A search for long-lived particles with the ATLAS Experiment

It is well assessed that the Standard Model (SM) correctly predicts the behaviour of the Universe at the infinitely small scale, but is not the definitive theory that we are looking for. In this thesis a search for new phenomena is performed using 139 fb−1 of pp collision data provided by the Large Hadron Collider (LHC) and collected by the ATLAS Experiment, aiming to identify light neutral particles that decay outside the innermost region of the detector. Such exotic decays yield collimated bundles of SM leptons or quarks, identified exploiting dedicated triggers, custom reconstructed objects and discriminated against the SM and non-collision background with sophisticated deep-learning-based classifiers. The results of the search show no disagreement with the expected backgrounds and are used to constrain simplified models that predict the existence of long-lived particles. To further extend the constraint put by the searches for visible decays of long-lived particles, the result of an ATLAS search for events with large energetic jets and missing transverse momentum is reinterpreted in terms of two different models. Thanks to this approach, the case where such exotic particles are recoiling against an initial-state-radiation jet and leave ATLAS without being detected can be constrained. Finally, this thesis will cover 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 High-Luminosity LHC. Studies related to the firmware logic and to the radiation-tolerance tests of the on-detector electronic boards are presented.