Not a jet all the way - an exploration of the strongly interacting dark sector in ATLAS and beyond

Collider searches for dark matter (DM) so far have mostly focussed on scenarios, where DM particles are produced in association with heavy standard model (SM) particles or jets. However, no deviations from SM predictions have been observed so far. Several recent phenomenology papers have proposed models that explore the possibility of accessing the strongly coupled dark sector, giving rise to unusual and unexplored collider topologies. One such signature is termed as semi-visible jet (SVJ), where parton evolution includes dark sector emissions, resulting in jets interspersed with stable invisible particles. Owing to the unusual MET-along-the-jet event topology this is still a largely unexplored domain within LHC. This thesis presents the first results from a search for SVJ in t-channel production mode in $pp$ collisions for an integrated luminosity of 139 fb$^{-1}$ at centre-of-mass energy corresponding to 13 TeV at the LHC, based on data collected by the ATLAS detector during 2015-2018. Additionally, studies are performed to explore the use of jet substructure methods to distinguish SVJ from SM jets in the first two scenarios, using observables in a IRC-safe linear basis, and ways forward are proposed for this approach to dark-matter at the LHC, including prospects for estimating modelling uncertainties.