Search for semileptonic vector boson scattering with the ATLAS Detector using LHC Run-II data

Considerations about unitarity violating vector boson scattering (VBS) processes were one of the main motivations for the Higgs mechanism. Only due to the precise cancellation of such processes with interactions involving the Higgs boson is unitarity restored within the Standard Model of particle physics. This intricate interplay makes the measurement of vector boson scattering an especially potent test for the Standard Model and an excellent place to search for new contributions beyond it. The presented analysis is a search for the electroweak production of a pair of vector bosons in association with a pair of jets with typical characteristics of vector boson scattering processes. The search is performed in the semileptonic channel with a 0-lepton final state and uses the full Run-II dataset of the ATLAS detector recorded at a center of mass energy of 13 TeV. Within the analysis vector boson tagging algorithms are used for the identification of hadronic decay structures with the help of jet substructure variables. Dedicated studies on such algorithms are presented. Using novel jet definitions potential improvements with respect to efficiency are shown. Potential methods of decorrelating the tagger decision from jet mass are studied. The characteristic decay structure of vector boson scattering processes yields jets in the forward detector region. The ATLAS ITk detector, a planned upgrade of the inner tracking detector, will extend the acceptance in these regions far beyond the current reach. Measurements on prototype sensors for the ITk are presented. They include geometrical, electrical, as well as hit efficiency measurement in an electron test beam setup. Prospects on how the ITk and further work on vector boson tagging could help to enhance the precision of future VBS searches are discussed.