Experimentelle Physik IV

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http://hdl.handle.net/2003/44

Arbeitsgebiete

Experimente am CERN (NOMAD, ATLAS, HARP), Siliziumdetektorentwicklung

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Search for same-sign top quark pairs with the ATLAS detector at the LHC
(2023) Cavalli, Noemi; Kröninger, Kevin; Sioli, Maximiliano
In this thesis, a search for same-sign top quark pairs produced according to the Standard Model Effective Field Theory (SMEFT) is presented. The analysis is carried out within the ATLAS Collaboration using collision data at a center-of-mass energy of s = 13 TeV, collected by the ATLAS detector during the Run 2 of the Large Hadron Collider, corresponding to an integrated luminosity of 140 fb−1. Three SMEFT operators are considered in the analysis, namely ORR, O(1)LR, and O(8)LR. The signal associated to same-sign top pairs is searched in the dilepton channel, with the top quarks decaying via t → W+b → ℓ+νb, leading to a final state signature composed of a pair of hightransverse momentum same-sign leptons and b-jets. Deep Neural Networks are employed in the analysis to enhance sensitivity to the different SMEFT operators and to perform signal-background discrimination. No significant excess with respect to the Standard Model predictions is observed, therefore upper limits are placed on the signal production cross section and on the Wilson Coefficients associated to the considered EFT operators. The observed upper limit at 95% of confidence level on same sign top pairs production cross section times Branching Ratio is 0.86 fb. The observed upper limits at 95% of confidence level on the SMEFT parameters are 0.005, 0.015 and 0.031 ([TeV/Λ]2) for the Wilson Coefficients associated to ORR, O(1)LR and O(8)LR respectively. This is the first result of the ATLAS Collaboration concerning the search for same-sign top quark pairs production in proton-proton collision data at s = 13 TeV, in the framework of the SMEFT.
The radiosensitizing effect of platinum nanoparticles in proton irradiations is not caused by an enhanced proton energy deposition at the macroscopic scale
(2022-07-29) Behrends, Carina; Bäcker, Claus Maximilian; Schilling, Isabelle; Zwiehoff, Sandra; Weingarten, Jens; Kröninger, Kevin; Rehbock, C.; Barcikowski, Stephan; Wulff, Jörg; Bäumer, Christian; Timmermann, B.
Objective. Due to the radiosensitizing effect of biocompatible noble metal nanoparticles (NPs), their administration is considered to potentially increase tumor control in radiotherapy. The underlying physical, chemical and biological mechanisms of the NPs' radiosensitivity especially when interacting with proton radiation is not conclusive. In the following work, the energy deposition of protons in matter containing platinum nanoparticles (PtNPs) is experimentally investigated. Approach. Surfactant-free monomodal PtNPs with a mean diameter of (40 ± 10) nm and a concentration of 300 μg ml−1, demonstrably leading to a substantial production of reactive oxygen species (ROS), were homogeneously dispersed into cubic gelatin samples serving as tissue-like phantoms. Gelatin samples without PtNPs were used as control. The samples' dimensions and contrast of the PtNPs were verified in a clinical computed tomography scanner. Fields from a clinical proton machine were used for depth dose and stopping power measurements downstream of both samples types. These experiments were performed with a variety of detectors at a pencil beam scanning beam line as well as a passive beam line with proton energies from about 56–200 MeV. Main results. The samples' water equivalent ratios in terms of proton stopping as well as the mean proton energy deposition downstream of the samples with ROS-producing PtNPs compared to the samples without PtNPs showed no differences within the experimental uncertainties of about 2%. Significance. This study serves as experimental proof that the radiosensitizing effect of biocompatible PtNPs is not due to a macroscopically increased proton energy deposition, but is more likely caused by a catalytic effect of the PtNPs. Thus, these experiments provide a contribution to the highly discussed radiobiological question of the proton therapy efficiency with noble metal NPs and facilitate initial evidence that the dose calculation in treatment planning is straightforward and not affected by the presence of sensitizing PtNPs.
Characterisation of irradiated and non-irradiated silicon sensors with a table-top two photon absorption TCT system
(2022-08-16) Pape, Sebastian; Fernández García, M.; Moll, M.; Montero, R.; Palomo, F.R.; Vila, I.; Wiehe, M.
A tabletop Two Photon Absorption-Transient Current Technique (TPA-TCT) set-up built at CERN was used to investigate a non-irradiated PIN diode, an irradiated PIN diode, and a non-irradiated 5 × 5-multipad HPK LGAD. The intrinsic three dimensional spatial resolution of this method is demonstrated under normal incidence of the laser probe. A charge collection versus depth profile of the non-irradiated PIN diode is presented, where reflection on the rear silicon-air interface was observed. It is found that the time-over-threshold versus depth profile is particularly suitable to determine the boundaries of the DUT's active volume. A depth scan of the irradiated PIN diode is discussed and a method to omit the single photon absorption background is presented. Finally, a charge collection measurement in the inter-pad region of the 5 × 5-multipad HPK LGAD is presented and it is demonstrated that TPA-TCT can be used to image the implantation and the electric field of segmented silicon devices in a three dimensional manner.
