Search

Given the omnipresence and economical availability of silicon semiconductor processes, it is no surprise that direct detection particle pixel detectors are commonly fabricated using these processes. However, silicon is not the ideal sensor material for certain applications, such as X-ray imaging, due to its low stopping power of higher energy particles. For these applications, high-Z semiconductor materials, e.g., GaAs, SiGe and CdZTe, enable a more efficient conversion of incident particles into electrical signals. For economic reasons, it is not feasible to implement the highly integrated detector electronics in such high-Z materials. Therefore, in order to build detectors using these materials, one has to combine a high-Z sensor layer with the readout electronics implemented in silicon. Traditionally, such hetero-detectors are built using bump bonding, which connects the sensor and the readout electronics using a grid of small solder joints. This thesis investigates the use of surface activated wafer bonding as an alternative technique to build hetero-structure detectors. This low-temperature covalent bonding technique allows to directly fuse different semiconductor layers without any additional materials at the bonding interface. This enables the creation of hetero structures by directly bonding a high-Z sensing layer to a silicon-based readout layer. Imposed by the bonding technique, the transition between the two bonded layers is not abrupt, but includes a thin (nm) layer of amorphous material. This amorphous region has a high concentration of crystalline defects and thus influences the electrical behaviour of the assembled detectors. Previous studies proposed that the effect of these defects could be minimized by placing the P/N junction of the detector at the bonding interface. In order to investigate this claim, covalently bonded pad diodes are fabricated by bonding high resistivity P- to N-type silicon wafers as commonly used in pixelized particle detectors. TEM measurements of these samples show the formation of a ca. 3nm wide amorphous layer at the bonding interface. EDXS measurements of the samples highlight the presence of additional metal contaminants, which can be traced back to the bonding process. The feasibility of using such structures for particle detection is investigated based on TCT measurements. The TCT measurements show a one-sided depletion behaviour of the samples, whereas only the P-type wafer can be depleted. This one-sided depletion indicates that the bonding interface of the fabricated samples behaves like a highly doped N++ layer, strongly influencing the electrical behaviour of the sensor structure. Based on this observation, a model, which includes an extension to the Shockley-Ramo theorem, is developed to predict the time domain signal shape produced by particles interacting in the fabricated samples. This model is shown to accurately predict the signals measured with an Am-241 alpha-source from first principles.

Associated production of the Higgs boson with a top quark-antiquark pair (ttH) provides the best direct probe of the top-Higgs Yukawa coupling at tree-level, and increasingly precise measurements of its production cross section are a major ongoing thrust at the CMS experiment. They are being performed in several decay channels for the Higgs boson. The H to bb decay channel is particularly interesting and promising, as it has the largest branching fraction. Optimization of data inputs and estimation of backgrounds for measurements of ttH production with H to bb are the topics of this thesis. In 2019, the CMS collaboration presented evidence of ttH, H to bb using 2016 (35.9 1/fb) and 2017 (41.5 1/fb) data. Contributions to this measurement in terms of development of new triggers are described. A more precise measurement using the full Run-2 data (2016–2018 : 137.2 1/fb) is underway. One particularly challenging background limiting the precision of this important measurement arises from direct tt + bb production. A novel estimation method for this background and the resulting improvement expected in the ttH, H to bb measurement are presented. Finally, improvements and optimization of operation and data acquisition of the CMS pixel detector will be described, which enhance the b-tagging capability, crucial for ttH, H to bb measurements.

The data collected by the CMS experiment at CERN in proton-proton collisions during 2016 are analyzed to characterize the Higgs boson in its decay channel to two photons. The signal strengths relative to the standard model cross section are measured inclusively and for individual production modes, employing an event categorization optimized for the best precision. Production cross sections are then measured in a fiducial phase space, differentially with respect to the Higgs boson production and decay kinematic properties. The cross section is also measured inclusively in the fiducial phase space, as well as in sub-regions enriched in events originating from specific production modes (ttH, VH). For these measurements, an approach aiming at minimizing the model dependence of the measurements is adopted. The measurements are found to be in agreement with the standard model predictions, within experimental and theoretical uncertainties. A new upgraded pixel detector has been installed in the CMS experiment in early 2017. The upgraded detector, while introducing several improvements with respect to the previous system, allows highly-efficient data taking at the increasing instantaneous luminosities of the LHC. The procedures designed to thoroughly test and calibrate the 1200+ detector modules are presented in this thesis, together with the quality criteria defined to identify modules suitable for installation in the final system. The results of the module production are reviewed, with a focus on modules employed for the construction of the second layer of the central part of the detector.

