Browsing by Author "Keaveney, James"

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    A search for tWZ production in the Full Run 2 ATLAS dataset using events with four leptons
    (2021) Reich, Jake; Keaveney, James; Yacoob, Sahal
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    A search for tWZ production in the trilepton channel using Run 2 data from the ATLAS experiment
    (2021) Warren, Benjamin; Keaveney, James
    This dissertation describes an analysis of events containing three leptons from 136 fb−1 of proton-proton collision data, with a centre of mass energy of 13 TeV, recorded by the ATLAS detector between 2016 and 2018. The aim of the analysis was to search for evidence of a top quark produced in association with a W boson and a Z boson (tWZ). An event selection scheme was developed using simulation to broadly suppress background events and to select signal events. Events were separated into mutually-exclusive regions of phase space to increase the ratio of signal to backgrounds and to calibrate the modelling of the backgrounds. Background events were further suppressed through the use of the Gradient Boosted Decision Tree (GBDT) machine learning algorithm. First, a hadronically-decaying W boson candidate was identified using a GBDT; this was used to suppress WZ background events. Then, an event-level GBDT was used to suppress all background events. A maximum likelihood fit was used to estimate the signal strength µ of tWZ production, where nuisance parameters were assigned to theoretical and experimental systematic uncertainties. The work presented here forms the basis of an official ATLAS experiment analysis, thus, the signal region was blinded to avoid potential biases in the future development of the official ATLAS analysis. The best fit value of µ resulting from a fit to a modified Asimov dataset was µˆ = 2.08+1.48 −1.45. This corresponds to an expected significance of Z exp µ = 0.72 σ. An expected upper limit µ exp up = 2.77+2.39 −1.28 was also determined from the fit to the modified Asimov dataset. Thus, this analysis has the potential to put the strongest ever constraint on tWZ production, but does not have the potential to observe tWZ production as predicted in the Standard Model. These constraints are limited by statistical uncertainties.
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    Characterizing Single Event Upsets within the lpGBT-based End-of-Substructure Card
    (2021) Naidoo, Joash Nicholas; Wyngaard, Janet; Keaveney, James
    The CERN ATLAS particle physics experiment is currently undergoing a significant system upgrade (ATLAS Phase II upgrade). As a result of the upgrade the experiment's Inner Tracker (ITk) and the front-end electronics of the ITk are being redesigned to handle increased data rates and a higher radiation environment. Within the ITk, the End Of Substructure (EoS) card is a new custom designed digital board that will provide the data, command, and power interface between on and off-detector electronics. Each EoS card makes use of one or two custom CERN designed low power Gigabit Transceivers (lpGBTs) ASICS that have been created for the purposes of supporting high bandwidth optical links in high radiation environments throughout CERN experiments. An estimated 1552 EoS cards will be installed in the ITk, each representing a potential point of failure. Given the complexity and quantity of new hardware designs involved, and that the EoS cards will be not be accessible or serviceable after the upgrade has been completed, there is a need for rigorous quality assurance (QA) and quality control (QC) testing. This thesis therefore describes an independent test setup commissioned, by the author, at the University of Cape Town (UCT) Physics Department for characterising aspects of EoS card's operation under representative radiation conditions. Specifically, the radiation environment of the ITk poses a challenge to electronics as energetic particles can deposit their energy within the circuit material resulting in an erroneous change in logic known as a Single Event Upset (SEU). The lpGBT is a radiation tolerant ASIC and employs digital signal processing (DSP) and triple modular redundancy (TMR) techniques to mitigate against the effects of SEUs on transmitted data. This thesis presents an experiment setup which tests this hypothesis that the DSP stages are susceptible to data corruption caused by SEUs. In addition the setup also attempts to characterize the susceptibility of the scrambler, encoder, and interleaver stages within the lpGBT to SEUs. This experiment is carried out by actively irradiating an EoS card with a neutron source (energy spectrum of up to 11 MeV), while emulating each stage on a non-irradiated off-board FPGA. Additionally and in support of this experiment, the existing firmware and LabView automation software developed at DESY are extended. Results from this thesis indicate that the DSP stages within the lpGBT are susceptible to data corruption caused by SEUs. It was also shown that the susceptibility of the experiment itself did not effect the measured SEU rates. Finally, preliminary results suggest that susceptibility of the DSP stages within the lpGBT can be characterized as the Bit Error Rate (BER) increases depending on the number of active stages.
