3. Electronic Theses and Dissertations (ETDs) - All submissions

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A serendipitous MeerKAT discovery of a neutral hydrogen (H I) rich galaxy group with megaparsec-scale filamentary-like structure
(2024) Lawrie, Graham David
Neutral hydrogen (H I) makes up a significant portion of cold gas in galaxies, and is an important component within galaxy formation and star formation. Environmental effects within cosmological overdensities such as galaxy clusters and groups are known to dramatically influence galaxy evolution, and H I content and morphology are important diagnostics thereof. This thesis reports the discovery of an H I galaxy group of 32 galaxies at z ∼ 0.041, as well as detail the H I and multiwavelength properties. The group, being H I-rich and spatially-resolved, allows for a detailed study of the role of the environment in the early stages of transforming its members’ gas content. Star formation rate (SFR) for individual galaxies is derived from near and mid-infrared photometry, and host galaxy study is done using sub-arcsecond imaging with Dark Energy Survey. The group appears to have a Mpc-scale filamentarylike structure. The data used in this project was observed as part of the MeerKAT Galaxy Cluster Legacy Survey (MGCLS), a radio continuum survey, but shows the potential for commensal spectral line and continuum science with modern radio interferometers.
Beam shaping and amplifiers
(2024) Harrison, Arthur Justin
Tailored or structured laser light has received much attention from the photonics community globally. Structured forms of laser light have the potential to enhance many aspects of laserenabled processes across a myriad of applications, such as industrial manufacturing processes, optical communication, quantum computing, medical sciences, and space sciences. While the laser itself is well over 60 years old, the range of obtainable "structured" laser outputs has been restricted for many decades since its inception. With recent advances in micro-mechanical electro-optics, liquid crystal display, and advanced lithographic processing technologies, it is possible for us to create highly complex forms of structured light with exotic spatial, phase, and polarization characteristics. As we make the inevitable transitions towards updating the existing laser technology with structured light systems, there are certain parameters that need to be met such as average laser power, which is crucial for performing micro and macro material processing, accounting for 15% of the global demand for lasers. Therefore, in addition to the selection of structured light, there is a clear need for obtaining high-power structured light which generally involves the need for optical power amplifiers. This dissertation explores the generation, power scaling, and characterization of structured light for application areas that demand high-power laser outputs. To fully characterize the process of power scaling of structured laser light, an accurate 3D numerical model is required. Here we present a new 3D model for an end-pumped cylindrical rod Master Oscillator Power Amplifier (MOPA) system, using contemporary analytical expressions and novel approximations which demonstrate significant improvements over current comparative 3D modelling approaches. Then, we explore the amplification of structured light fields, in the form of higher-order Laguerre-Gaussian modes, using a novel polarization-based dual-pass MOPA. This system was specifically developed to maximize amplification efficiency while maintaining the purity and complex structures of high-order Laguerre-Gaussian modes and showed excellent agreement with the 3D simulated results. Finally, we investigate the thermallyinduced aberrations resulting from end-pumping bulk solid-state gain media. We show that amplification of higher-order Laguerre-Gaussian modes in this aberrated system, specifically those possessing orbital angular momentum, results in the separation of the phase singularities, otherwise known as "vortex splitting". We fully study this effect and describe the cause and rectification of this phenomenon
Characterization of radiochromic film to measure small photon field output factors
(2024) Mabhengu, Thulani
AIM: This study aimed to investigate radiochromic (EBT3 type) film’s characteristics for the measurement of small photon field output factors and beam profiles for the field sizes ranging from 0.5 x 0.5 cm2 to 10 x 10 cm2 for the three photon energies, 6MV flattening filter free (FFF), 6MV flattening filter (FF), and 10MV FF. MATERIALS AND METHODS: Small-field dosimetry poses significant challenges and remains an active area of research in medical physics. In this study, radiochromic film (RCF), specifically Gafchromic EBT3 was used for small field measurements. The performance of RCF was compared with other conventional detectors such as ionization chambers, and a two-dimensional (2- D) detector amorphous silicon electronic portal imaging device (a-Si EPID). In addition, a comparison of the film measurements with software-calculated data, i.e, treatment planning system (TPS) was conducted. RESULTS: The field output factors (FOF) measured with EBT3 film compared to the TPS data for the smallest field size of 0.5 x 0.5 cm2 was found to be less or equal to 0.06% in all three photon energies. Volumetric detectors in 6MV FF beam showed an increase in % difference of up to 0.3 higher than the EBT3 film and TPS data. A higher quantitative percentage difference of 0.6% in FOF measured with a-Si EPID compared with the RCF data was observed in the smallest field size of 0.5 x 0.5 cm2 in higher energy beam, 10MV FF beam. CONCLUSION: The properties of RCF such as tissue-equivalent, less photon perturbation as well as volume averaging effects improve the accuracy of FOF measurements in small-fields compared to the ionization chambers utilized in this study. Therefore, RCF could serve as a reliable reference detector for small-field measurements.
