Extending the Reach of Collider Searches for Dark Matter

The Standard Model (SM) has done an excellent job at predicting the outcomes of experiments, but many questions remain unanswered. Neutrino masses and the existence of dark matter (DM) are two of these, both of which are not included in the Standard Model. The particle physics community, together with the rest of the world, faces difficult decisions and obstacles presented by the increasing threat of climate change. Making the most of the experiments, which have already been built, is therefore an important endeavour for exploiting all possibilities of New Physics (NP). This thesis presents two projects which are opportunities to extend the reach of two existing particle colliders in the search for dark matter. The searches for dark matter particles at the ATLAS detector at the LHC have a limited reach for long-lived Particles (LLPs) due to the finite width of the detector. The ANUBIS detector will extend these searches through measuring decays in the cavern above ATLAS. SET-ANUBIS sets out to calculate the sensitivity projections for the detector for various long-lived particle models. In this thesis, the development of the sensitivity study, with the considerations and choices made, are presented. The specific case of heavy neutral leptons is used for the development and is discussed. Considering a wider dark sector for which an invisible state is the portal, the production of the invisible state $X$ together with a photon, $e^+e^- \to \gamma +X$, at electron positron colliders is one of the strongest searches for $m_X \in [0.1,10]~$GeV. This thesis presents measurement strategies that can detect the spin of the invisible state as well as the underlying production mechanism. Based on the angular distribution of the final state photon, the cross-sections for polarised initial states and the photon polarisation, the measurement strategy can be used to identify whether the invisible state is a dark photon or an axion-like particle (ALP). The results are compared with a detailed analysis of the Standard Model background, and the sensitivity reach for searches for axion-like particles and dark photons at Belle II are calculated. It is found that the sensitivity of the ALP coupling the electrons can be improved when both incoming beams are polarised

How to measure the spin of invisible states in e + e − → γ + X

We examine the production of an invisible state X together with a photon, eþe− → γ þ X, at electron positron colliders and present measurement strategies that can detect the spin of the invisible state as well as the underlying production mechanism, based on the angular distribution of the final-state photon, the cross sections for polarized initial states, and the photon polarization. Our measurement strategy can be used to identify whether the invisible state is a hidden photon or an axion. The results are compared with a detailed analysis of the Standard Model background, and we calculate the sensitivity reach for searches for axionsand hidden photons at Belle II