Search for new invisible particles in events with a Z boson at CMS

This thesis documents a search for new particles at the Compact Muon Solenoid experiment (CMS) at the CERN Large Hadron Collider (LHC). Proton-proton collisions with reconstructed Z bosons are analyzed for evidence of the production of new unreconstructed (“invisible”) particles. The search strategy is centered around the use of the missing trans-verse momentum $p$$_{T}^{miss}$, which provides an indirect handle on the kinematic properties of particles that are not reconstructed directly in the detector. The production of invisible particles would be detected as an excess of events with large $p$$_{T}^{miss}$ over the known background processes. Analysis results are obtained based on the data sets collected by the CMS collaboration in the first half of the $Run-II$ period of LHC operation in 2015 and 2016. The 2015 dataset, corresponding to an integrated luminosity of 2.3 fb$^{−1}$, provides a first, coarse-grained glimpse of physics at the world record center-of-mass energy of 13 TeV. Although relatively small in size, the high collision energy allows to rival the sensitivity achieved in previous, larger data sets at lower energies. In 2016, a larger data sample of 35.9 fb$^{−1}$ could be recorded, providing additional statistical precision and sharpening the view of the high-energy landscape. The search for invisible particles is performed in both of these milestone data sets. After accounting for contributions from known backgrounds, no significant signal is observed in either sample. A special focus is set on the interpretation of the experimental results in a number of models for the production of invisible particles. As part of this thesis, results from the Z+$p$$_{T}^{miss}$ topology are for the first time interpreted in terms of $simplified$ $models$ of dark matter (DM) production, which are more robust than the previously used effective field theories. As a significant improvement over the interpretations provided in previous results ,a stronger focus is put on simplified models with an extended scalar sector, for which the Z+$p$$_{T}^{miss}$ signature provides competitive sensitivity. In a scenario with a second Higgs doublet and a pseudoscalar dark matter mediator, new portions of parameter space can be excluded for the first time. In addition to models of DM production, the analysis results are interpreted in terms of production of scalar unparticles, as well as gravitons in a scenario of large extra dimensions. In the case of unparticle production, significant errors have been discovered in the literature, leading to a revised view of the sensitivity of the Z+$p$$_{T}^{miss}$ topology, which is smaller than previously thought. Nevertheless, the unparticle interpretation derived here provides leading sensitivity in parts of the parameter space. Beyond the analysis of Run-II data, a study of the future analysis sensitivity at the high-luminosity LHC (HL-LHC) is performed. It is expected that the HL-LHC will provide a final proton-proton data set corresponding to 3 ab$^{-1}$ with √ s = 14 TeV over the next two decades. The effects of increased center-of-mass energy and integrated luminosity, as well as $p$$_{T}^{miss}$ reconstruction performance are studied in detail. It is found that an analysis of this expected data set will result in a mass reach which is increased by approximately a factor of two compared to the present-day sensitivity. Notably, the large data set would enhance the sensitivity so significantly that a number of signals that are not probed today could reach discovery sensitivity at the end of the HL-LHC program.