• vacuum decay
• Higgs phenomenology
• hep-ph

In the Standard Model the electroweak symmetry is broken due to the existence of an elementary scalar particle: the Higgs boson.
The Higgs boson was for a long time the missing piece of the Standard Model puzzle. Furthermore in the absence of direct signal of new physics, the Higgs boson couplings might indirectly indicate a portal to Beyond Standard Model theories. In this context, the discovery made at LHC of a new particle with a mass of 125.66 ± 0.34 GeV and the characteristics of the Higgs boson is of great importance: it could be the conclusive achievement of the Standard Model, or it could give a renewed impulse to the search for new physics. The discovery of this new particle was announced by the ATLAS and CMS collaborations during 2012. After that, all LHC and TeVatron collaborations presented at the Moriond 2013 conference the results based on the full collected data.
In the first part of this thesis we study the properties of this particle
in order to be sure that the new resonance is, indeed, the Higgs boson. In particular we study its couplings with the other particles: the SM has definite predictions for the gauge boson and fermion couplings with the Higgs boson. Those affect both the Higgs boson production mechanism at the LHC as well as its dominant decay modes. We find that these couplings are compatible with the SM ones.
Thus, although the discovery of the Higgs boson was expected to be the herald of new physics soon to be found at the TeV scale, so far no signal of new physics nor any clear deviation from the SM Higgs properties have been detected at the LHC. Moreover, the Higgs mass has not provided unambiguous indications for new physics.
Thus, in the second part of the thesis, we try to extrapolate the SM to high energy in search for clues. In order to do that we needed a precise computation of the corrections to the SM parameters.
The intriguing result is that, assuming the validity of the SM up to very high energy scales, the measured value of the Higgs mass is near-critical, in the sense that it places the EW vacuum right at the border between absolute stability and metastability.