A Very Narrow Shadow Extra Z-boson at Colliders
  We consider the phenomenological consequences of a hidden Higgs sector extending the Standard Model (SM), in which the ``shadow Higgs'' are uncharged under the SM gauge groups. We consider a simple U(1) model with one Higgs singlet. One mechanism which sheds light on the shadow sector is the mixing between the neutral gauge boson of the SM and the additional U(1) gauge group. The mixing happens through the usual mass-mixing and also kinetic-mixing, and is the only way the ``shadow $Z$'' couples to the SM. We study in detail modifications to the electroweak precision tests (EWPTs) that the presence of such a shadow sector would bring, which in turn provide constraints on the kinetic-mixing parameter, $s_\epsilon$, left free in our model. The shadow $Z$ production rate at the LHC and ILC depends on $s_\epsilon$. We find that observable event rate at both facilities is possible for a reasonable range of $s_\epsilon$ allowed by EWPTs. 
We-Fu Chang, John N. Ng, and Jackson M. S. Wu