The Higgs Boson from Extended Symmetries

Motivated by the desire to ameliorate the hierarchy problem without conflicting with electroweak precision tests, we investigate the idea that the Higgs doublet is a pseudo-Goldstone boson of an extended symmetry of the Standard Model. This thesis consists of two parts. The first chapter is devoted to an effective description of the phenomenology by means of a "minimal" SO(5) symmetry in the Higgs-top sector. We discuss the spectrum, electroweak precision tests, B-physics and naturalness of this model. We show the diffculty to comply with the different experimental constraints. In the second chapter we explore a scenario where the Higgs is still a pseudo-Goldstone boson but the spacetime symmetries are enlarged to include the supersymmetry. We discuss a model based on a SU(3) SU(2)W symmetry group. The tuning required is milder than 10% for a Higgs that can be as heavy as 135 GeV if non decoupling D-terms of an extra U(1) are taken into account. The model exhibits an interesting spectrum which could be visible at LHC, including a heavy top-like fermion at the TeV scale. Ordinary MSSM states could be as light as 400 GeV. This thesis is based on our recent papers [13, 17, 18]. During my PhD studies I have also published the papers [48, 49, 50, 51] on a parallel topic which is not discussed here.