Effect of the early kinetic decoupling in a fermionic dark matter model

Abstract

We study the effect of the early kinetic decoupling in a model of fermionic dark matter (DM) that interacts with the standard model particles only by exchanging the Higgs boson. There are two DM-Higgs couplings, namely CP-conserving and CP-violating couplings. If the mass of the DM is slightly below half of the Higgs boson mass, then the couplings are suppressed to obtain the measured value of the DM energy density by the freeze-out mechanism. In addition, the scattering processes of DM off particles in the thermal bath are suppressed by the small momentum transfer if the CP-violating DM-Higgs coupling is larger than the CP-conserving one. Due to the suppression, the temperature of the DM can differ from the temperature of the thermal bath. By solving coupled equations for the number density and temperature of the DM, we calculate the DM-Higgs couplings that reproduce the right amount of the DM relic abundance. We find that the couplings have to be larger than the one obtained without taking into account the difference in the temperatures. A consequence of the enhancement of the DM-Higgs couplings is the enhancement of the Higgs invisible decay branching ratio. The enhancement is testable at current and future collider experiments.

Dark matter relic abundance beyond kinetic equilibrium

“…In fact, it has repeatedly been pointed out that chemical and kinetic decoupling can be intertwined in a way that significantly affects the result of relic density calculations [24][25][26][27][28][29][30][31][32][71][72][73][74][75]. To provide context, we have therefore devoted a large part of this article (Sect.…”

“…For Scalar Singlet masses slightly smaller than half of the Higgs mass it was found that a larger coupling to the Higgs is required to obtain the right relic abundance, which interestingly implies that future measurements of the Higgs-to-invisible decay width can probe more of the parameter region than expected from the standard relic abundance computation, see Refs. [25,29,51] for experimental probes and also for other Higgs resonance scenarios with similar effects.…”

“…There exist however various wellmotivated scenarios where this assumption is not satisfied, at least not for all relevant parts of the parameter space, and where the standard approach is thus not directly applicable. A possible solution that has received increased attention is to derive coupled equations for the number density and higher moments of the DM phase-space distribution, making use of the Boltzmann hierarchy [24][25][26][27][28][29]. More recently, there have also been attempts to solve the full Boltzmann equation directly at the phase-space level [30][31][32].…”

A closer look at CP-violating Higgs portal dark matter as a candidate for the GCE

“…If the dark matter is a thermal relic, then the present-day dark matter abundance, Ω χ h 2 = 0.11, sets the annihilation cross section at the time of freeze-out, which is the well-known O(1) pb weak-scale cross section [56][57][58][59][60][61][62][63][64][65][66][67]. Recent work [68,69] has shown that for models with a hierarchy between annihilation and scattering strengths, early kinetic decoupling before freeze-out alters this number, requiring a larger cross section to achieve the observed abundance. At most extreme, a ∼ 20 pb annihilation cross section may be needed for a ∼ 57 GeV dark matter with purely imaginary couplings, though this is quite sensitive to the details of the QCD phase transition.…”

Pseudo-Nambu-Goldstone Dark Matter Model Inspired by Grand Unification

“…We comment on the allowed parameter space m χ m h i /2. For more precise calculations in the region m χ m h i /2, the effect of the early kinetic decoupling from the SM thermal bath should be taken into account [73,74]. If this effect is included, one can expect that the red line in Fig.…”