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    Investigating the Optical Link Performance of the End-of Substructure Card and Susceptibility to SEUs
    (2023) Van, Der Merwe Max; Wyngaard, Jane; Keaveney, James
    Particle physics experiments carried out by CERN attempt to investigate the fundamental forces of matter. One of these experiments is the ATLAS experiment, which studies the proton-proton collisions in the LHC. A series of upgrades are planned to increase the luminosity by a factor of five, leading to the high-luminosity LHC (HL-LHC). This upgrade will increase the potential for new discoveries but brings with it design challenges in relation to the harsh radiation environment and significant data throughput required. The ATLAS experiment is building a new detector to cope with these challenges, titled the Inner Tracker (ITk). A crucial part of this new detector is the End-of-Substructure (EoS) card, which constitutes the interface between the ondetector electronics and the off-detector systems. In addition to the operational challenges, the HL-LHC does not allow for repairs or replacing of EoS cards once operation commences, emphasizing the need for thorough testing and qualification of this component. This thesis focuses on characterizing the performance of the EoS card in the presence of radiation, under non-ideal operating conditions and the impact of optical link parameters. The first set of tests is centered on qualifying the radiation tolerance of the EoS card. The radiation environment within the ITk poses a threat to the stable operation of electronics as energetic particles have the potential to cause erroneous changes in device logic, known as Single Event Upsets (SEU). The SEU susceptibility of the EoS card, with a focus on the Versatile Link Plus Transceiver (VTRx+) component, is studied by irradiating the EoS card with a neutron source with a distributed energy spectrum and a peak energy of 11MeV while performing a bit error rate (BER) test to monitor for radiation induced errors. The second set of tests deals with characterizing the impact of an irregular power supply on the EoS card's performance through simulating noise on the supply lines and monitoring the response in BER. The final set of tests investigates the impact the VTRx+ configuration parameters have on the quality of the optical signal. These tests were carried out at the University of Cape Town (UCT) with the support of DESY, a national research institute in Germany, responsible for the production of the EoS cards. A number of new firmware, software and hardware modules were developed as part of this work in order to carry out the tests required. The most significant of which comprised a novel firmware addition allowing for the evaluation of the optical signal quality with an FPGA. This contribution is now being integrated into the quality control proceedings at DESY, to be used in assessing optical signal quality of the entire set of approximately 1552 EoS cards being produced.
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    Measurement of the leptonic charge asymmetry in ttW± production using the trilepton final state in proton-proton collisions at √ s = 13 TeV using the ATLAS experiment
    (2022) Garvey, Cameron; Keaveney, James; Yacoob, Sahal
    In this dissertation, a measurement of the leptonic charge asymmetry (A ` C ) in top quark pair production in association with a W boson (t ¯tW±) is presented. The A ` C is sensitive to new physics beyond the standard model, such as the axigluon and as a result, a measurement of the A ` C could prove useful in searches for new physics. The data set used in this measurement consists of proton-proton collisions at the Large Hadron Collider (LHC) at √ s = 13 TeV, which was recorded using the ATLAS experiment and corresponds to an integrated luminosity of 139 . An event selection scheme was put in place to optimally select for t ¯tW± events in the trilepton final state while suppressing background events. The A ` C is calculated using the pseudorapidities of the two leptons that decay from a top quark and a top anti-quark. A lepton-top association was implemented using machine learning which was used to correctly identify the leptons which decay from top quarks in 72% of t ¯tW± events. Using the results obtained from the lepton-top association, the A ` C was measured using a method called the fit across regions (FAR) method. This method makes use of machine learning to differentiate between signal (t ¯tW±) and all of the backgrounds to increase the sensitivity to the A ` C . Using the FAR fit, a leptonic charge asymmetry of A ` C = -9 % ± 22 % was extracted from Asimov data which is consistent with Standard Model prediction of the A ` C , as expected. Results of a fit to the ATLAS data remain blinded as this analysis forms the basis of an official ATLAS measurement which is yet to be published. The dominant source of uncertainty results from the limited size of the data set. Further data acquired at the LHC over the next decade should reduce the impact of the dominant uncertainty of the measurement of the A ` C in t ¯tW±.