Making the invisible visible Searches for invisible Higgs decays at LHC with the ATLAS detector

The Standard Model (SM) of particle physics is a theoretical model which describes matter and its interaction with the three fundamental forces of nature: electromagnetic, nuclear strong and weak. This model, extremely precise and predictive, was completed by the discovery of the Higgs boson in 2012. Despite its great success, the SM has several shortcomings such as the absence of any description of the pieces of evidence from observational astrophysics that indicate the existence of Dark Matter. Following the Standard Cosmological Model, Dark Matter accounts for the majority of matter in the universe (around 75%) compared to baryonic matter. Its discovery would then be a major turning point for the understanding of the universe.
In the last years, multiple theories and experimental techniques have been developed to detect Dark Matter, such as the production of Dark Matter at high-energy colliders.
The work of this thesis will be focused on the search of Dark Matter at the Large Hadron Collider (LHC) using 36.1 fb⁻¹ of proton-proton interactions delivered at a center-of-mass energy of 13 TeV, recorded by “A Toroidal LHC Apparatus” (ATLAS) detector during 2015 and 2016. The channel at study is the invisible Higgs decay in associated production with a Z boson with a final signature of two leptons and missing transverse energy. Two different analysis techniques are presented, one following a cut-based approach, the other using a multivariate technique based on a boosted decision tree.
An additional chapter will describe the ATLAS fast physics TAG data monitoring system (TADA).