Abstract
In some models of thermal relic dark matter, the relic abundance may be set by inelastic scattering processes (rather than annihilations) becoming inefficient as the universe cools down. This effect has been called coscattering. We present a procedure to numerically solve the full momentum-dependent Boltzmann equations in coscattering, which allows for a precise calculation of the dark matter relic density including the effects of early kinetic decoupling. We apply our method to a simple model, containing a fermionic SU(2) triplet and a fermionic singlet with electroweak-scale masses, at small triplet-singlet mixing. The relic density can be set by either coannihilation or, at values of the mixing angle θ ≲ 10−5, by coscattering. We identify the parameter ranges which give rise to the observed relic abundance. As a special case, we study bino-like dark matter in split supersymmetry at large μ.
Highlights
The conditions for coscattering to occur are that (1) the dark matter particle χ is in equilibrium with the thermal bath at early times, (2) there exists one or more state(s) ψ carrying the same charge as χ under the symmetry which stabilizes χ, (3) the couplings and masses are such that χψ → X annihilations as well as χχ → X annihilations decouple earlier than χX → ψX conversions, and these in turn decouple earlier than ψψ → X annihilations.1 It has been argued [2] that, in the case that ψ and χ are coupled via an extra mediator particle φ with mφ + mψ > 2 mχ, the dark matter mass should be exponentially below the weak scale
We present a procedure to numerically solve the full momentum-dependent Boltzmann equations in coscattering, which allows for a precise calculation of the dark matter relic density including the effects of early kinetic decoupling
We have presented a precise calculation of the dark matter relic abundance from thermal freeze-out in a scenario where the usual coannihilation formalism cannot be applied, as the relic density is set by coscattering
Summary
We will proceed with a brief review of the singlet-triplet model studied in [10, 11], emphasizing its possible origins in split supersymmetry. Since the dark matter particle is to be χ-like, we assume that M > |m| This model can be UV-completed with a Z2-odd Dirac fermion doublet Ψ with hypercharge. Since we are not assuming any particular model of supersymmetry breaking, this is a possible choice of parameters. We will study O(1) Wilson coefficients λ and mass differences M − m which are not parametrically smaller than the electroweak scale, such that the mixing angle is roughly of the order v/Λ. (In split supersymmetry, these would correspond to a bino-like lightest neutralino χ01, a wino-like next-to-lightest neutralino χ02 and a wino-like chargino χ±1 respectively, with the higgsino-like states at the higher mass scale |μ|). The mixing-induced mass difference is even smaller for the mixing angles of interest to us
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