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@PHDTHESIS{Albert:762542,
      author       = {Albert, Andreas},
      othercontributors = {Hebbeker, Thomas and Wiebusch, Christopher},
      title        = {{S}earch for new invisible particles in events with a {Z}
                      boson at {CMS}},
      school       = {RWTH Aachen University},
      type         = {Dissertation},
      address      = {Aachen},
      reportid     = {RWTH-2019-05664},
      pages        = {1 Online-Ressource (viii, 183 Seiten) : Illustrationen,
                      Diagramme},
      year         = {2019},
      note         = {Veröffentlicht auf dem Publikationsserver der RWTH Aachen
                      University; Dissertation, RWTH Aachen University, 2019},
      abstract     = {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 pTmiss, 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 pTmiss 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+pTmiss topology are for the first time interpreted in
                      terms of simplified models of darkmatter (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+pTmiss 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+pTmiss
                      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 3-1
                      ab with √ s = 14 TeV over the next two decades. The
                      effects of increased center-of-mass energy and integrated
                      luminosity, as well as pTmiss 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.},
      cin          = {133110 / 130000},
      ddc          = {530},
      cid          = {$I:(DE-82)133110_20140620$ / $I:(DE-82)130000_20140620$},
      typ          = {PUB:(DE-HGF)11},
      doi          = {10.18154/RWTH-2019-05664},
      url          = {https://publications.rwth-aachen.de/record/762542},
}