With bottom-up to the top and beyond
(2022) Sedlaczek, Kevin; Kröninger, Kevin; Albrecht, Johannes
In this thesis, the measurement of the inclusive and differential production cross-sections of a top quark in association with an anti-top quark at large transverse momenta in proton-proton collisions, as well as one of the first direct interpretations of such a measurement in the framework of effective field theories (EFT) are presented. The measurement is based on data collected with the ATLAS detector at the Large Hadron Collider at CERN and corresponds to an integrated luminosity of 139 fb-1. Events of the process tt → WbWb→lvbqqb in the region of large transverse momenta are analyzed. After unfolding the data to particle level, the cross-section is presented as function of several kinematic variables relevant to this process. In order to validate the predictions based on the Standard Model of particle physics, the data is compared to several predictions and effects of higher order contributions in quantum chromodynamics are investigated. In this measurement, the impact of systematic uncertainties on the energy reconstruction of jets is reduced by employing a novel method. It introduces a scaling of the energies of the jets in order to decrease discrepancies in the reconstruction of jet energies in data and the different predictions. Using this precision measurement, an indirect and model-independent search for physics beyond the Standard Model (BSM) in the framework of EFT is performed. In particular, the effects of two EFT operators on the coupling of the top quark to gluons and other quarks are investigated. A model based on the two operators OtG and O(8)tq in the top-quark sector of the EFT framework is employed to interpret the measurement in the context of BSM physics. Bayesian statistics is used in order to set limits on the effects of these operators via their respective Wilson coefficients CtG and C(8)tq. This analysis is among the very first direct EFT interpretations of a differential cross-section measurement in the top-quark sector. The determined limits on C(8)tq are competitive with recent global combinations, while soundly treating all parameters of the interpretation and without employing any of the assumptions, typical in global combinations. Both Wilson coefficients are found to be compatible with the SM.
Nuclear physics in proton therapy: new approaches for proton beam range verification using delayed gamma-rays
(2022) Bäcker, Claus Maximilian; Kröninger, Kevin; Bäumer, Christian
In-vivo monitoring of the delivered dose is desirable in radiation therapy. Radionuclides generated during the proton irradiation can be used for imaging methods from nuclear medicine after the treatment session. In order to improve these techniques, this work focuses on three different aspects of nuclear physics from basic nuclear physics measurements to a clinical validation. The most relevant nuclear interaction is the production of C-11 from carbon as this nuclear interaction is used as a monitor reaction for the measurement of several other radionuclides. Furthermore, this is one of the relevant residuals produced from the tissue nuclei. As the data from the literature scatter by about 15%, a new reference cross section value of (68 ± 3) mb at 97 MeV is determined from the cross sections measured in different settings. In a second part, the activation of iodine during proton irradiation is investigated. Several radionuclides are produced from iodine which decay by the emission of positrons and can potentially be used for PET imaging subsequent to the fractional treatment. In the last step, the activation of titanium implants is investigated. A benchmarking test including MC simulations is performed which study reveals the limited applicability of PET imaging with implants for field verification in proton therapy.
The silicon strip detector of the ATLAS Inner Tracker: from individual sensing units to multi-module petal structures
(2022) Renardi, Alessia; Kröninger, Kevin; Gregor, Ingrid-Maria
Nowadays particle detector technology is taking big steps forwards and new devices dedicated to particle physics show very high performance. Particularly the semiconductor detectors have advanced significantly and are used for tracking purposes in the A Toroidal LHC ApparatuS (ATLAS) experiment at CERN thanks to their excellent spacial resolution: the compact size of the silicon and its high granularity allow to reach a precision measurement of few tens of microns. This thesis is focused on the upgrade of the ATLAS tracking detector required for the High Luminosity Large Hadron Collider (HL-LHC), starting in 2027. The HL-LHC foresees an integrated luminosity of L = 3000 fb−1, which comes with an unprecedented rate of proton collisions, with a pile-up of h i = 200, and very high radiation doses. As the current inner detector has not been designed for the HL-LHC environmental conditions, an all-silicon Inner Tracker (ITk) will take its place during Phase-II upgrade of the ATLAS experiment. The ITk strip endcap sub-detector is the main topic of this PhD project. The investigation covers the assembly of silicon strip endcap modules and their loading on a local support structure. The building and loading procedures are presented as well as results of quality control (QC) tests carried out on prototyping components to establish their working performance and the fulfillment of the specifications. This work provides the procedure optimization in order to achieve the requirements imposed by the collaboration. Results on prototyping components, such as a fully electrical module and a semielectrical petal, both built and tested at DESY, are presented. They are followed by tests on an electrical petal performed at low temperature with the evaporative CO2 cooling technique. The QC tests carried out on all prototypes have demonstrated that they have been properly assembled and are fully functional. Moreover they fulfil the respective requirements validating therefore the components design and the building methods.
