ABSTRACT:
The decay and annihilation cross-sections of dark matter particles may depend on the value of a chameleonic scalar field that both evolves cosmologically and takes different values depending on the local matter density. This possibility introduces a separation between the physics relevant for freeze-out and that responsible for dynamics and detection in the late universe. We investigate how such dark sector interactions might be implemented in a particle physics Lagrangian and consider how current and upcoming observations and experiments bound such dark matter candidates. A specific model allows for an increase in the annihilation cross-section by a factor of $10^6$ between freeze-out and today, while different choices of parameters allow for scattering cross-sections near the astrophysical bounds.