Very narrow shadow extraZboson 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 that the presence of such shadow sector would bring to the electroweak precision tests, which in turn provide constraints on the kinetic-mixing parameter, $s_{ϵ}$, left free in our model. The shadow $Z$ production rate at the CERN LHC and the International Linear Collider depends on $s_{ϵ}$. We find that the observable event rate at both facilities is possible for a reasonable range of $s_{ϵ}$ allowed by electroweak precision tests.