Experimental consolidation and absolute measurement of the nat C(p,x)11 C nuclear activation cross section at 100 MeV for particle therapy physics
(2021-08-02) Bäcker, Claus Maximilian; Horst, Felix; Adi, Wihan; Bäumer, Christian; Gerhardt, Marcel; Jentzen, Walter; Kazek, Sandra Laura; Kröninger, Kevin; Schuy, Christoph; Verbeek, Nico; Weingarten, Jens; Wulff, Jörg; Timmermann, Beate
The natC(p,x)11C reaction has been discussed in detail in the past [EXFOR database, Otuka et al. (Nuclear Data Sheets 120:272–276, 2014)]. However, measured activation cross sections by independent experiments are up to 15% apart. The aim of this study is to investigate underlying reasons for these observed discrepancies between different experiments and to determine a new consensus reference cross section at 100 MeV. Therefore, the experimental methods described in the two recent publications [Horst et al. (Phys Med Biol https://doi.org/10.1088/1361-6560/ab4511 [Titel anhand dieser DOI in Citavi-Projekt übernehmen] , 2019) and Bäcker et al. (Nuclear Instrum Methods Phys Res B 454:50–55, 2019)] are compared in detail and all experimental parameters are investigated for their impact on the results. For this purpose, a series of new experiments is performed. With the results of the experiments a new reference cross section of (68±3) mb is derived at (97±3) MeV proton energy. This value combined with the reliably measured excitation function could provide accurate cross section values for the energy region of proton therapy. Because of the well-known gamma-ray spectrometer used and the well-defined beam characteristics of the treatment machine at the proton therapy center, the experimental uncertainties on the absolute cross section could be reduced to 3%. Additionally, this setup is compared to the in-beam measurement setup from the second study presented in the literature (Horst et al. 2019). Another independent validation of the measurements is performed with a PET scanner.
Development of tools for Bayesian data analysis and their application in the search for physics beyond the Standard Model
(2021) Grunwald, Cornelius; Kröninger, Kevin; Caldwell, Allen
In this thesis, methods for Bayesian data analysis are employed in the search for physics beyond the Standard Model (BSM). A new implementation of the Bayesian Analysis Toolkit in Julia (BAT.jl) is introduced as a modern data analysis framework providing algorithms for Bayesian inference. The EFTfitter.jl package for interpreting measurements in the context of effective field theories (EFTs) is presented. It facilitates combining measurements and estimating underlying parameters with Bayesian inference. Both tools are employed for indirect BSM searches using EFTs. Wilson coefficients of dimension-six operators from the top-quark sector of the Standard Model effective field theory (SMEFT) are constrained, and approaches for enhancing fits of SMEFT coefficients are investigated. Studies on the effects of correlations between the uncertainties of measurements on the results of SMEFT fits are conducted. It is demonstrated that the correlations can significantly impact the resulting constraints and can be the crucial components deciding whether deviations from the Standard Model of particle physics are observed or not, in particular when assuming future measurements with reduced uncertainties. Moreover, studies on combining measurements of top-quark and flavor physics observables for SMEFT interpretations are performed, and the steps necessary for a combined fit are discussed. Powerful synergies between top-quark processes and b → s transitions are observed when constraining SMEFT Wilson coefficients of the top-quark sector. It is demonstrated that due to complementary sensitivities, combining observables from different energy scales can tighten the constraints significantly. Future scenarios assuming measurements from HL-LHC, Belle II, and CLIC are investigated, and their potential for improving constraints on up to eleven SMEFT Wilson coefficients is pointed out. The benefits of orthogonal constraints in multidimensional phase spaces for resolving ambiguous solutions are highlighted. The SMEFT studies presented in this thesis demonstrate the capabilities of the new BAT.jl and EFTfitter.jl packages for BSM analyses.
