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The subject of this thesis is the development and test of silicon strip detectors for the high luminosity upgrade of the tracking detector of the ATLAS experiment at the Large Hadron Collider. Special emphasis is devoted to the understanding of the impact of mechanical stress on the electrical properties and the particle detection performance of detector modules.
First simulations were done to estimate the maximum expected stress on a sensor when operated at -30 °C within the future silicon strip tracking detector ITk, at ATLAS. The maximum stress in a worst case scenario is expected to be 27 MPa. Tensile strength tests were done to estimate the maximum stress which can be applied to a silicon strip sensor. Silicon shards, with a thickness and dopand concentration corresponding to the ITk sensor specifications break at >23 MPa, wafers at >700 MPa and sensors at ~ 400 MPa. The huge variations lead to the assumption, that the tensile stregth, which is highly dependent on the quality of the crystal lattice, is due to different cutting technology. Wafers, irradiated with a fluence equivalent of a lifetime dose of an ITk sensor, show no stress dependency of the youngs modulus. The tensile strength of irradiated wafers is decreased by ~ 6,6 %. No damage on silicon sensors from mechanical stress is expected for sensor modules installed it the ITk.
The electrical properties of silicon strip sensors were studied for applied mechanical stress on ATLAS07 sensors up to 60 MPa. The specifications of the sensors are similar to the specification of strip sensors in the future silicon strip tracker barrel region of the ATLAS detector. The leakage current changes at 50 MPa by -1.7 %, the bias resistance by +0.8 % and the interstrip resistance by -25 %. The depletion voltage and the implant resistance are not affected by mechanical stress. Except for the interstrip resistance the results can be explained by piezoresistive effects.
Silicon strip modules were build and studied in particle test beams. These modules consists of an ATLAS07 or an ATLAS12 sensor and an analogue readout to study the influence of stress on the module performance. The sensor module noise is independent from the applied stress. An effect of stress on the signal strength was seen. The ATLAS07 sensor module signal strength was decreased and the ATLAS12 sensor module signal strength was increased with a slope of ~0,6 MPa^-1. The average cluster size of the ATLAS07 sensor module was increased by 0,25 % MPa^-1 and the average cluster size of the ATLAS12 sensor module was decreased by 0,06 % MPa^-1 with applied stress.
Luminosity is a key quantity of any collider, since it allows for the determination of the absolute cross sections from the observed rates in a detector. Since the Higgs boson discovery in 2012, the highest priority at the Large Hadron Collider (LHC) has been given to an accurate understanding of the electroweak scale and a search for new physics. Precise luminosity measurements in such conditions are of crucial importance, as they determine the precision of any physics cross section measurement.
To increase the production of particles of low cross section the LHC is running at the highest possible luminosity. After the first Long Shutdown (LS1) the original performance goal for the luminosity of was reached with 1011 protons per bunch and a bunch spacing of 25 ns. In such conditions radiation hard detectors with extremely fast response time are required, especially for instrumentation near the beam.
The Compact Muon Solenoid experiment is equipped with three online luminometers, which fulfill the listed requirements: the Fast Beam Conditions Monitor (BCM1F), the Pixel Luminosity Telescope (PLT) and the Forward Hadron calorimeter (HF).
The BCM1F was upgraded during LS1 from 8 to 24 diamond sensors and is read out by a dedicated fast ASIC. The back-end comprises a deadtime-less histogramming unit, with 6.25 ns bin width and analog-to-digital converters with 2 ns sampling time in the VME standard. A microTCA system with better time resolution is in development. Because of its excellent time resolution BCM1F measures separately both luminosity and machine induced background particles.
The performance of the detector in the first running period and radiation damage monitoring of the sensors and electronics chain form the first part of this thesis.
Calibration of the luminometers at the LHC is done using van der Meer (VdM) scans. The proton beams are scanned against each other. The effective width of the beams is measured and the visible cross section, the key quantity for the luminosity measurement, is determined.