The present dissertation documents contributions to two complementary aspects of high- energy particle physics research with the Compact Muon Solenoid (CMS) experiment at the Large Hadron Collider (LHC). The work includes an analysis of proton-proton collisions to search for physics beyond the standard model (SM) and radiation tolerance assurance studies conducted in the context of the development of a new pixel detector. The first part of the thesis describes a search for so-far undiscovered physics processes, which are predicted for example by supersymmetric extensions of the SM. To this end, events with at least three charged leptons as well as hadronic jets and missing transverse momentum are analyzed at the unprecedented center-of-mass energy of 13 TeV. The study is based on a data set corresponding to an integrated luminosity of 12.9 fb^-1 of proton-proton collisions that has been recorded with the CMS detector in 2016. The examined final state features very low SM background contributions. The resulting high sensitivity for different beyond the SM processes can be further enhanced by categorizing events of interest into 32 exclusive signal regions. Reducible and irreducible SM background contributions to these signal regions are estimated with data and Monte Carlo simulations, respectively. The observed data are found to agree with the SM predictions within the assigned uncertainties. The results are therefore interpreted in terms of exclusion limits on masses of supersymmetric particles in the context of simplified model topologies that feature gluino pair production and a neutralino as the lightest supersymmetric particle (LSP). In a model producing four top quarks and two LSPs, gluino masses up to 1 200 GeV could be excluded, extending the exclusion limit set by a similar search at sqrt{s} = 8 TeV by about 200 GeV. Gluino masses up to 1 000 GeV could be excluded in a model with light-flavor jets, vector bosons, and two LSPs in the final state. The second part of the thesis is dedicated to irradiation studies with the new readout chip (ROC) for the Phase I Upgrade of the CMS pixel detector. The replacement of the pixel detector with an improved version has been completed in February 2017 and aims at main- taining and improving the performance of the detector at increased instantaneous luminosities of up to 2 × 10^34 cm^-2 s^-1 . One of the key components of the new detector is a revised ROC, which is used in the forward part and in layers 2-4 of the central detector, designed for hit detection efficiencies above 99% at pixel hit rates up to 120 MHz/cm^2 . Because of the close proximity to the collision point, the new ROC is faced with stringent demands in terms of tolerance against ionizing radiation damage. Studies conducted in the context of this disser- tation significantly contributed to improving the radiation tolerance of the chip during its development phase. Moreover, it could be confirmed that the performance of the final ver- sion of the chip meets the design specifications even after accumulating the expected lifetime doses for layers 2 and 1 of 0.5 MGy and 1 MGy, respectively. Several important performance figures of the ROC have been shown to comply with the specifications even after heavy irradi- ation of up to 4.2 MGy. The thesis not only describes comprehensive functionality tests after irradiation, but also provides recommendations for dose dependent adjustments of operation parameters that will be necessary to optimize the performance of the chip in the experiment.

The multi- TeV e+e− Compact Linear Collider (CLIC) is one of the options for a future high-energy collider for the post-LHC era. It would allow for searches of new physics and simultaneously offer the possibility for precision measurements of standard model processes. The physics goals and experimental conditions at CLIC set high precision requirements on the vertex detector made of pixel detectors: a high pointing resolution of 3 μm, very low mass of 0.2% X0 per layer, 10 ns time stamping capability and low power dissipation of 50 mW/cm2 compatible with air-flow cooling. In this thesis, hybrid assemblies with thin active-edge planar sensors are characterised through calibrations, laboratory and test-beam measurements. Prototypes containing 50 μm to 150 μm thin planar silicon sensors bump-bonded to Timepix3 readout ASICs with 55 μm pitch are characterised in test beams at the CERN SPS in view of their detection efficiency and single-point resolution. A digitiser for AllPix, a Geant4-based simulation framework, has been developed in order to gain a deeper understanding of the charge deposition spectrum and the charge sharing in such thin sensors. The AllPix framework is also used to simulate the beam telescope and extract its tracking resolution. It is also employed to predict the resolution that can be achieved with future assemblies with thin sensors and smaller pitch. For CLIC, a full coverage of the vertex detector is essential while keeping the material content as low as possible. Seamless tiling of sensors, without the need for overlaps, by the active-edge technology allows for extending the detection capability to the physical edge of the sensor and thereby minimising the inactive regions. Thin-sensor prototypes containing active edges with different configurations are characterised in test-beams in view of the detection performance at the sensor edge. Technology Computer-Aided Design (TCAD) finite- element simulations are implemented to reproduce the fabrication and the operation of such devices. The simulation results are compared to data for different edge terminations.

This work involves an analysis of data recorded at the Large Hadron Collider combined with a program to develop detectors for future collider experiments. Using the full 4.9 fb$^{-1}$ of 7 TeV data collected in 2011 and the 19.2 fb$^{-1}$ of 8 TeV data collected in 2012, the $B_c(2S)$ meson has been observed with the ATLAS detector in the hadronic decay mode $B_c(2S)\rightarrow B_c\pi^{+}\pi^{-}$, $B_c\rightarrow J/\psi\pi$. This new state has been found in the mass difference distribution with invariant mass $6845 \pm 7_{stat.} \pm 4_{syst.}$ \MeV. To prepare for the high radiation environment at the High Luminosity LHC, diamond sensors are being developed. Their leakage current and resistivity are measured at fluences and temperatures relevant to the ATLAS upgrade. No evidence of dependence of the resistivity on fluence or temperature has been observed for the ranges [-10 $^\circ$C, +20 $^\circ$C] and [0, $1.0\ imes10^{16} \rm{n_{eq}/cm^2}$]. To study the radiation damage of the sensors in the ATLAS Pixel Detector, their leakage current is monitored at the level of single pixel modules. The result agrees with a prediction based on the Hamburg model.

A search for supersymmetry in a sample of proton-proton collision events with a center-of-mass energy of 8 TeV is presented. The sample, collected with the Compact Muon Solenoid detector at the Large Hadron Collider, corresponds to an integrated luminosity of 19.4 fb[-]1 . Events are required to have large missing transverse energy, at least three jets, and at least one identified bottom-quark jet. Numbers of events in exclusive bins of the scalar sum of jet transverse momentum values, missing transverse energy, and bottom-quark jet multiplicity are found to be consistent with the standard model expectations. The results are interpreted as 95% confidence level upper limits on simplified supersymmetric models approximating gluino mediated bottom- and top-squark production.