Search for a Dark Sector with the ATLAS Detector

Approximately 85% of the mass contents of the universe is in the form of dark matter, which could be composed of new particles. The search for such particles, which could eventually be produced in the proton-proton collisions at the LHC, is underway with the ATLAS detector. These searches have so far mostly focused on supersymmetric particles, whose decay chains contain dark matter candidates (for ex. the neutralinos), and on the direct production of dark matter particles in so-called 'simplified' models by looking for mono-X final states. With the start of Run-3 in 2021, it will be possible to strengthen the limits provided by these types of searches, but the discovery space beyond the already existing limits will be reduced. It is therefore important to cover less explored scenarios which require a very good understanding of the objects in the detector, understanding which can now benefit from many years of data taking. One of these scenarios is the existence of a 'hidden' or 'dark' sector (including a dark matter candidate) which can be evinced by the presence of jets with unusual characteristics of their associated inner detector tracks and/or of their calorimeter energy deposition pattern, as these jets come from the decay of dark-sector particles which can have long lifetimes. The work will focus on the search for particles from the dark sector with the ATLAS detector, in the context of dark matter searches. The work will first focus on the searches with the full Run-2 dataset. If these do not reveal any sign of new physics, the work will then focus on identifying possible weaknesses in the parameter space coverage in order to prepare the Run-3 searches. This will necessarily include work on performances (for ex. displaced vertices associated to atypical calorimeter energy deposition, possibility of new triggers,...). (author)