The impact of detector instability, beam-beam effects, correlations of the particle density distributions in the X and Y planes, and satellite and ghost bunches are studied in detail and systematic uncertainties are derived. A comparison of the VdM scans of 2015 and 2016 completes the second part of the thesis.
As a contribution to the upgrade of the beam instrumentation for the high luminosity LHC, a novel single crystal sapphire detector was designed, built and studied in a test-beam. The detector comprises a stack of sapphire plates. The response depends on the direction of the incident particles. The performance of the detector is described in the third part of the thesis. It is demonstrated that this sapphire detector can be used for the detection of single relativistic particles. The results point to the dominant contribution of the electrons to the signal generation in sapphire.
The LHC is the largest particle accelerator and storage ring in the world, used to investigate fundamentals of particle physics and to develop at the same time the technology of accelerators and detectors. Four main experiments (ATLAS, ALICE, CMS and LHCb) , located around the LHC ring, provide insight into the nature of particles and search for answers to as yet unexplained phenomena in the universe. Two proton or heavy ion beams circulate in the LHC and are brought into collision in the four experiments.
The physics potential of each experiment is determined by the luminosity, which is a ratio of the number of the events during a certain time period to the cross section of a physics process. A measurement of the luminosity is therefore essential to determine the cross section of interesting physics processes.
In addition, safe and high-quality data-taking requires stable beam conditions with almost no beam losses. Each experiment has its own detectors to measure beam losses, hereafter called machine induced background. One such detector is installed in CMS, BCM1F. Based on diamond sensors it was designed and built to measure both, the luminosity and the machine induced background. BCM1F ran smoothly during the first LHC running period from 2009-2012 and delivered valuable beam loss and luminosity information to the control rooms of CMS and LHC. At the end of 2012 the LHC was shut down to improve the performance by increasing the proton energy to 7TeV and decreasing the proton bunch spacing to 25ns. Due to the success of BCM1F an upgrade of its sensors and readout components was planned in order to fulfil the new requirements.
The upgrade of the sensors comprises a two pad instead of one pad metallization. 24 instead of the previous 8 single crystal diamond sensors were foreseen for the new BCM1F to enhance the robustness and redundancy. To instrument BCM1F, 59 sensors were electrically characterized by measuring the leakage current, signal stability and charge collection efficiency. Quality criteria were defined to select sensors for the final installation. An overview of these measurements including a summary of the results is given in this thesis. In addition, an upgraded amplifier was developed within the collaboration in 130nm CMOS technology. It has a peaking time of 7ns instead of the 22ns of the one previously installed. A BCM1F prototype comprising a two pad sensor and the upgraded amplifier was tested at the DESY-II accelerator in a 5GeV electron beam. Results of these test-beam measurements are presented in this thesis as well as simulations to interpret the measurements.
The installation of the upgraded BCM1F was completed in 2014. In 2015 BCM1F was commissioned and started to measure luminosity and machine induced background. At the end, the thesis will describe both types of measurements with the focus on machine induced background demonstrating the functionality of BCM1F.
The Higgs mechanism is responsible for the spontaneous breaking of the electroweak symmetry leading to the generation of masses of fermions and weak bosons. This thesis is devoted to search for the neutral supersymmetric Higgs particles Φ=h, H and A decaying into b quarks produced in association with at least one more b quark at the Large Hadron Collider. Here the study of the final states characterized by three b-tagged jets is described in detail. The analysis was performed using data corresponding to 2.7 − 4.8 fb^−1 integrated luminosity of pp collisions with a centre-of-mass energy of 7 TeV collected in 2011 with the CMS detector. Two analysis scenarios were adopted to perform a search for neutral Higgs bosons of low and medium masses, 90 GeV ≤ MΦ < 180 GeV and 180 ≤ GeV MΦ ≤ 350 GeV, respectively. Two-dimensional templates, built up from double-b-tagged data and based on the invariant mass of the two leading b jets and a variable reflecting b-jet properties of three leading jets, are used to model the background. The signal is modeled by templates obtained from Monte Carlo simulation. Various systematic effects affecting the signal efficiency and changing shapes of the signal and background templates were investigated.