Development of a dosimetry audit methodology for advanced radiotherapy
(2024) Maselesele, Humbulani Vincent
Radiotherapy treatment technologies are advancing rapidly and this necessitates the establishment of new dosimetry audit methodologies for quality improvements. The International Atomic Energy Agency / World Health Organization introduced a postal dosimetry audit service for high energy photon beams in reference conditions in 1969 using thermoluminescent dosimetry. Together with other dosimetry audit networks including Imaging and Radiation Oncology Core Houston, more complex methodologies to audit a range of radiotherapy activities have been developed over the years. Most routine audits are carried out for high energy photon reference beam output only, owing to the coverage and resources needed. End-to-end dosimetry audits include the entire radiotherapy process associated with a particular technique. This study focuses on the development of a remote end-to-end audit methodology using ionization chamber and radiophotoluminescent glass dosimeters, for complex radiotherapy treatments and requires participation of the entire radiotherapy team. The methodology that was developed uses a head and neck phantom (with inserts for an ionization chamber or solid-state dosimeters) that was scanned, planned and treated in accordance with a given patient histology and treatment prescription based on a typical diagnosis. Five clinical teams were requested to use their local scanning protocol, delineate target volumes and other organs at risk associated with the treatment site using the local clinical and margin protocols, and then prepare a treatment plan. After treatment planning, local quality control checks and final plan approval were carried out and the phantom was then treated on a linear accelerator. During treatment, the dose at a specified point (defined in the centre of the phantom in the vicinity of the target and represented by three external markings on the phantom), was measured using a PTW 3- D pinpoint ionization chamber (Type: 31022). A second treatment delivery was then performed with a set of radiophotoluminescent dosimeters embedded at a fixed position in the phantom. In addition, small field output factors were measured for a range of selected equivalent square field sizes and compared with the onsite treatment planning iii commissioning data from the centres. The data were then anonymized and centrally analyzed. Major differences were found in the delineation of target volumes and as a result, comparative data analysis of the planning was limited, which highlighted the need for clinicians to participate in external audit teams. Differences of up to 7.5% were observed when comparing the treatment planning calculated doses to the doses obtained from the dosimeters placed in the phantom. Differences of up to 17.0% were observed in the small field output factors for equivalent square field sizes of 0.5 cm x 0.5 cm. The study also demonstrated that the end-to-end audit methodology was not sensitive to the errors in the extrapolation of small field output factors in the planning systems. The results of this study have shown that there is a need for additional steps to be audited separately prior to performing end-to-end dosimetry audits for complex treatment techniques. In addition, differences in clinical approaches, which include clinical interpretation of a prescription, volume definitions, and the evaluation and reporting of dose objectives and constraints, need to be addressed separately. Following this, methodologies that provide pre-defined target and organ at risk volumes are likely to be more efficient for an end-to-end audit programme. On-site patient specific quality assurance alone is not sensitive to errors in the commissioning of the treatment planning systems and some treatment planning modelling parameters should be audited separately
Gauge gravity dualities from group representation theory
(2022) Gandote, Sonagnon Eunice Edwige
This thesis considers two distinct problems. First we discuss scrambling and equilibration in N = 4 super Yang-Mills theory using operators that have a very large dimension, of order N2 . A basis for these operators, is provided by the so-called Gauss graph operators. The operators are labelled by a pair of Young diagrams and a graph. We characterize the typical graph and the dynamics associated to it. We show that the resulting dynamics is that of a fast scrambler. Our system equilibrates in a time scale given by t ∼ p λ where p is an order N number equal to the number of nodes in the graph and λ is the ’t Hooft coupling. Finally we use bilocal holography to explore the duality between the free O(N) vector model and higher spin gravity. We demonstrate a mapping between the CFT and the higher spin gravity that is determined by the symmetry of the problem. We then turn to a study of the geometry of this mapping. Using a specific code subspace, we demonstrate that bilocal holography reproduces the entanglement wedge reconstruction. We also make contact with ideas that have been influential in the holographic computation of entanglement entropy.
