Quartic couplings, masses and thresholds in the basic extension of the standard bodel

The Standard Model (SM), despite of being phenomenologically extremely successful, presents some fundamental questions, like the big differences in the values of the masses of the quarks (hierarchy of masses), and the possible generation of flavour changing neutral currents (inspired by the evidence about the oscillations of neutrinos). Hints how these questions might be answered may be obtained by the study of the Higgs sector of models beyond the standard model. The simplest extension of the SM known as the two-Higgs-doublet-model (2HDM) involves a second Higgs doublet and predicts the existence of five scalar particles: three neutral (A⁰), (h⁰, H⁰) and two charged (H^plus). In this paper we focus our attention on the basic results of the model, the masses of the five particles, and the theoretical constraints imposed by vacuum stability and the triviality principle. We address, on one side, the range of validity in the energy scale of the 2HDM by means of the renormalization group equations, and on the other, the consequences of a top/bottom Yukawa coupling unification, assuming that the hierarchy of the quark masses is atributted to the vacuum expectation values v¹ and v² of the Higgs fields and not to the Yukawa couplings.