Development and calibration of an s-tagging algorithm and its application to constrain the CKM matrix elements |Vts| and |Vtd| in top-quark decays using ATLAS Run-2 Data
(2021) Zeißner, Sonja Verena; Kröninger, Kevin; Albrecht, Johannes
In this thesis, the development and calibration of an algorithm used to identify jets from strange quarks as well as a measurement constraining the CKM matrix elements $|V_{ts}|$ and $|V_{td}|$ in top-quark decays are presented. The thesis considers data from proton-proton collisions at the Large Hadron Collider at a center-of-mass energy of 13 TeV recorded by the ATLAS Experiment during Run 2. First, the maximally achievable separation between jets from strange quarks and jets from down quarks at hadron colliders given different idealized detector designs is studied using recurrent neural networks containing Long Short-Term Memory layers. Afterwards, an algorithm to select jets from strange quarks for the application at the ATLAS Experiment is developed using deep neural networks. Its efficiency for these jets from strange quarks and mis-tag rates for jets of other flavors is determined in semileptonic decays of top-antitop pairs selected from data. The algorithm to identify jets from strange quarks is then applied in events containing decays of top-antitop pairs with an electron and a muon of opposite-sign electric charge in the final state in order to study its potential to constrain the CKM matrix elements $|V_{ts}|$ and $|V_{td}|$ in the two-dimensional plane spanned by them. In this study, limits of $|V_{ts}|^2+|V_{td}|^2<0.06$, $|V_{ts}|<0.21$, and $|V_{td}|<0.24$ are derived at 95% confidence level assuming unitarity of the CKM matrix. Kurzfassung Diese Dissertation befasst sich mit der Entwicklung und Kalibrierung eines Algorithmus zur Identifikation von Jets aus strange-Quarks sowie einer Messung zur Bestimmung von oberen Schranken auf die CKM-Matrixelemente $|V_{ts}|$ und $|V_{td}|$ in top-Quark-Zerfällen. In dieser Arbeit werden Daten aus Proton-Proton-Kollisionen am Large Hadron Collider, die bei einer Schwerpunktsenergie von 13 TeV am ATLAS-Experiment während des Run-2 aufgenommen wurden, verwendet. Zuerst wird die maximal mögliche Trennung von Jets aus strange-Quarks und Jets aus down-Quarks an Hadronenkollidern unter der Annahme einer Nutzung von unterschiedlichen, idealisierten Detektordesigns untersucht. Hierfür werden Recurrent Neural Networks verwendet, deren Hauptkomponente Long Short-Term Memory Layers sind. Anschließend wird ein Algorithmus zur Selektion von Jets aus strange-Quarks zur Anwendung am ATLAS-Experiment entwickelt, der Deep Neural Networks verwendet. Die Identifikationseffizienz dieses Algorithmus in Bezug auf Jets aus strange-Quarks und die Wahrscheinlichkeit, Jets eines anderen Ursprungs fehl zu identifizieren, werden in semileponischen Zerfällen von top-antitop-Paaren, die aus dem aufgenommenen Datensatz selektiert wurden, bestimmt. Schlussendlich wird der Algorithmus zur Identifikation von Jets aus strange-Quarks in Ereignissen angewendet, die dileponischen Zerfällen von top-antitop-Paaren mit einem Myon und einem Elektron im Endzustand beinhalten. Hierbei wird sein Potenzial in der Bestimmung von Schranken auf die CKM-Matrixelemente $|V_{ts}|$ und $|V_{td}|$ im zweidimensionalen Raum untersucht. Daraus folgen Schranken von $|V_{ts}|^2+|V_{td}|^2<0.06$, $|V_{ts}|<0.21$ und $|V_{td}|<0.24$ mit einem Konfidenzintervall von 95% unter der Annahme von CKM-Matrix-Unitarität.
Study of interference effects in the search for flavour-changing neutral current interactions involving the top quark and a photon or a Z boson at the LHC
(2020-03-30) Barros, Maura; Castro, Nuno Filipe; Erdmann, Johannes; Geßner, Gregor; Kröninger, Kevin; La Cagnina, Salvatore; Peixoto, Ana
Flavour-changing neutral-current interactions of the top quark can be searched for in top-quark pair production with one top quark decaying to an up-type quark and a neutral boson, and they can be searched for in the single production of a top quark in association with such a boson. Both processes interfere if an additional up-type quark is produced in the case of single production. The impact of these interference effects on searches for flavour-changing neutral currents at the LHC is studied for the case where the neutral boson is a photon or a Z boson. Interference effects are found to be smaller than variations of the renormalisation and factorisation scales.