H.E.S.S. observations of the Gamma-ray Binary LMC P3
(2024) Fisher, Lalenthra
Context: The first extra-galactic gamma-ray binary, LMC P3, is a gammaray binary comprising of an unconfirmed compact object and an O-star and is located in the Large Magellanic Cloud. Initially discovered in Fermi-LAT data, it shows an orbital period of 10.3 days. H.E.S.S. has reported the detected Very High Energy (VHE) gamma-ray emission during only 20% of the orbit (Abdalla et al. 2018), between orbital phases 0.2 and 0.4, which roughly corresponds to the inferior conjunction of the compact object. Purpose: H.E.S.S. has continued the observations of this object since then. This work aims to produce a data analysis with a much deeper data set. The new data allowed for a more precise measurement of the location of the VHE gamma-ray peak along the orbit of the system to be made. Methods: Using LMC observations from the HESS array from 2003- 2022, a spectral and temporal analysis was conducted using the analysis package Gammapy. N 157B is used as a standard candle for a consistency check. The analysis incorporates the latest orbital solutions for the system. Results: The peak of the VHE emission was found at a phase 0.268 - 0.366, which with the current orbital solution, places this after inferior conjunction. Conclusions: The VHE light curve is better constrained w.r.t. the previous H.E.S.S. publication(Abdalla et al. 2018). The peak of emission is reduced from 20% to just under 10% of the orbital period. In-depth physical modelling is required to explain the physical origin of the VHE gamma-ray emission.
Investigating the strength of the scissors resonance in 151Sm
(2022) Magagula, Sebenzile Pretty Engelinah
The isotopic chain of samarium is one of the isotopic chains with a number of stable isotopes. The scissors resonance (SR) of 151Sm was studied with the aim of understanding the evolution of the SR along the Sm isotopic chain. Change in deformation of a nucleus from prolate through spherical to oblate, leads to changes in statistical properties, particularly the -strength function (SF) and nuclear level density (NLD). The evolution of the resonance modes such as the SR depends on the deformation of the isotopes. The experiment was performed at the Oslo Cyclotron laboratory where a 152Sm selfsupporting target was bombarded with a 13.5 MeV deuteron beam. The reaction 152Sm(d,t)151Sm populated the nucleus of interest. An array of Sodium Iodine NaI(Tl) detectors, CACTUS, detected -rays and the silicon particle telescope array, SiRi, was used to detect charged particles in coincidence. The NLD and SF were extracted below the neutron separation energy, Sn, using the Oslo Method. These results were used to investigate the SR in the 151Sm and the extracted B(M1) was compared to that of previously measured samarium isotopes. This study provides a near complete picture of the evolution of the SR for the Sm isotopic chain.
