Early kinetic decoupling of dark matter: When the standard way of calculating the thermal relic density fails

Abstract

Calculating the abundance of thermally produced dark matter particles has become a standard procedure, with sophisticated methods guaranteeing a precision that matches the percent-level accuracy in the observational determination of the dark matter density. Here, we point out that one of the main assumptions in the commonly adopted formalism, namely local thermal equilibrium during the freeze-out of annihilating dark matter particles, does not have to be satisfied in general. We present two methods for how to deal with such situations, in which the kinetic decoupling of dark matter happens so early that it interferes with the chemical decoupling process: i) an approximate treatment in terms of a coupled system of differential equations for the leading momentum moments of the dark matter distribution, and ii) a full numerical solution of the Boltzmann equation in phase-space. For illustration, we apply these methods to the case of Scalar Singlet dark matter. We explicitly show that even in this simple model the prediction for the dark matter abundance can be affected by up to one order of magnitude compared to the traditional treatment.

Thermodynamic evolution of secluded vector dark matter: conventional WIMPs and nonconventional WIMPs

“…where the hidden scalar scattering with all relativistic SM fermions is taken into account. Under the typical condition m S T ∼ ω, this term can further reduce to a semi-relativistic Fokker-Planck-type equation [38][39][40][41][42][43],…”

Forbidden frozen-in dark matter

“…The simplest case is the usual freeze-in, where m S > 2m χ and Y DM gradually grows, with most of the production happening around T ∼ m S . This is shown in 14 Kinetic equilibrium is an extremely good assumption in the parameter space studied in this work since away from the Higgs boson resonance elastic scatterings of S off particles of the SM plasma are much more frequent than annihilations of S. In a different model where this assumption would be violated one would be required to solve also for the temperature of S or even its full phase space density, see [37]. This would also bring additional complication to the forbidden freeze-in case as the thermal mass of S would need to be computed out of equilibrium.…”

Gravitational wave signals of pseudo-Goldstone dark matter in the Z3 complex singlet model

“…Note that the BR inv can exclude points with m χ > m h /2, because the Higgs boson can decay invisibly also to two h 2 if it is light enough. Early kinetic decoupling [92,93] may additionally enhance BR inv several times [31], but in practice this would not change the parameter space of GW signals. In particular, all our points with a potentially measurable GW signal have m χ , m 2 > m h /2.…”