Dark sector searches in final states with long-lived or prompt neutral particles with the ATLAS detector and upgrade of the L0 muon trigger for HL-LHC

Dark Matter (DM) is one of the outstanding questions to which the Standard Model (SM) of particle physics still has no answer. There is a large body of evidence which points to the existence of a non-baryonic matter component in the Universe, interacting with ordinary matter only through gravity. Weakly Interactive Massive Particles (WIMPs) have always been an appealing DM candidate, as they naturally account for the amount of DM seen nowadays. For this reason, different experiments have been developed in order to detect these elusive particles, but so far they have all proved to be inconclusive. In recent years, a new paradigm has started to be investigated, in which a new sector of particles, the so-called Dark Sector (DS), is foreseen. In such a scenario, the DM would be a stable particle among those predicted in the DS. Particles in this new sector are neutral under all SM forces and the connection between the dark and the visible sector is granted through different portals, in which SM particles mix with Beyond the Standard Model (BSM) ones. This work exploits the simplest DS model, foreseeing a dark interaction which is similar to the electromagnetic one, mediated by a so-called Dark Photon (DP). In this case, the connection between the two sectors can be granted via the vector portal, where the dark and the visible photons mix kinematically, so that the DP can decay into SM particles. Such models have been investigated thoroughly and no evidence of DPs have been found up to now. The region with a small coupling between the two sectors and large DP masses is accessible only at Higgs factories, such as the Large Hadron Collider (LHC), where an additional connection between the dark and the visible sector can be granted via the Higgs portal. Through such portal, the SM Higgs boson can decay into BSM particles, this possibility being viable since Higgs boson decays into invisible particles are still largely unconstrained. Using the dataset collected during Run-2 by the A Thoroidal LHC ApparatuS (ATLAS) experiment at the LHC,, this work investigates the scenario where the DPs decay inside the ATLAS innermost detector, yielding collimated jet-like structures. This search aims to extend the sensitivity of the Run-1 search that was targeting the same final state, by improving the reconstruction techniques (both at the online and at the offline level) and by exploiting new background estimation strategies. Preliminary results already show better sensitivities than those obtained in the Run-1 search, probing a larger DP mass range. This work investigates in addition for the first time the possibility that the coupling between the SM photon and the dark one is so suppressed that DPs decay outside the ATLAS detector. DPs escaping detection would leave missing energy as signature of their passage, which could be sizeable if recoiling against highly energetic jets. Final states with this signature, referred to as monojet, are thus here used to probe this scenario. The results of this search are presented in terms of both this DP already introduced and for two additional models foreseeing the production of Long Lived Particles (LLPs) too. Complementary sensitivities with respect to the ones of the different dedicated searches are observed, probing previously uncovered scenarios with detector stable LLPs. Searches for unconventional final states, like the ones described in this thesis, are mostly limited by the trigger system, which performs an online event selection that often discards non-standard events. Increasing the performance of this system is thus of utmost importance to improve the sensitivity to such BSM scenarios. Such performances will be even more crucial during the High-Luminosity (HL) phase of the LHC, where the trigger system will have to cope with 200 pp collisions occurring every event. Therefore, this thesis will cover as well a study dedicated to the upgrade of the trigger system of the ATLAS experiment, that is necessary for the correct operation under the conditions of the HL-LHC.