Novel GEM detectors for CMS and exotic physics searches at the HL-LHC Article Swipe

The LHC at CERN will be upgraded to become the HL-LHC during the first half of the2020s. The significant increase in instantaneous luminosity allows probing particle physics with even greater sensitivity. At the same time, the physics environment will become even harsher, compared to the present LHC conditions. To cope with these conditions, the CMSdetector at the LHC needs to be upgraded. This thesis focuses on the transition of CMS tothe HL-LHC era. Two main topics will be discussed: 1) novel GEM detectors for the CMSdetector upgrades and 2) innovative strategies for displaced muon searches at the HL-LHC. For the installation in the first endcap stations of CMS (GE1/1), large-size triple-GEM detectorshave been mass-produced for the first time in high-energy physics. The CMS GEMgroup was able to assemble, test, and qualify the needed 144 chambers. The integration ofGE1/1 detectors in CMS is currently taking place. The GE1/1 GEM chambers show acceptablebehavior in terms of gas tightness and high-voltage integrity. The gas gain variations across the large detector area are found to be non-negligible. However, they do not exceed acceptable levels. To understand the origin of these variations, additional gas gain measurements with X-rays were performed while varying environmental and design parameters. Different sources of gain variations have been identified, listed in decreasing importance for the GE1/1 GEM chambers: induction field, drift field, thickness of GEM foil, contamination to nominal gas mixture. For the production of large-size GEM foils, the single-mask etching technique is used, as opposed to the double-mask technique. The influence of the etching technique on the gas gain is found to be non-negligible. The double-mask foils show the highest gain. The orientation of the single-mask foils with respect to the drift and inductionfields matters for the gas gain of the detector. To deepen the understanding of signal formation in a triple-GEM detector, microscopic simulations of the electron avalanches inside the complex, active volume were performed. The best configuration of hole pitch and diameteron the GEM foil with respect to the effective gas gain is the symmetric double-mask foil. In addition, the sensitivity of a physics study for displaced muons at the HL-LHC is discussed in this thesis. The HL-LHC offers a unique possibility to search for exotic, rare signatures.A possible search would significantly profit from the usage of dedicated tools for triggering, reconstruction, and analysis techniques. The study is constructed in a signature-drivenand model-independent way to be sensitive to long-lived particles decaying to muons from different beyond the standard model physics. For the smuon interpretation within GMSB SUSY models, a search is sensitive to masses around 100 GeV. In the case of DarkSUSY models, the study is sensitive to the discovery of a hypothetical dark photon with$10 \leq m_{\gamma_D}/GeV \leq 45$ and $10^8 < \epsilon < 10^7$, where $\epsilon$ gives the strength of the kinetic mixing between the standard model photon and the dark photon. This addresses a phase-space of dark photon models not yet covered by existing searches.