FROM DARK MATTER TO THE DARK SECTOR: SEARCH FOR NEW PHYSICS IN FINAL STATES WITH A PHOTON AND MISSING TRANSVERSE MOMENTUM, AT THE ATLAS EXPERIMENT

The Dark Matter (DM) problem is one of the main open questions in physics, and an hint of the incompleteness of the Standard Model (SM) of elementary particles. Among the several proposed DM models, one of the most explored DM candidate is the WIMP (Weakly Interacting Massive Particle), while an appealing scenario which is gaining growing interest, predicts the existence of a full Dark Sector, with its own internal gauge structure and particle content. This thesis focuses on WIMP and Dark Photon searches in proton-proton collisions at 13 TeV at the Large Hadron Collider (LHC), using the 139 fb-1 of full Run-2 data collected by the ATLAS experiment between 2015 and 2018. Final states involving a photon and missing transverse momentum (ETmiss), an imbalance in the total transverse momentum of the final state due to undetected particles, are considered, the latter being a potential signature of DM or Dark Sector particles. The first search targets mono-photon final states (photon+ETmiss), interpreting the results in terms of pair-production of weakly interacting DM candidates via an s-channel decay of an axial-vector or vector mediator. The second part of this work present two searches for a Dark Photon (H->yyd) (predicted as the gauge boson of a new U(1) symmetry group of the Dark Sector) produced via the decay of a (SM or BSM) Higgs boson, . The search is performed, at first, targeting Higgs boson production in association with a leptonically decaying Z boson (), and then reinterpreting the mono-photon analysis in terms of the same decay for a BSM Higgs with masses between 400 GeV and 3 TeV, in the gluon-gluon fusion and Vector Boson Fusion production modes. These analyses are part of a huge effort carried on within the ATLAS collaboration, towards the understanding of DM nature. Collider searches have a key role in this field, ensuring good complementarity with Direct Detection and Indirect Detection strategies, which is of fundamental importance to extend the possible reach of research in probing the extremely wide range of possible DM models and masses.