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Implications of the Stability and Triviality Bounds on the Standard Model with Three and Four Chiral Generations

We revisit the stability and triviality bounds on the Higgs boson mass in the context of the Standard Model with three and four generations (SM3 and SM4, respectively). In light of the recent results from LHC the triviality bound in the SM3 has now become obsolete, and the stability bound implies for a Higgs mass of e.g. mH=115 GeV the onset of new physics before 650 TeV, whereas there are no limits for mH>133 GeV. For the SM4, the stability and triviality curves intersect and bound a finite region. As a consequence, the fourth generation fermions place stringent theoretical limits on the Higgs mass, and there is a maximal scale beyond which the theory cannot be perturbatively valid. We find that the Higgs mass cannot exceed 700 GeV for any values of the fourth generation fermion masses. Turning the argument around, the absence of a Higgs signal for mH<600 GeV excludes a fourth generation with quark masses below 300 GeV and lepton masses below 350 GeV. In particular, the quark bounds also hold for the small mixing scenarios for which the direct limits from Tevatron and LHC are not applicable, and the lepton bounds we obtain are stronger than the collider limits. If a Higgs boson lighter than 700 GeV is not observed, a fourth generation of chiral fermions with perturbative Yukawa couplings will be conclusively excluded for the full range of parameters.