Z-portal dark matter

We propose to generalize the extensions of the Standard Model where the Z boson serves as a mediator between the Standard Model sector and the dark sector chi. We show that, like in the Higgs portal case, the combined constraints from the recent direct searches restrict severely the nature of the coupling of the dark matter to the Z boson and set a limit m(chi) greater than or similar to 200 GeV (except in a very narrow region around the Z-pole region). Using complementarity between spin dependent, spin independent and FERMI limits, we predict the nature of this coupling, more specifically the axial/vectorial ratio that respects a thermal dark matter coupled through a Z-portal while not being excluded by the current observations. We also show that the next generation of experiments of the type LZ or XENON1T will test Z-portal scenario for dark matter mass up to 2TeV. The condition of a thermal dark matter naturally predicts the spin-dependent scattering cross section on the neutron to be sigma(SD)(chi n) similar or equal to 10(-40) cm(2), which then becomes a clear prediction of the model and a signature testable in the near future experiments.