Non-relativistic susceptibility and a dark matter application

Understand

When thermal rate equations are derived for the evolution of slow variables, it is often practical to parametrize the right-hand side with chemical potentials.

• To close the system, the chemical potentials are subsequently re-expressed in terms of the slow variables, which involves the consideration of a "susceptibility".
• Here we study a non-relativistic situation in which chemical potentials are large compared with the temperature, as is relevant for late-time pair annihilations in dark matter freeze-out.
• An order-of-magnitude estimate and a lattice simulation are presented for a susceptibility dominated by bound states of stop-like mediators.