PhD dissertation: ‘Shedding light on extended scalar sectors’

The Standard Model has a number of traits which are surprising from the theoretical perspective and may hint the presence of some underlying hidden structure beyond the SM. In this thesis, we explored some of the open problems which materialize from those traits and could be solved by the presence of an extended scalar sector. First, we considered axion-like particles (ALPs) using an Effective Field Theory (EFT) approach and imposed improved limits on their CP-odd couplings to fermions. The remainder of the thesis was devoted to the Electroweak hierarchy problem (EWHP). We presented two original works which follow two different guiding principles, one traditional and the other rather innovative: symmetry and near-critical dynamics. The first among these works presents non-abelian discrete symmetries as able to suppress large radiative corrections and stabilize the masses of scalar particles. In the second contribution, we argue that the small observed mass of the Higgs could be a trademark of the near-criticality of the Higgs sector, backed by the observation that every parameter in the Higgs potential seems in fact to also be near-critical.  We explore the consequences of this fact using the so-called “Higgs metastability bound” in the case of a concrete model—the low-scale Majoron model—and demonstrate that this justification of the mass of the Higgs implies new experimental signals in future experiments and in particular, in the future colliders. We argue that this conclusion is actually more general and should apply to wider contexts than the concrete model analysed.