## Bisherige Themen

Dies sind die Titel und Abstracts der Bachelorarbeiten, die bisher im Going Abroad Programm entstanden sind.

## Inbetriebnahme eines laserbasierten Testsystems für CMS Phase-1 Pixelsensoren

To further increase the accuracy and efficiency of the CMS Inner Tracker a new generation (Phase-2) of pixel sensors is required. However, new approaches must first be validated on the current sensor generation (Phase-1) with test systems.

This bachelor thesis commissions such a laser-based test system, which can perform the Alive and the S-curve test as well as the Trim calibrations. Furthermore, the use of the laser enables the spatially resolved simulation of particle radiation without exposing the sensor to high-energy radiation. With a sample sensor and the software pxar the required functionalities are implemented, and the focal point of the laser is determined.

During the analysis of the sample’s IV-characteristic, an unexpected hysteresis was measured, most probably caused by thermal effects. Furthermore, examination of the laser focal point with Gaussian optics yields a focal length of $9 \,\mathrm{mm}$. However, the determination of the focal length is limited by the pixel size and charge sharing effects. Nevertheless, the results indicate that the setup can perform tests and calibrations that could be used in the development process of the next generation of pixel sensors. In addition, a future upgrade could offer the possibility to test also manufactured Phase-2 pixel sensors.

## Kritische Schichtanalyse der Unterstützungsstruktur von CMS Phase-2 Pixelsensoren

The following bachelor thesis discusses a thermal analysis of the support and readout structure of Phase-2 pixel detectors from the Inner Tracker of the CMS detector. The structure is analyzed with a finite element analysis in ANSYS. The model is simulated with different heat loads in the heat-generating parts and the temperature distribution is discussed. Besides, there a critical layer analysis on the different layers of the model is performed. Criteria for the criticality of a material layer are derived and applied to the model. The results show that the current detector design is sufficient to achieve temperatures below $-20\,^\circ\mathrm{C}$.

## Study of the Sensitivity of Top Quark Spin Correlations to Supersymmetric Top Quark Partners

The study discussed in this thesis considers the hypothetical production of top squarks at hadron colliders, with the assumption that top squarks will always decay into top quarks and neutralinos of a small mass of approximately $0\,\mathrm{GeV}$. Targeting the dileptonic final states, the $\Delta{\phi}$ distribution between the two charged lepton final products in the laboratory frame are used to set limits on the production of top squarks of various masses. Limits are first derived from the results of the CMS measurement using $35.9\,\mathrm{ fb }$ of data recorded at the LHC in 2016, with a center of mass energy of $13\,\mathrm{TeV}$. Then, an extrapolation is made by scaling the 2016 data to a higher luminosity of $300\,\mathrm{fb^{-1}}$ and reducing the systematic uncertainties. It is shown that no exclusion on the top squark range can be made for the combination of a top squark mass range of $160\,\mathrm{GeV}$ to $300\,\mathrm{GeV}$ with a fixed small neutralino mass almost equal to zero.

## Studies on event selection criteria for the non-resonant $HH\rightarrow b\bar{b}b\bar{b}$ decay channel

In this thesis, methods of event selection for non-resonant Di-Higgs production in proton-proton collisions are discussed and a comparison of the different trigger paths for the Compact Muon Solenoid (CMS) and ATLAS experiment is made.

An event selection based on selection criteria used by ATLAS experiment is applied to Monte Carlo simulated data for the CMS experiment, with a center-of-mass energy of $\sqrt{s}={13}\,\mathrm{TeV}$ and an integrated luminosity of $36\,\mathrm{fb^{-1}}$ , that only contains Di-Higgs events that decay exclusively via the dominant channel $H\!H\rightarrow b\bar{b}b\bar{b}$ . To determine the efficiency and purity, selection criteria are applied on the Monte Carlo simulated data sample, and the result is compared with the generator level information of the simulation.

Furthermore, the background rejection provided these criteria, namely the dominant QCD-multijet background and the sub-dominant $t\bar{t}$ background, are studied. Based on the expected signal and background rates, experimental upper limits for the non-resonant Di-Higgs cross section are studied.