A selection framework for LHCb’s upgrade trigger
(2020) Nolte, Niklas; Albrecht, Johannes; Erdmann, Johannes
Das LHCb Experiment am Large Hadron Collier am CERN wird momentan für die nächste Datennahme verändert und modernisiert. Die instantane Luminosität wird um einen Faktor fünf erhöht, damit mehr Daten in kürzerer Zeit aufgenommen werden können. Die erste Stufe der Datennahme, der Hardwaretrigger, wird entfernt. LHCb muss nun eine Kollisionsrate von 30MHz in Echtzeit verarbeiten. In dieser Arbeit werden drei Projekte vorgestellt,die signifikant zu der Entwicklung eines schnellen und effizienten Triggersystems beitragen. Der erste Beitrag ist ein Scheduling Algorithmus mit vernachlässigbarem Overhead in der neuen Trigger-Applikation. Der Algorithmus steuert das Multi-Threading des Systems und ist der erste Algorithmus in LHCb, der den technischen Spezifikationen des Systems genügt. Durch die Restriktion auf Inter-Event Parallelismus können die meisten teuren Entscheidungen schon vor der Laufzeit der Applikation getroffen werden. Der zweite Beitrag besteht aus mehreren Algorithmen zur Filterung und Kombination von Teilchen in der Kollision. Diese Algorithmen sind bis zu mehreren Größenordnungen schneller als die aktuellen, etablierten Algorithmen. Der Einsatz der neuen Algorithmen in der zweiten Trigger-Phase (HLT2) ist ein wichtiger Schritt zur Vervollständigung eines Trigger-Systems, dass den erhöhten Anforderungen entspricht. Das letzte Projekt beschäftigt sich mit der Bandbreite, mit der der Trigger Kollisionen abspeichert. Dazu wird die wichtigste Selektion im HLT2 betrachtet, der topologische Trigger. Dieser Trigger versucht, Zerfälle von beauty Hadronen inklusiv zu selektieren. Zuerst wird der Selektionsalgorithmus selber optimiert. In einem zweiten Schritt werden die Kollisionen, die der Selektion entsprechen, getrimmt. Irrelevante Information für die Analyse von beauty Hadronen in diesen Kollisionen werden entfernt. Damit kann die Bandbreite pro gespeicherter Kollision verringert werden.
Investigations of planar n+-in-n ATLAS silicon sensors with modified pixel implantations
(2020) Wagner, Mareike; Kröninger, Kevin; Rhode, Wolfgang
In hoch energetischen Physikexperimenten werden kontinuierlich verbesserte Sensoren und Module benötigt, um neuen Anforderungen gerecht zu werden. Die REINER-Pixelmodule weisen verschiedene Pixelvariationen auf, um die Sensoreigenschaften, vor allem nach Bestrahlung, zu untersuchen. In dieser Arbeit werden die Labor- und Teststrahlergebnisse von mehreren REINER- Pixelmodulen präsentiert. Insgesamt wurden dazu neun Module mit Protonen und Neutronen in Bestrahlungseinrichtungen bestrahlt, um die Strahlenhärte und die beabsichtigte Ladungsverstärkung zu untersuchen. Vor der Bestrahlung zeigten alle Pixelvariationen ähnliche Effizienzen in der Ladungssammlung und des Teilchennachweises, was sich jedoch nach der Bestrahlung änderte. Für fast alle Module wurden nach der Bestrahlung die höchsten Teilchennachweiseffizienzen bei niedrigen Spannungen für das Standardpixeldesign gemessen. Nur bei einem mit Neutronen hoch bestrahlten Modul (5x1015neq/cm²) erreicht die Pixelvariation V5 deutlich höhere Nachweiseffizienzen bei niedrigen Spannungen. Die anschließenden Studien, bei denen ein anderes Modul erhitzt wurde, haben gezeigt, dass die höhere Anzahl an Ladungsträgern und die erhöhte Teilchennachweiseffizienz von Pixelvariation V5 ein Aufheizeffekt sind: Für dieses mit Neutronen hoch bestrahlte Modul wurden nach langen Aufheizzeiten eine erhöhte Ladungssammlung und eine höhere Teilchennachweis-effizienz gemessen. Zusätzlich wurde im Zuge dieser Arbeit ein Aufbau in Betrieb genommen, mit dem die laserinduzierte Ladungssammlungseffizienz eines Sensors mit einer Auflösung im Subpixelbereich im Labor untersucht werden kann. Erste vielversprechende Ergebnisse dieses Aufbaus sind konsistent zu Pixeleffizienzen aus Teststrahlmessungen: Die erhöhte Ladungssammlung- und Teilchennachweiseffizienz von Pixelvariation V5 nach Bestrahlung und Aufheizen ist vermutlich auf elektrische Feldstärkenmaxima zurückzuführen, die an den Ecken und Kanten der n⁺-Implantation auftreten. Dieses stärkere elektrische Feld verbreitert die Verarmungszone und ein Ladungsvervielfältigungseffekt tritt auf.
