Search for High-Mass Diphoton Resonances in Proton-Proton Collisions at 13 TeV and Radiation Studies for Calorimetry at the High-Luminosity LHC

In this dissertation, two different topics are addressed which are part of the main areas of research of modern high-energy physics experiments at the Large Hadron Collider (LHC): a search for new physics and the development of new detectors. The first part of this dissertation presents the search for high mass diphoton resonances in proton-proton collisions at a center-of-mass energy of 13 TeV with the Compact Muon Solenoid (CMS) experiment. Particular attention is paid to the assessment of the background. The results are interpreted in terms of spin-0 and spin-2 resonances with masses between 0.5 and 4.5 TeV and widths, relative to the mass, between 1.4x10^(-4) and 5.6x10^(-2). Limits are set on scalar resonances produced through gluon-gluon fusion, and on Randall–Sundrum gravitons. Two results are presented, both following the same search strategy, but one employing a dataset of 3.3 1/fb, the other 16.2 1/fb. Both are statistically combined with results obtained by the CMS collaboration at 8 TeV with 19.7 1/fb. For the combination with the dataset of 3.3 1/fb, a modest excess of events compatible with a narrow resonance with a mass of about 750 GeV and a global significance of 1.6 standard deviations is observed. This excess could not be confirmed with the larger dataset of 16.2 1/fb. The production of RS-gravitons is excluded at leading order at a 95% CLs up to 3.85 and 4.45 TeV for coupling parameters of 0.1 and 0.2, respectively. These are the most stringent limits on Randall-Sundrum graviton production to date. For the High-Luminosity LHC (HL-LHC), the forward electromagnetic calorimeter (ECAL) of CMS has to be replaced. A sampling calorimeter, using an inorganic scintillator as an active medium, was one suitable option. In the second part of this dissertation, Monte-Carlo simulations with the particle-physics toolkit FLUKA determine aspects of the behavior of such a sampling calorimeter in the radiation environment of the upgraded CMS detector at the HL-LHC. Measurements performed for LYSO, YSO and cerium fluoride crystals, exposed to a proton fluence of up to 5x10^14 cm^2, are compared to dedicated FLUKA simulations. The main drivers of their residual dose are determined. It is found that LYSO and cerium fluoride crystals show similar levels of residual dose as lead-tungstate. Based on these results, an extrapolation to the behavior of the sampling calorimeter, located in the CMS detector, is performed. Characteristic parameters such as the induced ambient dose, fluence spectra for different particle types and the residual nuclei are studied, and the suitability of these materials for a future calorimeter is surveyed. Particular attention is given to the creation of isotopes in an LYSO-tungsten calorimeter that might contribute a prohibitive background to the measured signal. The harsh radiation environment in CMS induces high levels of radioactivity in the exposed materials. The radiological hazard to personnel during the upgrade of the electronic system of the central part of ECAL for the HL-LHC is quantified.