MeerKAT’s view of radio morphologies in the nearby X-shaped and dual-AGN system NGC 326
(2024) Rasakanya, William Tabudi
Hierarchical structure formation in the Universe naturally predicts frequent galaxy mergers, and as a result, supermassive black hole mergers. A dual-AGN phase will then occur during the course of those mergers. Constraining the timescales spent at this stage provides predictions for supermassive black hole merger event rates and constraints on galaxy formation and evolution. Supermassive black holes at the centres of their host galaxies actively accrete matter in some systems, resulting in a production of radiation in a broad wavelength range, and in a fraction of those, radio-bright jets. The relativistic jets are usually paired and can span from parsec to megaparsec scales. A subclass of those have exotic radio morphologies, including the so-called ‘X-shaped’ sources. The study of such systems is important to better understand the intrinsic and extrinsic physical properties and feedback mechanisms influencing the evolution of radio jets, as well as the identification of exotic systems which may be used as physical laboratories. In this thesis, we present MeerKAT’s first look at the prototypical, dual-AGN and previously characterised X-shaped system, NGC 326. The extended large-scale diffuse structure of NGC 326, revealed first by the LOFAR 144 MHz maps, is unveiled for the first time at GHz frequencies. Analysis of the MeerKAT L-band map, in combination with the LOFAR 144 MHz map, suggests that the production of the wings and the extended structure is a consequence of hydrodynamical backflows within the radio structure residing inside a merging cluster. Due to the high imaging fidelity, sensitivity, and dynamic range, the large-scale structure was able to be captured in the gigahertz observation, despite the low surface brightness and steep spectral index of the older plasma of the source. Detailed, multi-band imaging of nearby universe X-shaped sources such as this will directly assist in discerning the relevant physical drivers of the increasing number of exotic radio source morphologies being identified in SKA precursor and pathfinder survey
Multiwavelength study of the 2020 and 2021 flares of S5 1803+784 and BL Lacertae
(2022) Omojola, Joseph
This thesis presents a detailed spectral and temporal analysis of two blazars (S5 1803+784 and BL Lacertae) during their recent flaring states from 2020 to 2021. They are both low synchrotron peak (LSP) BL Lac objects and reached their highest daily fluxes yet observed during these flares. The time-resolved spectral energy distribution (SED) and the spectral characteristics are used to study the high-energy emission mechanism and the nature and location of the emitting region in the framework of the single-zone leptonic jet model. The phenomenological spectral fits provide some constraints on the characteristics of the jet and emitting region structure, gamma-ray emission process, and the likely driver of particle acceleration. The two sources have unique emission characteristics that suggest the existence of external soft photons from outside the jet (EC) undergoing inverse Compton scattering in addition to synchrotron self-Compton scattering of the synchrotron photons intrinsic to the jet. In the case of S5 1803+784, the temporal analysis indicates six distinct flares, four of which have symmetric profiles, while two are asymmetric. The broadband SED of S5 1803+784 during the period labeled as Flare A, which peaked on April 12, 2020, was fit to a single-zone model using both synchrotron+SSC only and SSC+EC components. The X-ray emission in the quiescent state comes from inverse Compton scattering, while during the flare, the X-ray forms part of the upper tail of the synchrotron emission due to the acceleration of the emitting particles in the photon radiation field. The bestfit parameters show that the dusty torus (DT) may be the primary source of the external photons responsible for the inverse Compton emission. The synchrotron+SSC only model only fits the data if some parameters have unphysical values, favouring the SSC+EC model. The singlezone leptonic jet model could fit the SED during the flare with an emitting region size of 𝑅 = 3.42 × 1016 cm and a distance from the central black hole 𝑅𝐻 = 5. 00 × 1017cm. The size of the emitting region becomes more compact during the flare; one possible scenario is a collapse of the emitting region size, which could be triggered by sudden rearrangement of the magnetic field lines within the emitting region. The hard power-law spectrum of the low energy spectral slope is consistent with magnetic dissipation as the likely driver of particle acceleration during the flare. The highest daily flux ever observed from BL Lacerate (64.86± 3.16) × 10−7 cm−2 s −1 in the 0.1-300GeV energy range was observed with a flare duration of less than six days. The shortest flare has a duration of 2.1 days; the short variability time scale during the flares was also observed in both the X-ray and the optical wavebands. The BL Lacertae flare labeled as Flare A1 has an asymmetric flare profile with flare-time gamma-ray flux variability of 84%, the highest observed up to that time from BL Lacertae, with a less than a day variability timescale. The size of the emitting region of 𝑅 < 2.70 × 1015 cm, estimated using both the VLBI measured Doppler factor and the variability time scale (𝑡𝑣𝑎𝑟) from the gamma-ray flux cannot fit the SED with either the single-zone synchrotron+SSC-only or the SSC+EC model; the inability of the single-zone model to satisfy this constraint points to the possibility of multiple emitting blobs and that the gamma-ray flux variability timescale (𝑡𝑣𝑎𝑟) is likely from such blobs. The single-zone SED model produces the best fit with an emitting region of size 𝑅 = 1.38 × 1017 cm and the distance of the emitting region from the black hole 𝑅𝐻 = 1. 