Precision measurements of IVB parameters and bounds on new physics

Abstract

The main purpose of this thesis is to discuss the impact of electroweak precision measurements on the present knowledge of particle physics, both in the framework of the Standard Model and of its most straightforward extensions. In Chapter 1 we give a general overview of the electroweak sector of the SM and of SUSY phenomenology. In Chapter 2 we deal with different definitions of the electroweak mixing angle, with particular attention to the relation between the phenomenological angle $θ$ and the MS parameter $\hatθ$. In Chapter 3 we use results found in the previous chapter to discuss in detail how both the decoupling and the non-decoupling approach to the running of coupling constants in the MS scheme produce the same numerical value of $m_{GUT}$, despite of the different initial conditions. In Chapter 4 we derive simple analytical formulas for the contribution of a chargino almost degenerate with the lightest neutralino to electroweak observables, showing that in the case of wino domination the study of oblique corrections allows a concrete improvement of experimental bounds on chargino mass. Finally, in Chapter 5 we consider the possibility of extra fermion generations, proving that they are strongly disfavoured by the present experimental data if all particles are heavier that Z-boson, while for the specific case of the extra neutrino around 50 GeV in mass (still allowed by experimental data) the quality of the fit is not worse than the SM.