector that at early times is ultra-relativistic but
far-underpopulated relative to thermal equilibrium (such initial conditions
often arise, e.g., from inflaton decay). Although elastic scatterings can
establish kinetic equilibrium we show that for a broad variety of
self-interactions full equilibrium is never established despite the DM yield
significantly evolving due to $2\to k$ ($k>2$) processes (the DM carries no
conserved quantum number nor asymmetry). During the active phase of the
process, the DM to Standard Model temperature ratio falls rapidly, with DM
kinetic energy being converted to DM mass, the inverse of the
recently-discussed `cannibal DM mechanism'. Potential observables and
applications include self-interacting DM signatures in galaxies and clusters,
dark acoustic oscillations, the alteration of free-streaming constraints, and
possible easing of $\sigma_8$ and Hubble tensions.
