Axial dark matter: The case for an invisible Z′Z′

We consider the possibility that fermionic dark matter (DM) interacts with the Standard Model fermions through an axial Z′Z′ boson. As long as Z′Z′ decays predominantly into dark matter, the relevant LHC bounds are rather loose. Direct dark matter detection does not significantly constrain this scenario either, since dark matter scattering on nuclei is spin-dependent. As a result, for a range of the Z′Z′ mass and couplings, the DM annihilation cross section is large enough to be consistent with thermal history of the Universe. In this framework, the thermal WIMP paradigm, which currently finds itself under pressure, is perfectly viable.