Abstract
In face of the small-scale structure problems of the collisionless cold dark matter (DM) paradigm, a popular remedy is to introduce a strong DM self-interaction which can be generated nonperturbatively by a MeV-scale light mediator. However, if such a mediator is unstable and decays into SM particles, the model is severely constrained by the DM direct and indirect detection experiments. In the present paper, we study a model of a self-interacting fermionic DM, endowed with a light stable scalar mediator. In this model, the DM relic abundance is dominated by the fermionic DM particle which is generated mainly via the freeze-out of its annihilations to the stable mediator. Since this channel is invisible, the DM indirect detection constraints should be greatly relaxed. Furthermore, the direct detection signals are suppressed to an unobservable level since fermionic DM scatterings with a nucleon appear at one-loop level. By further studying the bounds from the CMB, supernovae and BBN on the visible channels involving the dark sector, we show that there is a large parameter space which can generate appropriate DM self-interactions at dwarf galaxy scales, while remaining compatible with other experimental constraints.
Highlights
In order to generate so large dark matter (DM) self-scattering cross section, one interesting scenario is to introduce a light O(MeV)-scale scalar or vector particle to mediate this interaction so that the corresponding DM cross section can be boosted nonperturbatively [17, 27,28,29,30,31,32,33,34,35,36]
In the model with a scalar mediator, even though the DM indirect detection constraints can be avoided since the fermionic DM annihilation is p-wave dominated [54], the DM direct detection upper bounds [55] imply the longevity of the scalar mediator, which would modify the primordial abundances of light elements during Big-Bang nucleosynthesis (BBN) [56,57,58]
In order to generate such a large DM self-scattering, one popular strategy is to introduce a light mediator to enhance the cross section nonperturbatively. This leads to the velocity-dependent DM self-interactions which help to evade the strong constraint at the scale of galaxy clusters
Summary
Our model is a simple extension of the SM by including a Dirac fermion χ and two real scalars S and φ. In order to avoid unnecessary parameters in the Lagrangian, we impose an extra Z2 symmetry: φ → −φ, which implies the following potential terms. Note that the latter Z2 symmetry would be broken by the VEV of φ = vφ so that the perturbation φ = φ − vφ can mi√x with the neutral component h of the Higgs doublet which is defined as H ≡ (0, (vH + h)/ 2)T in the unitary gauge with vH = 246 GeV. We further expand the potential written in terms of perturbation fields S, h, and φ up to the second order, determining the mass squared of S as m2S = μ2S + (κHSvH2 + κSφvφ2)/2 and the following mass squared matrix for h and φ: M2hφ =.
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