37 × 1017cm, which were used to constrain and model the SED in two-zone emission SSC+EC models. The two-zone model can reproduce the emitting region's physical features and spectral characteristics during the flare. In both blazars' SED models, the emitting region is particle-dominated. The gamma-ray emitting region electron low-energy spectral slopes return a hard power-law consistent with magnetic turbulence driven by reconnecting field lines. The two blazars share some similarities in that they are both BL Lac objects and LSP in their quiescent states, and the same mechanism may be responsible for enhancing particle acceleration in the emitting region during the flares; they require different approaches to explain the emitting region structure and emission processes and the observed fluxes during the flares. While S5 1803+784 SED S5 shows synchrotron-dominated X-ray emission during the flare for the first time, BL Lacertae requires a two-zone model to satisfy the variability time scale constraint.
Not a jet all the way: an exploration of the strongly interacting dark sector in ATLAS and beyond
(2024) Sinha, Sukanya S
Collider searches for dark matter (DM) so far have mostly focussed on scenarios, where DM particles are produced in association with heavy standard model (SM) particles or jets. However, no deviations from SM predictions have been observed so far. Several recent phenomenology papers have proposed models that explore the possibility of accessing the strongly coupled dark sector, giving rise to unusual and unexplored collider topologies. One such signature is termed as semi-visible jet (SVJ), where parton evolution includes dark sector emissions, resulting in jets interspersed with stable invisible particles. Owing to the unusual MET-along-the-jet event topology this is still a largely unexplored domain within LHC. This thesis presents the first results from a search for SVJ in t-channel production mode in pp collisions for an integrated luminosity of 139 fb−1 at centre-of-mass energy corresponding to 13 TeV at the LHC, based on data collected by the ATLAS detector during 2015-2018. Additionally, studies are performed to explore the use of jet substructure methods to distinguish SVJ from SM jets in the first two scenarios, using observables in a IRC-safe linear basis, and ways forward are proposed for this approach to dark-matter at the LHC, including prospects for estimating modelling uncertainties.
Probing heavy-quark production vs charged-particle multiplicity in pp collisions at √ s = 5.02 TeV with ALICE
(2024) Mdhluli, Joyful Elma
The differential heavy-flavour (charm and beauty quark) production as a function of the charged-particle multiplicity is investigated in pp collisions at √ s = 5.02 TeV. The heavy-quark production cross section as a function of transverse momentum (pT) and pseudorapidity (η) is also evaluated. The measurement of the inclusive yield of heavy quarks via the single-muon (µ) decay channel is done in the muon spectrometer at forward pseudorapidity (−4 < η < −2.5) and the charged-particle multiplicity using the Silicon Pixel Detector at central pseudorapidity (|η| < 1). These measurements are important in order to gain more insight into which processes are involved in the collision at a partonic level as well as provide information on the interplay between hard and soft mechanisms during particle production. Reference data taken during Run 2 in 2015 and 2017 in pp collisions at √ s = 5.02 TeV are analyzed for measurements of the inclusive heavy-flavour decay muon differential cross-section and heavy-flavour decay muon yield as a function of the charged-particle multiplicity. A total number of 102.7 M (21.34 M) events for the single muon low (high)- pT triggers are analyzed for both measurements. Minimum bias triggered events with multiplicity tracklets > 0 are used for multiplicity estimation. Two methodologies are utilized, the data-driven method and the ”Official Framework”. Results of inclusive heavy-flavour decay muon differential cross-section measured as a function of pT are compared with the theoretical model FONLL, the data and theoretical model are in agreement within systematic uncertainties. The results are also compared with measurements in pp collisions at √ s = 7 TeV, an energy dependence is observed. Results of inclusive heavy-flavour decay muon yield as a function of the charged-particle multiplicity indicated an approximately faster than linear correlation. Comparisons with other heavy-quark measurements vs charged-particle multiplicity is made and the measurements are in agreement at low charged-particle multiplicities. This thesis also reports on the Service Task work done. This entailed tests performed on detector elements of the Muon Chambers (MCH) that suffered high-voltage trips that were repaired during the long shutdown (LS2). The detector elements were successfully repaired and are currently stored as spares for the Run 3 data-taking period.
