Collisionless Self-interacting Dark Matter
Ani Prabhu
Princeton University

Self-interacting dark matter (SIDM) is a well-motivated alternative to the standard cold dark matter paradigm. In these models, self-interactions enable heat transport within dark matter halos, leading to the formation of extended isothermal cores and, on longer timescales, gravothermal core collapse. Efficient heat transport requires the mean free time between hard collisions to be comparable to the dynamical (crossing) time, implying characteristic scattering cross sections of order sigma/m ~ cm^2/g, comparable to nucleon elastic scattering cross sections in the Standard Model. In this talk, I will argue that this picture can be significantly broadened. Dark matter with long-range interactions can undergo rapid collisionless relaxation driven by collective fluctuations of the mean field, providing an alternative route to effective dark matter ?thermalization? without frequent particle collisions. This mechanism is closely analogous to violent relaxation in self-gravitating systems and to collisionless relaxation processes in Standard Model plasmas. As an illustrative example, I will consider a dark sector with a feeble dark electromagnetic interaction and show that plasma-like instabilities, seeded by position- and velocity-space anisotropies, can generate fluctuating fields that efficiently scatter particle orbits. These collective effects can produce effective relaxation rates comparable to those required to modify halo structure, even when the underlying force is only modestly stronger than gravity.

Date:	Mardi, le 31 mars 2026
Heure:	15:30
Lieu:	Université McGill
	Ernest Rutherford Physics, R.E. Bell Conference Room (room 103)