Theoretical predictions for Higgs boson decay into four leptons.

After the discovery of the Higgs boson at the LHC, the precise determination of its properties and the search for Beyond the Standard Model (BSM) signals are two of the most important tasks in particle physics. The investigation of the Higgs sector relies on the comparison between theoretical predictions and experimental data. In order to compare the SM predictions with very precise data, the reduction of theoretical uncertainties is mandatory. This is the main motivation for the work presented in my Ph.D. dissertation. I have focused my attention on the Higgs boson decay into four charged leptons. In the thesis I detail the calculation of the full Next-to-Leading Order (NLO) electroweak corrections to Higgs boson decay into four charged leptons, by considering the gold-plated channel. The NLO corrections are matched with a QED Parton Shower (PS), in order to simulate exclusive multiple photon emission and provide novel results at NLOPS electroweak accuracy. I also provide some new results on the possible effects of new physics which have been obtained in an Effective Field Theory (EFT) framework.