Quality management in brachytherapy: a risk-based approach
(2024) Boroto, Motshelo Godwil
AIM: The aim of this dissertation was to use the prospective risk assessment methods: process mapping, failure modes and effects analysis (FMEA) and fault tree analysis (FTA) to identify areas of risk in the brachytherapy process and use the results to design a new quality assurance (QA) program. MATERIALS AND METHODS: The study was conducted in three phases in which a multidisciplinary team of professionals participated through completion of questionnaires. Results of the analysis of these feedback were then used to re-design the brachytherapy quality program of the case study private hospital. RESULTS: Over 20 potential failure modes (FMs) were identified along the brachytherapy process. 20% of these FMs were analyzed further based on their risk priority number (RPN). A collective of all these FMs were found to directly or indirectly contribute to the patient’s dose misadministration. Applicator in wrong position ranked the highest. Movement of patients on a stretcher in between departments was identified as one of the main causes. CONCLUSION: New protocols were recommended for dose prescription, imaging and treatment planning. Usage of the kV CBCT in the linac bunker was implemented for brachytherapy imaging and treatment planning. Checklists were developed as QA tools to intercept most of the identified FMs. Additional quality control (QC) tests were also suggested and some traditional tests deemed unnecessary by the prospective methods were excluded from the QA program. Application of risk-based methods helped in assigning resources to the most vulnerable parts of the brachytherapy process in the private hospital.
Reliability testing and upgrade of a low voltage power supply for the front-end electronics of the ATLAS Tile Calorimeter Phase-II Upgrade
(2022) Nkadimeng, Edward Khomotso
The LHC is planned to deliver five times the nominal instantaneous luminosity in the High-Luminosity LHC (HL-LHC). To address the projected greater detector occupancies and harsher radiation environment at the HL-LHC, considerable upgrades to most of the ATLAS detector sub-systems are required. The Phase-II Upgrade is planned to last approximately three years and is scheduled for 2026-2029. The ATLAS Tile Calorimeter (TileCal) will require the complete replacement of the readout electronics in order to accommodate its acquisition system to the increased radiation levels, trigger rates, and high pile-up conditions during the HL-LHC period. The Phase-II upgraded readout electronics will digitize the PMT signals from every TileCal cell for every bunch crossing and will transmit them directly to the off-detector electronics. In the counting rooms, the off-detector electronics will store the calorimeter signals in pipe-lined buffers while transmitting reconstructed trigger objects to the first level of trigger at 40 MHz. The replacement of the on-detector and off-detector read-out electronics for increased resolution, trigger performance, and radiation tolerance for the HL-LHC operation conditions is a major part of the Phase-II TileCal upgrading activities. The TileCal upgrade project has undergone an extensive research and development program as well as several test beam campaigns. Wits University’s Institute for Collider Particle Physics is responsible for 50% (1024 units) of the total upgrade low voltage power supply transformer-coupled buck converter (Bricks) that delivers 10 V to the front end system. In the year 2022, a large-scale production and testing is planned for around 1024 radiation-tolerant power supplies with the same output voltage. The LVPS Brick V7.5.0 currently installed within the Tile Calorimeter will be completely replaced as part of this production. The LVPS Bricks are located within the TileCal’s inner barrel and their operational reliability is off critical importance. As a result, performance screening will be implemented, which will include testing and certification of all LVPS Bricks manufactured. Radiation tests for Total Ionizing Dose (TID), Non-Ionizing Energy Losses (NIEL), and Single Event Effects (SEE) were also carried out to evaluate the radiation tolerance strategies used in the design and to qualify the LVPS Brick for the HL-LHC requirements, as per the ATLAS policy on radiation tolerant electronics.
