Fine-tuning the rho parameter with new gauge bosons.

Abstract

We explore the class of models containing a triplet Higgs-scalar representation as well as singlets and doublets under ${\mathrm{SU}(2)}_{L}$ in which the gauge group is also extended by (at least) an additional U(1) factor. In a large subclass of such models we show that the $\ensuremath{\rho}$ parameter, involving the lightest physical charged and neutral gauge bosons, can be fine-tuned to unity without the vanishing of any of the scalar vacuum expectation values. Two examples of such models, based on the groups ${\mathrm{SU}(2)}_{L}\ifmmode\times\else\texttimes\fi{}{\mathrm{U}(1)}_{R}\ifmmode\times\else\texttimes\fi{}{\mathrm{U}(1)}_{B\ensuremath{-}L}$ and ${\mathrm{SU}(2)}_{L}\ifmmode\times\else\texttimes\fi{}{\mathrm{SU}(2)}_{R}\ifmmode\times\else\texttimes\fi{}\mathrm{U}(1)$, are shown to be phenomenologically viable.