This thesis is a presentation of an analysis of the $ZZ\rightarrow 4\ell$ ($\ell = e,\mu$) process in proton-proton collisions with centre-of-mass energy $\sqrt{s}$ = 13 TeV at the LHC during 2015 and 2016 (a total integrated luminosity of 36.1 fb$^{-1}$), using the ATLAS detector. Candidate $ZZ \rightarrow 4\ell$ events are selected in the three decay channels, $4e$, $2e2\mu$ and $4\mu$. The cross section of the $ZZ\rightarrow 4\ell$ process is measured in four fiducial regions closely matching the detector acceptance: one for each decay channel ($4e$, $2e2\mu$ and $4\mu$) and one for the combination of all decay channels. The total cross section of $pp\rightarrow ZZ$ is measured in a phase-space in which both $Z$ bosons have a mass $m_{Z}$ in the range 66 GeV $ < m_{Z} < $ 116 GeV to be 16.5 $\pm 0.5$ (stat.) $\pm 0.4$ (syst.) $\pm 0.5$ (lumi.)$\,$ pb which is consistent with a next-to-next-to-leading-order prediction of $16.9^{+0.6}_{-0.5}$ pb. Observed event yields in four bins of transverse momentum are used to set 95{\%} CL$_{\text{s}}$ limits on four neutral triple gauge couplings ($f^{V}_{i}$, $V=Z,\gamma$, $i=4,5$) which parameterise an effective $ZZV$ vertex (assuming both $Z$ bosons are on shell). The obtained limits are of the order of $|f^{V}_{i}|<0.0017$.

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Am Large Hadron Collider und am ATLAS-Detektor werden umfangreiche Aufrüstungsarbeiten vorgenommen. Diese Arbeiten sind in mehrere Phasen gegliedert und umfassen unter Anderem Änderungen an der Ausleseelektronik der Flüssigargonkalorimeter; insbesondere ist es geplant, während der letzten Phase ihren Primärpfad vollständig auszutauschen. Die Elektronik besteht aus einem analogen und einem digitalen Teil: während ersterer die Signalpulse verstärkt und sie zur leichteren Abtastung verformt, führt letzterer einen Algorithmus zur Energierekonstruktion aus. Beide Teile müssen während der Aufrüstung verbessert werden, damit der Detektor interessante Kollisionsereignisse präzise rekonstruieren und uninteressante effizient verwerfen kann. In dieser Dissertation werden Simulationsstudien präsentiert, die sowohl die analoge als auch die digitale Auslese der Flüssigargonkalorimeter optimieren. Die Korrektheit der Simulation wird mithilfe von Kalibrationsdaten geprüft, die im sog. Run 2 des ATLAS-Detektors aufgenommen worden sind. Der Einfluss verschiedener Parameter der Signalverformung auf die Energieauflösung wird analysiert und die Nützlichkeit einer erhöhten Abtastrate von 80 MHz untersucht. Des Weiteren gibt diese Arbeit eine Übersicht über lineare und nichtlineare Energierekonstruktionsalgorithmen. Schließlich wird eine Auswahl von ihnen hinsichtlich ihrer Leistungsfähigkeit miteinander verglichen. Es wird gezeigt, dass ein Erhöhen der Ordnung des Optimalfilters, der gegenwärtig verwendete Algorithmus, die Energieauflösung um 2 bis 3 % verbessern kann, und zwar in allen Regionen des Detektors. Der Wiener Filter mit Vorwärtskorrektur, ein nichtlinearer Algorithmus, verbessert sie um bis zu 10 % in einigen Regionen, verschlechtert sie aber in anderen. Ein Zusammenhang dieses Verhaltens mit der Wahrscheinlichkeit fälschlich detektierter Kalorimetertreffer wird aufgezeigt und mögliche Lösungen werden diskutiert.:1 Introduction 2 An Overview of High-Energy Particle Physics 2.1 The Standard Model of Particle Physics 2.2 Verification of the Standard Model 2.3 Beyond the Standard Model 3 LHC, ATLAS, and the Liquid-Argon Calorimeters 3.1 The Large Hadron Collider 3.2 The ATLAS Detector 3.3 The ATLAS Liquid-Argon Calorimeters 4 Upgrades to the ATLAS Liquid-Argon Calorimeters 4.1 Physics Goals 4.2 Phase-I Upgrade 4.3 Phase-II Upgrade 5 Noise Suppression With Digital Filters 5.1 Terminology 5.2 Digital Filters 5.3 Wiener Filter 5.4 