A tale of a new generation - Searching for dimuon decays of the Higgs boson with the ATLAS detector

The current most comprehensive knowledge about the elementary particles and their interactions is incorporated in a theory called the Standard Model, which is validated and supported by a large number of experiments. However, its equations are far from providing a complete understanding of nature and many fundamental scientific questions, such as the origin of fermion masses, remain open. Measuring the properties of the Higgs boson, and in particular its interactions with other massive particles, is a pivotal way to find insights. Those with vector bosons and third-generation fermions have now been observed and found to be compatible with the Standard Model. In this experimental context, the dimuon decay of the Higgs boson represents the best candidate to perform further studies, offering a clean signature and a branching ratio that is finally becoming accessible with the current data, and allowing to investigate for the first time the Higgs interactions with second-generation fermions. This thesis presents a direct search for this process performed by the ATLAS experiment, using proton-proton collisions produced during Run 2 of the Large Hadron Collider, at a centre-of-mass energy of 13 TeV. The analysed dataset is still insufficient to claim a direct observation, and an upper limit on the decay branching ratio is established. However, this result contributes to providing the strongest hint to date of the Higgs coupling to second-generation fermions, and suggests the existence of an interaction between muons and the Higgs boson compatible with the Standard Model predictions.