The subject of this thesis is the mechanism through which leptons obtain their masses.
This is done via the search for the decay of Higgs bosons into pairs of the heaviest
kind of leptons, the tau leptons.
In the first chapter, the theoretical backgrounds are shortly recalled. Chapter 2
summarizes the analyses that led to the discovery of the Higgs boson and presents a
description of the CMS experiment. Chapter 3 presents the progress achieved in
the reconstruction of undetectable particles. The last chapter presents in detail the
H → τ τ analysis and its results, addressing the initial question of the lepton mass