Extension of the Sm

A new type of gauge extension of the SM is proposed. It is based on a SU (2) × SU (2) × U (1) group with the peculiarity that the gauge bosons of the extra SU (2) do not couple to fermions. This feature relaxes some of the constraints on the masses of the new gauge bosons, leaving the possibility of having lighter masses than in traditional extensions. The model exhibits several interesting properties, it is anomaly free and at tree level it does not have FCNC while loop induced effects are strongly suppressed. Also, from the analysis of ∆ρ at one loop, two configurations for the vevs giving rise to a screening phenomenon are identified. One of these configurations can also be related with the Bess model. A tree level fit to the most recent electroweak data is performed confirming the possibility of having new light gauge boson masses. The constraints coming from different FCNC processes like b → sγ, B 0 − ¯ B 0 and K 0 − ¯ K 0 mixing are also taken into account. Finally, a generalization of this model for the case of having several extra copies of SU (2) groups is commented, focusing on the presence of screening configurations and the corresponding mass spectrum.