The Hunt for Physics Beyond the Standard Model: Dark Matter, Invisible Higgs Boson Decays, Large Extra Dimensions and Unparticles in pp Collisions at √s = 13 TeV

The standard model of particle physics is a theory that describes the fundamental structure of matter in the universe, as well as providing a classification for all known elementary particles and their quantum properties. While dark matter has not been directly observed, its existence is implied from astrophysical evidence that cannot be explained by our current understanding of gravity unless more matter is present than is observed. This dissertation explores several models that provide a coupling mechanism between dark matter and ordinary matter. These searches are performed using events with a Z boson candidate and large missing transverse momentum. This dissertation is based on proton-proton collision data at a center-of-mass energy of 13 TeV, collected by the CMS experiment at the LHC in 2016–2018, corresponding to an integrated luminosity of 137 fb−1 . The search uses the decay channels Z → ee and Z → µµ. No significant excess of events is observed over the background expected from the standard model. Limits are set on dark matter particle production in the context of simplified models with vector, axial-vector, scalar, and pseudoscalar mediators. For the simplified models, the results are translated to provide limits for spin-dependent and spin-independent scattering cross sections and are compared to those from direct-detection experiments. In addition, results are interpreted for the two-Higgs-doublet model with an additional pseudoscalar. An upper limit is set on the branching fraction of invisible decays of the Higgs boson. Outside the context of dark matter, models that invoke large extra dimensions or propose the production of unparticles could contribute to the same signature and the observed limits on the production cross sections are used to constrain the parameters of these models.