Higgs and beyond in the LHC era

The last few years witnessed some major breakthroughs in the field of fundamental particle physics, which had a big impact in our understanding of Nature at a microscopic level. On March 30th, 2010, the first proton-proton collisions took place at the Large Hadron Collider (LHC), marking the beginning of a new era in particle physics. The excellent performance of the machine and the detectors, due to the fantastic work of all the researchers involved in the experiments, lead, in only two years, to the announcement of the discovery of the Higgs boson on July 4th, 2012. This event could be considered as the peak of success for the Standard Model (SM) of elementary particles, which predicted the existence of this particle – as well as all its properties – since more than forty years before. In the following two years the ATLAS and CMS experiments at the LHC measured the properties of the Higgs particle with a good accuracy, showing no significant deviation from the SM. In the meanwhile, also the numerous direct searches for other new particles turned out to give only negative results, against all expectations from the theory community, pushing the scale of new physics to higher and higher values. Also, while the cosmological evidence for Dark Matter (DM) is now stronger than ever, so far all direct and indirect searches provided negative results (albeit with some isolated exceptions which, however, are still much debated in the literature and seem to be incompatible with other negative results) and the bounds on weakly interacting massive particle DM are extremely strong. In neutrino physics an important event took place in June 2011, when the Tokay-to-Kamioka (T2K) collaboration reported an evidence for a non-zero, and sizable, value of the reactor neutrino mixing angle, θ13. This was confirmed in March 2012 by the Daya Bay collaboration, which measured this mixing angle with a very high precision, confirming that its value lies on the high-end of previous upper bounds. Since many popular and well motivated models of neutrino mixing predicted a zero, or very small, value of the reactor angle, this result was very important and offered a new insight in the quest for understanding the origin of flavor in the lepton sector. Also, since CP violation in the lepton sector effects vanish in the θ13 → 0 limit, the fact that this angle is sizable opens up many interesting possibilities for measuring CP violation in the neutrino sector. The work presented in this thesis was largely stimulated by these two major breakthroughs in particle physics. On the one hand the Higgs discovery and the measurement of its properties, in particular its mass, lead us to study the consequences of these measurements for a specific class of models beyond the SM: composite Higgs models (and also in supersymmetric versions of these models). In particular, we found that a very definite (and testable) prediction for the spectrum of new physics can be obtained: fermionic top partners are expected to be near the ∼1TeV scale. Also, the measurements of the Higgs couplings and the fact that the bounds for the new physics scale are often much higher than the electroweak scale, open up the possibility of studying possible deformations from the SM in an effective field theory framework. In this context we studied the possibility of linking the properties of the Higgs with other electroweak observables, very well constrained by LEP, via renormalization group effects, finding that they already allow to derive constraining, and independent, bounds on some Higgs properties. In the future, when some deviation from the SM will be – hopefully – observed, these effects could provide a new window on the new physics sector. On the other hand, we studied how the measured value of θ13 can be accommodated in some motivated models of neutrino mixing by exploiting corrections due to the mixing among the charged leptons. Such corrections are expected, for example, in Grand Unified Theories, which allow to link the charged lepton sector with the quark sector, and therefore the neutrino mixing matrix with the quark mixing one. This analysis allowed us to obtain a precise prediction for the value of the Dirac CP violating phase in neutrino mixing, testable by future neutrino experiments.