First evidence of standard model pp → tttt production and performance studies of the ATLAS tile calorimeter for HL-LHC
(2020) Rustige, Lennart; Kröninger, Kevin; Meyer, Andreas
In diesem Dokument werden zwei Analysen auf dem Gebiet der Teilchenphysik vorgestellt. Erstens, Studien über die Performanz der Rekonstruktion von Myonen unter Verwendung von Kalorimeterinformationen während der HL-LHC Phase des ATLAS Detektors. Zweitens, die Suche nach der simultanen Produktion von vier Top-Quarks, wie vom Standard Modell (SM) prognostiziert, unter Verwendung des vollständigen Run-II-Datensatzes, der von ATLAS aufgenommen wurde. Dieser Datensatz entspricht einer integrierten Luminosität von L = 139 fb-1 von Proton–Proton Kollisionen bei einer Schwerpunktsenergie von √s = 13 TeV. Für diese Arbeit wird die Performanz der Rekonstruktion von Myonen für verschiedene pile-up Szenarien untersucht, wie sie für die HL-LHC-Phase erwartet werden, und im Hinblick auf verschiedene Rauschszenarien, die den Verlust der Energieauflösung und die Verschlechterung der Detektorakzeptanz aufgrund von Alterung und Bestrahlung der Detektorkomponenten nachbilden. Diese Studie wird durchgeführt, um vorgeschlagene Szenarien für die Aufrüstung des Detektors vor ihrer Implementierung zu testen. Die Suche nach der vom SM prognostizierten Produktion von vier Top-Quarks, konzentriert sich auf die Zerfallsmoden mit zwei Leptonen gleicher Ladung oder mehr Leptonen im Endzustand. Die Suche nach diesem Prozess ist unter anderem durch die sehr hohen involvierten Energien motiviert und durch die Tatsache, dass derzeit verfügbare Datensätze möglicherweise für eine Entdeckung ausreichen. Die finalen Messergebnisse werden mit einem Profile-Likelihood-Fit erzielt, der das Ergebnis eines Boosted-Decision-Trees beinhaltet, welcher darauf trainiert ist zwischen Signal und Untergrund zu unterscheiden. Der Fit führt zu einem Wirkungsquerschnitt von σ(p→tt ¯tt ¯ )=24(+7-6)fb, was einer beobachteten (erwarteten) Signifikanz von Z = 4,3 (Z = 2,4) entspricht. Dies repräsentiert den ersten Nachweis (Evidence) für diesen Prozess. Das erhaltene Ergebnis ist mit der SM Vorhersage innerhalb von 1,7 Standardabweichungen kompatibel. Auf diesen ersten Nachweis aufbauend, wird die Möglichkeit der Rekonstruktion des vier Top-Quark Systems unter Verwendung eines kinematischen Likelihood Fits entwickelt und getestet. Die Entwicklungen und Tests werden mit dem KLFitter Software Paket durchgeführt und erzielen eine Effizienz der korrekten Zuordnung aller vier Top-Quarks von ε = 33 ± 4 % unter optimalen Bedingungen für den Endzustand mit einem Lepton.
Searches for vector-like quarks with 13 TeV at the ATLAS experiment and development of a boosted-object tagger using a deep neural network
(2020) Freundlich, Elena M.; Kröninger, Kevin; Rhode, Wolfgang
In this thesis, two searches for the pair production of vector-like top (VLT) and vector-like bottom (VLB) quarks are presented using 36.1 fb⁻¹ and 139 fb⁻¹ of data. The data was taken at √s = 13 TeV in proton-proton collisions with the ATLAS detector, which is one of the major experiments at the Large Hadron Collider at CERN. The decay signature of this search for TT and BB production is characterized by the presence of a Z-boson, which is reconstructed from two opposite-charge same-flavor leptons (e or μ) and is produced alongside a third-generation quark. In addition, the analyses target final-state objects with high transverse momentum, pT. Within the scope of this thesis, the Multi-Class Boosted-Object Tagger (MCBOT) is developed and characterized. It uses a deep neural network, which identifies reclustered large-radius jets originating from V-bosons, H-bosons, top-quarks and background unambiguously. For the search with 139 fb⁻¹, the multiplicity of hadronic high-pT objects is exploited by defining multiple regions according to the number of MCBOT- and b-tags. No deviation from the Standard Model expectation is found in the observed data for 36.1 fb⁻¹ and 139 fb⁻¹, and exclusion limits on the pair production cross-section and the mass of VLT and VLB quarks are set depending on various branching-ratio assumptions. With 36.1 fb⁻¹, VLT quark masses below 1102 GeV are excluded with respect to the (T,B) and (X,T) doublet models. For VLB quarks, lower mass limits of 1063 GeV are set for the (B,Y) doublet. The results of this search significantly contribute to the sensitivity of two combination searches for TT and BB production performed at the ATLAS experiment with 36.1 fb⁻¹, which constrain the phase-space for vector-like quark existence further. For the full Run-2 search with 139 fb⁻¹ of data, lower limits on the VLT quark mass of 1350 GeV are observed and VLB quark masses below 1316 GeV are excluded assuming the mentioned doublet models. The observed limits of this search for TT and BB production represent the highest exclusion limits to date assuming the 100% branching ratio into Zt/b final states and – in case of VLB quarks – for the (B,Y) doublet model.