Matched Wiener Filter 5.5 Matched Wiener Filter Without Bias 5.6 Timing Reconstruction, Optimal Filtering, and Selection Criteria 5.7 Forward Correction 5.8 Sparse Signal Restoration 5.9 Artificial Neural Networks 6 Simulation of the ATLAS Liquid-Argon Calorimeter Readout Electronics 6.1 AREUS 6.2 Hit Generation and Sampling 6.3 Pulse Shapes 6.4 Thermal Noise 6.5 Quantization 6.6 Digital Filters 6.7 Statistical Analysis 7 Results of the Readout Electronics Simulation Studies 7.1 Statistical Treatment 7.2 Simulation Verification Using Run-2 Data 7.3 Dependence of the Noise on the Shaping Time 7.4 The Analog Readout Electronics and the ADC 7.5 The Optimal Filter (OF) 7.6 The Wiener Filter 7.7 The Wiener Filter with Forward Correction (WFFC) 7.8 Final Comparison and Conclusions 8 Conclusions and Outlook Appendices / The Large Hadron Collider and the ATLAS detector are undergoing a comprehensive upgrade split into multiple phases. This effort also affects the liquid-argon calorimeters, whose main readout electronics will be replaced completely during the final phase. The electronics consist of an analog and a digital portion: the former amplifies the signal and shapes it to facilitate sampling, the latter executes an energy reconstruction algorithm. Both must be improved during the upgrade so that the detector may accurately reconstruct interesting collision events and efficiently suppress uninteresting ones. In this thesis, simulation studies are presented that optimize both the analog and the digital readout of the liquid-argon calorimeters. The simulation is verified using calibration data that has been measured during Run 2 of the ATLAS detector. The influence of several parameters of the analog shaping stage on the energy resolution is analyzed and the utility of an increased signal sampling rate of 80 MHz is investigated. Furthermore, a number of linear and non-linear energy reconstruction algorithms is reviewed and the performance of a selection of them is compared. It is demonstrated that increasing the order of the Optimal Filter, the algorithm currently in use, improves energy resolution by 2 to 3 % in all detector regions. The Wiener filter with forward correction, a non-linear algorithm, gives an improvement of up to 10 % in some regions, but degrades the resolution in others. A link between this behavior and the probability of falsely detected calorimeter hits is shown and possible solutions are discussed.:1 Introduction 2 An Overview of High-Energy Particle Physics 2.1 The Standard Model of Particle Physics 2.2 Verification of the Standard Model 2.3 Beyond the Standard Model 3 LHC, ATLAS, and the Liquid-Argon Calorimeters 3.1 The Large Hadron Collider 3.2 The ATLAS Detector 3.3 The ATLAS Liquid-Argon Calorimeters 4 Upgrades to the ATLAS Liquid-Argon Calorimeters 4.1 Physics Goals 4.2 Phase-I Upgrade 4.3 Phase-II Upgrade 5 Noise Suppression With Digital Filters 5.1 Terminology 5.2 Digital Filters 5.3 Wiener Filter 5.4 Matched Wiener Filter 5.5 Matched Wiener Filter Without Bias 5.6 Timing Reconstruction, Optimal Filtering, and Selection Criteria 5.7 Forward Correction 5.8 Sparse Signal Restoration 5.9 Artificial Neural Networks 6 Simulation of the ATLAS Liquid-Argon Calorimeter Readout Electronics 6.1 AREUS 6.2 Hit Generation and Sampling 6.3 Pulse Shapes 6.4 Thermal Noise 6.5 Quantization 6.6 Digital Filters 6.7 Statistical Analysis 7 Results of the Readout Electronics Simulation Studies 7.1 Statistical Treatment 7.2 Simulation Verification Using Run-2 Data 7.3 Dependence of the Noise on the Shaping Time 7.4 The Analog Readout Electronics and the ADC 7.5 The Optimal Filter (OF) 7.6 The Wiener Filter 7.7 The Wiener Filter with Forward Correction (WFFC) 7.8 Final Comparison and Conclusions 8 Conclusions and Outlook Appendices