Search for New Physics in Rare Higgs Boson Decays with the CMS Detector at the Large Hadron Collider

A new boson with a mass of 125 GeV was discovered at the large hadron collider (LHC) in July 2012. The properties of this particle are so far consistent with the standard model (SM) expectation. Differences in the Higgs boson decay rates and predicted by the SM might indicate the presence of new particles and forces between them. Particularly, rare exclusive decays of the Higgs boson are a promising laboratory to study physics beyond the standard model. Searches for decays of the Higgs boson into a Z boson and a J/ψ meson or into pairs of J/ψ or Υ mesons are performed for using a proton proton collision data set collected by the compact muon solenoid (CMS) experiment at the LHC at a center of mass energy of √s = 13 TeV that corresponds to an integrated luminosity of about 137 fb⁻¹. This class of rare Higgs boson decays can challenge the SM of particle physics. The branching fraction of these decays can be enhanced by the beyond SM particles or phenomena such that they could be observed in the LHC. Higgs decay candidate events with Z bosons decaying into an electron or muon pair, or with quarkonium resonances decaying into muon pairs are selected using online event filters. Longitudinal polarization is expected for the Z boson and assumed for the decay mesons. As different polarization states affect the signal acceptance, scenarios with uniform and transverse polarization are considered. No significant excess is observed, and therefore upper limits at 95% confidence level are placed on the branching fractions of these decays. The observed upper limit branching fractions at 95% CL for the Higgs boson decaying into the final states under investigation range from 10⁻⁴ to 10⁻³. The decay of the Z boson into J/ψ or Υ pairs is also searched for and found to be 10.8 × 10⁻⁷ for Z → J/ψJ/ψ and 3.9 × 10⁻⁷ for Z → Υ(nS)Υ(mS) (n, m = 1, 2, 3). Furthermore, higher mass quarkonium decays are included in the searches via their inclusive decay into the reconstructed ground state quarkonia. The high luminosity LHC is expected to reach Higgs boson decay branching fraction upper limits at values of about 10⁻⁵, about factors of 1 to 10 above the SM predictions, but expected by several extension of the SM.

Luminosity measurements are crucial for the physics program of the CMS experiment. The pixel luminosity telescope (PLT) provides input for integrated luminosity and real-time feedback on instantaneous luminosity. The instrument must maintain stable operational conditions over a long time period. This stability can be monitored with the measured position of the collision points or beamspot. From the raw data collected by the PLT, tracks are reconstructed from hits in the three silicon detector planes. Corrections for the alignment of the planes within the telescopes are applied. These tracks are extrapolated to the center of the CMS detector to obtain an estimate of the beamspot position. This beamspot position remained within a radius of 300 μm for most of the proton proton collisions recorded during the year 2016. Beamspot positions outside this circle are used as indicators for adverse beam conditions.

The final version of the dissertation thesis