Search for emerging jets within dark QCD models with the ATLAS detector during the LHC's Run-3

The Standard Model (SM) of particle physics is a very successful theory describing the particle contents and interactions governing the dynamic of the infinitely small in the universe. It is a theory that allowed the prediction of many fundamental particles and the understanding of the fundamental interactions between them, which culminated with the recent discovery of the Higgs boson by the ATLAS and CMS experiments at the Large Hadron Collider (LHC) in 2012. The SM is however acknowledged not to be a complete theory as several inconsistencies and limitations are still present. In particular, the lack of a candidate particle for dark matter is commonly seen as one of its limitations within the particle physics community. Many particle physics experiments thus aim at discovering new particles that could explain the seemingly missing mass density highlighted in the universe. As searches for a new weakly interacting massive particle have not yet produced a discovery, the search for dark matter has recently broadened to more diverse dark matter particles, such as composite particles. Such composite particles are assumed to lay within a distinct and complex dark sector dynamic in analogy to the SM sector. These kinds of theories are very interesting as they are well motivated and yield very exotic signatures in particle physics experiments. In this context, this thesis aims at studying a particular theory of dark sector with a strong dynamic resembling the SM strong interaction, and containing some unstable dark particles with long lifetimes. Such a dark sector is supposed to be accessible through a new massive mediator that can be produced in SM particle collisions and produce a jet-like signature with many displaced objects. This search and this thesis have been conducted with the ATLAS detector at the LHC and with the first proton-proton collision data of \RunThr recorded during the years 2022 and 2023.