Light Z′ and dark matter from U(1)X gauge symmetry.

We consider a U(1) X gauge symmetry extension of the Standard Model (SM) with a Z ′ -portal Majorana fermion dark matter that allows for a relatively light gauge boson Z ′ with mass of 10 MeV− a few GeV and a much heavier dark matter through the freeze-in mechanism. In a second scenario the roles are reversed, and the dark matter mass, in the keV range or so, lies well below the Z ′ mass, say, ∼1 GeV. We outline the parameter space that can be explored for these two scenarios at the future Lifetime Frontier experiments including Belle-II, FASER, LDMX and SHiP. [ABSTRACT FROM AUTHOR]