Recent dark matter (DM) direct searches place very stringent constraints on the possible DM candidates proposed in extensions of the Standard Model. There are however models where these constraints are avoided. One of the simplest and most striking examples comes from a straightforward Higgs portal pseudoscalar DM model featured with a softly broken U (1) symmetry. In this model the tree-level DM-nucleon scattering cross section vanishes in the limit of zero momentum-transfer. It has also been argued that the leading-order DM-nucleon cross section appears at the one-loop level. In this work we have calculated the exact cross section in the zero momentum-transfer at the leading-order i.e., at the one-loop level of perturbative expansion. We have concluded that, in agreement with expectations, the amplitude for the scattering process is UV finite and approaches zero in the limit of vanishing DM masses. Moreover, we made clear that the finite DM velocity correction at tree-level is subdominant with respect to the one-loop contribution. Based on the analytic formulae, our numerical studies show that, for a typical choice of model parameters, the DM nuclear recoiling cross section is well below O(10 −50 cm 2 ), which indicates that the DM direct detection signal in this model naturally avoids the present strong experimental limits on the cross section.

Multi-component dark matter scenarios constitute natural extensions of standard single-component setups and offer attractive new dynamics that could be adopted to solve various puzzles of dark matter. In this work we present and illustrate properties of a minimal UV-complete vector-fermion dark matter model where two or three dark sector particles are stable. The model we consider is an extension of the Standard Model (SM) by spontaneously broken extra U (1) X gauge symmetry and a Dirac fermion. All terms in the Lagrangian which are consistent with the assumed symmetry are present, so the model is renormalizable and consistent. To generate mass for the dark-vector X µ the Higgs mechanism with a complex singlet S is employed in the dark sector. Dark matter candidates are the massive vector boson X µ and two Majorana fermions ψ ± . All the dark sector fields are singlets under the SM gauge group. The set of three coupled Boltzmann equations has been solved numerically and discussed. We have performed scans over the parameter space of the model implementing the total relic abundance and direct detection constraints. The dynamics of the vector-fermion dark matter model is very rich and various interesting phenomena appear, in particular, when the standard annihilations of a given dark matter are suppressed then the semi-annihilations, conversions and decays within the dark sector are crucial for the evolution of relic abundance and its present value. Possibility of enhanced self-interaction has been also discussed.

We study a vector dark matter (VDM) model in which the dark sector couples to the Standard Model sector via a Higgs portal. If the portal coupling is small enough the VDM can be produced via the freeze-in mechanism. It turns out that the electroweak phase transition have a substantial impact on the prediction of the VDM relic density. We further assume that the dark Higgs boson which gives the VDM mass is so light that it can induce strong VDM self-interactions and solve the small-scale structure problems of the Universe. As illustrated by the latest LUX data, the extreme smallness of the Higgs portal coupling required by the freeze-in mechanism implies that the dark matter direct detection bounds are easily satisfied. However, the model is well constrained by the indirect detections of VDM from BBN, CMB, AMS-02, and diffuse γ/X-rays. Consequently, only when the dark Higgs boson mass is at most of O(keV) does there exist a parameter region which leads to a right amount of VDM relic abundance and an appropriate VDM self-scattering while satisfying all other constraints simultaneously.

Motivated by the possibility of enhancing dark matter (DM) self-interaction cross-section σ self , we have revisited the issue of DM annihilation through a Breit-Wigner resonance. In this case thermally averaged annihilation cross-section has strong temperature dependence, whereas elastic scattering of DM on the thermal bath particles is suppressed. This leads to the early kinetic decoupling of DM and an interesting interplay in the evolution of DM density and temperature that can be described by a set of coupled Boltzmann equations. The standard Breit-Wigner parametrization of a resonance propagator is also corrected by including momentum dependence of the resonance width. It has been shown that this effects may change predictions of DM relic density by more than order of magnitude in some regions of the parameter space. Model independent discussion is illustrated within a theory of Abelian vector dark matter. The model assumes extra U (1) symmetry group factor and an additional complex Higgs field needed to generate a mass for the dark vector boson, which provides an extra neutral Higgs boson h 2 . We discuss the resonance amplification of σ self . It turns out that if DM abundance is properly reproduced, the Fermi-LAT data favor heavy DM and constraint the enhancement of σ self to the range, which cannot provide a solution to the small-scale structure problems.The dark matter (DM) constitutes 84.5% of total Universe matter [1], nevertheless its origin is still unknown in spite of unprecedented effort made both by experimentalists and theoreticians. Even more, the standard WIMP (weakly interacting massive particles) paradigm seems to suffer from various difficulties when confronted with observations on small cosmological scales. For instance "too-big-to-fail" [2, 3] and the "cusp-core" [4][5][6][7] problems are widely discussed in the literature. In particular, the DM densities inferred in central regions of DM dominated galaxies are usually smaller than expected from WIMP simulations [8,9]. It turns out that an appealing alternative is to assume that dark matter may self-interact strongly [10]. The assumption of self-interacting dark matter (SIDM) implies that central (largest) DM density could be reduced. The numerical simulations have shown that SIDM halos are consistent with observations if DM particles have a nuclear-scale self-interaction cross-section 0.1 cm 2 /g < σ self /M DM < 10 cm 2 /g within halos [8,[11][12][13][14][15][16][17][18][19]. The largest values of σ self /M DM are in contradiction with the cluster limit, σ self /M DM < 1.25 cm 2 g −1 [20], therefore scenarios with velocity-dependent σ self /M DM are preffered [21].The aforementioned problems directly encouraged us to study the possibility for the Breit-Wigner resonant enhancement of dark matter annihilation and self-scattering [22][23][24][25][26][27][28][29][30][31][32]. However, it turns out that physics relevant in the vicinity of a resonance is interesting on its own and worth studying in details regardless of any phenomenological a...

Abstract:We explore an extension of the Standard Model by an additional U(1) gauge group and a complex scalar Higgs portal. As the scalar is charged under this gauge factor this simple model supplies a vector dark matter candidate satisfying the observed relic abundance and limits from direct dark matter searches. An additional Higgs-like state, that may be heavier or lighter than the observed Higgs, is present and satisfies LEP and LHC bounds whilst allowing for absolute stability of the electroweak vacuum in a range of the parameter space.