Perturbative unitarity constraints on general W′ models and collider implications

We study perturbative unitarity constraints on general W′ models by considering the high-energy behavior of fermion scattering into gauge bosons. In most cases we survey, a Z′ boson with a comparable mass must be present for the theory to be consistent, with fixed couplings to the standard model gauge bosons and fermions. Applying these results to a class of W′ models which explains the top quark forward-backward asymmetry observed at the Tevatron, we find that a Z′ must exist with a mass below 7-8 TeV and sizable coupling to the light quarks. While such a Z′ is strongly constrained by existing experiments, we show that the LHC can explore the entire mass range up to the unitarity limit. We also show how it is possible, by raising the Z′ mass consistent with unitarity, to explain the CDF Wjj excess in terms of a light W′, without generating an excess in Zjj events.