Detection and identification of electrons and photons
(2020) Arling, Jan-Hendrik; Kröninger, Kevin; Gregor, Ingrid-Maria
It is important to exploit particle detectors for the detection and identification of electrons and photons in the best possible way, as it is adressed in this thesis by discussing three different application examples of precisely measuring electrons and photons at high-energy physics and test beam experiments. Firstly, the optimization of the photon identification within the ATLAS detector at the LHC is investigated. Appropriate selection criteria are needed to efficiently discriminate prompt photons steming from the hard-scattering collision process from photons originating from hadronic jet decays. Systematic studies for the use of a multivariate optimization with a Boosted Decision Tree are presented and compared to the currently used rectangular cut approach. Secondly, the planned Inner Tracker (ITk) as an upgrade of the ATLAS detector for the HL-LHC is discussed. The focus of the investigations lies on the support structures foreseen for the silicon sensor modules, the petal core. The thermo-mechanical characterization of the petal is investigated using infrared thermography, dual-phase CO2 cooling and geometrical metrology, proving a well-performing design of the local support structure within the ITk detector specifications. Finally, measurements with multi-GeV electrons at the DESY II Test Beam Facility are presented to investigate the potential of the Material Budget Imaging technique. This technique aims to reconstruct the material distribution of samples by measuring the deflection angles of traversing electrons with the high-resolution EUDET-type beam telescopes exploiting the underlying effect of multiple Coulomb scattering. The results in terms of the determination of the characteristic radiation length of several materials as well as the two-dimensional material budget image of complex composite structures are shown.
High temperature thermoluminescence peaks – inﬂuence on the albedo principle for personal dosimetry and beneﬁts for the new TL-DOS neutron dosemeter
(2020) Heiny, Myriam; Kröninger, Kevin; Westphal, Carsten
The individual monitoring service at the Materialprüfungsamt North Rhine-Westphalia and the TU Dortmund are developing the compact dosemeter system TL-DOS based on thin-layer thermoluminescence detectors as well as an associated glow curve analysis tool. Various dosemeters of the system measure different dose quantities and types of radiation. This thesis focuses on the TL-DOS neutron dosemeter to measure the whole body dose. The system is optimized in terms of LiF:Mg,Ti detectors, an albedo badge, a readout device and the glow curve deconvolution. In order to gain more information about the irradiation scenario such as the particle type, the detectors are measured at 653K to include the high temperature peaks. After a design speci_cation was defined, the neutron dosemeter is characterized in di_erent photon and neutron radiation fields as well as in workplace fields. This thesis also presents an analysis that provides the individual induced signal of mixed field irradiations by analyzing the high temperature peaks of a glow curve. The signal induced by irradiation combinations like alpha+photon or neutron+photon fields is separated with only one detector. Finally, the current dealing with the significant energy dependence of neutron dosemeters is critically discussed and new approaches are presented.
Temperature scaling of leakage current in irradiated silicon sensors
(2019) Wizemann, Felix; Kröninger, Kevin; Albrecht, Johannes
Mit dem Ausbau zum High Luminosity LHC (HL-LHC) wird der Spurdetekor des ATLAS-Experiments durch den Inner Tracker (ITk) ersetzt, der Pixel- und Streifensensoren aus Silizium nutzen wird. In Dortmund werden Module für die Endkappe des ITK Streifendetektors gebaut werden. In einem Teil der Arbeit wird der Ablauf derProduktion dieser Module entwickelt. Während des Betriebs werden Strahlenschäden den Leckstrom der Siliziumsensoren erhöhen. Mit Hilfe des Parameters Eeff kann dieser mit der Temperatur skaliert werden. Studien haben gezeigt, dass dieser Parameter 1.21 eV für Sensoren beträgt, die zu einer Fluenz von bis zu 1 x 1015 neqcm-2 bestrahlt wurden. Bei höherer Bestrahlung wurde ein niedriger Wert für Eeff beobachtet. Um diese Änderung in der Skalierung mit der Temperatur zu untersuchen, wird Eeff in dieser Arbeit für bestrahlte Sensoren mit Fluenzen zwischen 6 x 1014 neqcm-2 und 3 x 1015 neqcm-2 in Abhängigkeit von der angelegten Spannung bestimmt. Es wird beobachtet, dass Eeff spannungsabhängig ist und 1:21 eV nur oberhalb der Depletionsspannung anwendbar ist.
