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.

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.

We calculate the Higgs decay rate of h → Zγ by including the contributions from new scalars with arbitrary quantum numbers of the weak isospin (T ) and hypercharge (Y ) in the standard model. We find that our general formula for the decay rate of h → Zγ matches with that for h → γγ in the limit of m Z = 0, but it is different from those in the literature. To illustrate our result, by taking the current 2σ excess of the h → γγ rate measured by the LHC, we examine the corresponding shift for the Zγ decay channel due to the new scalar. We show that the enhancement or reduction of the h → Zγ rate only depends on the relative size of T and the absolute value of Y .Explicitly, we predict 0.76 < R Zγ ≡ Γ(h → Zγ)/Γ SM (h → Zγ) < 2.05 by imposing the observed range of 1.5 < R γγ ≡ Γ(h → Zγγ)/Γ SM (h → γγ) < 2, which is independent of the number of multiplets and the couplings to the Higgs particle as long as the scalars are heavier than 200 GeV.This result provides a clear signature for the future LHC measurements to test physics beyond the standard model. a chianshu@phys.

We investigate and compare two simple models of dark matter (DM): a vector and a scalar DM model. Both models require the presence of two physical Higgs bosons h 1 and h 2 which come from mixed components of the standard Higgs doublet H and a complex singlet S. In the Vector model, the extra U (1) symmetry is spontaneously broken by the vacuum of the complex field S. This leads to a massive gauge boson X µ that is a DM candidate stabilized by the dark charge conjugation symmetry S → S * , X µ → −X µ . On the other hand, in the Scalar model the gauge group remains the standard one. The DM field A is the imaginary component of S and the stabilizing symmetry is also the dark charge conjugation S → S * (A → −A). In this case, in order to avoid spontaneous breaking, the U (1) symmetry is broken explicitly, but softly, in the scalar potential. The possibility to disentangle the two models has been investigated. We have analyzed collider, cosmological, DM direct and indirect detection constraints and shown that there are regions in the space spanned by the mass of the non-standard Higgs boson and the mass of the DM particle where the experimental bounds exclude one of the models. We have also considered possibility to disentangle the models at e + e − collider and concluded that the process e + e − → Z + DM provides a useful tool to distinguish the models.

No abstract