The fitting machinery, based on a binned least-squares fit of the signal and background templates and the systematics model dependent on the hypothesized mass of the pseudoscalar Higgs particle A, was developed to extract the signal and background yields from the data. No significant evidence for the production of the Higgs bosons is found.
Using the CLS method, we set cross section times branching fraction upper limits at 95% confidence level (CL) on the production of such neutral Higgs bosons Φ in the mass range from 90 GeV to 350 GeV. The observed exclusion limits are well within the expected ±2σ band.
The benchmark scenario of the Minimal Supersymmetric Standard Model, denoted as mmax , with the two choices of the Higgsino mass parameter, μ = +200 GeV and μ = −200 GeV is considered. The obtained 95% CL upper limits on σ (pp → bb + Φ) × BR(Φ → bb) are interpreted as the upper limits on the MSSM parameter tan β . Ranges 27 ≤ tan β ≤ 51 and 22 ≤ tan β ≤ 37 for the masses of the Higgs boson from 90 to 350 GeV are ruled out at μ = +200 GeV and μ = −200 GeV, respectively. The 95% CL limits on tan β obtained in this channel supersede previous tan β limits established by CDF and D0 experiments.
The Large Hadron Collider (LHC) currently in operation intends to explore particle physics on the TeV scale. The International Linear Collider (ILC) and the Compact Linear Collider (CLIC) are being designed to measure the properties of particles possibly discovered at the LHC with high precision. Very forward detector systems at these machines are needed for the precise measurement of the luminosity and to approach full polar angle coverage. In the current detector concepts for linear collider detectors two electromagnetic calorimeters, Beam Calorimeter (BeamCal) and Luminosity Calorimeter (LumiCal), are foreseen. Both calorimeters are designed as sandwich calorimeters with tungsten absorber layers instrumented with finely segmented sensors. Due to a large amount of beamstrahlung remnants hitting BeamCal at the innermost radii, the sensors must withstand up to 1 MGy radiation dose per year. In this thesis two types of sensor materials were investigated: single crystal chemical vapour deposition diamonds (scCVDD) and gallium arsenide doped by chromium (GaAs:Cr). The very forward calorimeters ensure coverage for high energy electrons, positrons and photons down to very low polar angles. Within this thesis, simulation studies are presented for different beam parameters of the ILC. A new sensor segmentation was proposed to achieve better reconstruction efficiency of single high-energy electrons, positrons and photons on top of the beamstrahlung background. Only for a few years ago polycrystalline diamond sensors have been used for beam diagnostics in high-energy physics experiments. The Compact Muon Solenoid experiment, CMS, at the LHC is instrumented with several detectors for the Beam Conditions and Radiation Monitoring. The Fast Beam Conditions Monitor (BCM1F) is part of these systems. Here for the first time single crystal diamond sensors have been used. Eight detectors, comprising each a single crystal sensor and front-end electronics, are positioned around the beam pipe on both sides of the interaction region. They monitor the beam halo to protect the inner CMS detectors from adverse beam conditions and ensure high quality data for CMS. In this thesis, BCM1F data is evaluated for intrinsic time resolution and performance under harsh radiation conditions. Furthermore, it is investigated if it can be used for a bunch by bunch on-line luminosity measurement. The second type of sensor, made of GaAs:Cr, was produced in Tomsk State University and tested as a candidate for the BeamCal for future ILC and CLIC detectors. Several GaAs:Cr sensors were characterized in the laboratory for leakage current and capacitances and used for test beam investigations. Two sensors were assembled with a fan-out, front-end and ADC ASICs to build a fully functional prototype of a sensor plane. Several test beam campaigns were done to measure the performance of the system.