Search for flavour-changing neutral currents in processes with a single top quark in association with a photon using a deep neural network at the ATLAS experiment at √s = 13TeV
(2019) Geßner, Gregor; Kröninger, Kevin; Wagner, Wolfgang
In this thesis, a search for flavour-changing neutral currents in processes involving a singly produced top quark and a photon is presented. In this search, proton-proton collision data are used which were collected by the ATLAS experiment at the LHC and correspond to an integrated luminosity of 81 fb⁻¹. The top quark is assumed to decay into a b quark and a W boson, which is supposed to decay leptonically. Events are selected whose final state contains exactly one photon, one charged lepton, one b-tagged jet and a certain magnitude of missing transverse momentum. The background contributions, that arise from events with either an electron or a jet misidentified as photon, are estimated using data-driven techniques. The signal is modelled in an effective field theory. The signal coupling is assumed to be either left- or right-handed and to involve either an up quark or a charm quark. Assuming only one of these signal couplings being present at a time, a discriminant is constructed using a deep neural network which classifies events into signal and background candidates. A profile likelihood fit is performed to improve the background prediction and to estimate the signal strength. Since no signal contribution is observed in data and the data are found to be consistent with the background expectation, upper exclusion limits are set on the branching ratios B(t→uγ) and B(t→cγ). The observed limits at a confidence level of 95 % on the branching ratio are B(t→uγ, left-handed) < 2.8 · 10⁻⁵, B(t→uγ, right-handed) < 6.1 · 10⁻⁵, B(t→cγ, left-handed) < 22 · 10⁻⁵ and B(t→cγ, right-handed) < 18 · 10⁻⁵. These limits are interpreted as limits on the cross section for the production of a top quark and a photon via a flavour-changing neutral current, and as limits on the absolute value of the sum of the Wilson coefficients CuBij* and CuWij* for the left-handed and CuBij and CuWij for the right-handed couplings.
The COBRA extended demonstrator – conception, characterization, commissioning
(2019-12-06) Temminghoff, Robert; Kröninger, Kevin; Rhode, Wolfgang
Der neutrinolose doppelte Beta-Zerfall ist ein hypothetischer Kernzerfall dessen Nachweis grundlegende Fragen über bislang unbekannte Eigenschaften von Neutrinos liefern und auf Physik jenseits des Standard Modells der Teilchenphysik hindeuten würde. Das COBRA Experiment sucht nach diesem Zerfall mit Hilfe von CdZnTe Halbleiter-Detektoren. In dieser Arbeit wird ein wesentlich verbesserter Aufbau des Experiments, genannt Extended Demonstrator (XDEM), präsentiert. Um diesen zur verwirklichen, wurden wichtige Eigenschaften der verwendeten Detektoren in Labormessungen bestimmt. Außerdem wurden Simulationen durchgeführt um Aufschluss über die Wahrscheinlichkeit einen doppelten Beta-Zerfall zu detektieren zu erhalten und eine Abschätzung des Hintergrundes durch andere Prozesse zu erreichen. Darauf basierend wurden Daten analysiert, welche über mehr als ein halbes Jahr am LNGS Untergrundlabor in Italien aufgezeichnet wurden. Im Vergleich zu früheren Versionen des COBRA Experiments wurde der Hintergrund um mehr als einen Faktor 30 reduziert bei gleichzeitiger Steigerung der Wahrscheinlichkeit ein Signal zu messen um 50 %. Es wurden keine Hinweise auf neutrinolosen doppelten Beta-Zerfall gefunden. Stattdessen wurden untere Grenzen für die Halbwertszeit der Zerfälle von 116Cd und 130Te aufgestellt, welche für 116Cd 2,7x10^21 yr und für 130Te 8.8x10^21 yr betragen. Dies sind die stärksten Grenzen auf diese Halbwertszeiten die im Rahmen des COBRA Experiments bestimmt wurden.
Search for Higgs boson production in association with a single top quark and constraints on the H+b-jets background in the H → γγ decay channel at the ATLAS experiment
(2019) Nitsche, Isabel; Kröninger, Kevin; Rhode, Wolfgang
In this thesis, the production of a single top quark in association with a Higgs boson (tH) is studied at √s = 13 TeV in the H → γγ decay channel at the ATLAS experiment. Similar to the production of a Higgs boson in association with a top-antitop quark pair (ttH), tH production allows for a direct measurement of the Yukawa coupling, Yt, of the top quark. Due to a destructive interference in tH production, it also provides sensitivity to negative values of the coupling strength modifier κt = Yt/Yt,SM, which describes deviations from the Standard Model (SM) expectation Yt,SM. For the first time at the ATLAS experiment, dedicated tH categories are included in the measurement of top-quark-associated Higgs boson production in the H → γγ channel, using 36.1/fb of data. The measured signal strength for ttH+tH production agrees with the SM expectation and no hint for a negative sign of κt was found. An important background for hadronic ttH and tH final states in the H → γγ channel is the production of a Higgs boson in association with additional b-jets (H+b-jets). This background is associated with a large systematic uncertainty, as the accuracy of the prediction from Monte Carlo (MC) simulations is not known. A measurement of the differential cross section of the number of b-jets, Nb-jets, for inclusive Higgs boson production in the H → γγ decay channel using 79.8/fb of ATLAS data is presented. The measured cross section for Nb-jets = 1, with the dominant contribution resulting from H+b-jets production, is in agreement with the MC expectation within the uncertainties. Additionally, an approach to improve the sensitivity to H+b-jets is presented, which is based on multivariate analysis techniques to discriminate between H+b-jets and the non-resonant background.

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