Search for a heavy resonance decaying into two photons in association with b-jets with the atlas detector and constraints on a 2HDM+S model
(2022) Shrif, Esra Mohammed
This thesis consists of two parts. The rst part, a two-Higgs doublet model extended with an additional singlet scalar (2HDM+S) is motivated and discussed. A brief introduction to the model and its parameters space is provided. Constraints are applied to the parameter space of this model in order to accommodate a number of discrepancies in the data that have been interpreted in [1] as the result of the H → Sh decay produced via gluon-gluon fusion and in association with top quarks. Implications on the phenomenology of the heavy pseudo-scalar (A) and charged scalar (H+) are discussed. In particular, the decays A → ZH and H+ → W+H become prominent. This leads to nal states with multiple leptons and b-quarks. The decay A → ZH → ZSh results in the production of a high transverse momentum Z produced in association with a lepton and two b-quarks with little additional jet activity. These predictions are compared to the data with the model's benchmark points. With the parameters obtained here, the model is able to accommodate the features at the LHC reported in [1]. Without varying these parameters additional excesses in the Zb¯b and tt¯invariant mass spectra, and the production of 3 leptons plus two b-tagged jets can be explained assuming mA ≈ 600 GeV. In the second part of this thesis, a search for a heavy resonance decaying into two photons in association with b-tagged jets is presented. The search uses proton proton collision data taken at centre-of-mass energy of 13 TeV with the ATLAS detector at the LHC during 2015 to 2018. The dataset used corresponds to a total of 139 fb−1 integrated luminosity. Three di erent models are tested in this search. A Higgs boson like heavy scalar X produced with top quarks, b quarks or Z boson decaying into b ¯b are examined. Limits on these models are set at 95% CL on the production cross-section times branching ratio into two photons, for masses ranging from 160 GeV-1.6 TeV.
Searching for multi-messenger counterparts to the sources in the H.E.S.S. Galactic Plane Survey
(2023) Batzofin, Rowan
The High Energy Stereoscopic System (H.E.S.S.) Galactic Plane Survey (HGPS) has detected very-high-energy (VHE) gamma-ray emission from 78 sources in the Milky Way. These sources belong to different object classes (pulsar wind nebulae, supernova remnants or binary systems) and some of these sources remain unidentified. The gamma-ray emission of these objects may be of leptonic or hadronic origin and gamma-ray observations alone cannot distinguish between these two scenarios. The detection of neutrino emission would provide evidence for a hadronic scenario and fitting a combined synchrotron and inverse Compton model would provide evidence of a leptonic scenario in these objects. MeerKAT surveyed the 1st and 4th quadrant of the Milky Way. There are 8 unidentified H.E.S.S. sources in the regions surveyed by MeerKAT. This process of searching for radio emission coincident with the unidentified sources in the HGPS can be expanded to the whole Galactic Plane once MeerKAT has surveyed it all. One unknown and two known H.E.S.S. sources (HESS J1843-033, HESS J1826-148 and HESS J1833-105) are modelled with a combined synchrotron and inverse Compton model using the MeerKAT radio data and H.E.S.S. VHE data. HESS J1843-033 is an unidentified source, HESS J1826-148 is a binary and HESS J1833-105 is a composite SNR. That these sources could be fit by a combined synchrotron and inverse Compton model suggest that they are leptonic in nature. Based on the observed gamma-ray spectra we predict the neutrino emission of all the sources in the HGPS under the hypothesis that the emission is solely of hadronic origin. This prediction relies entirely on observation and is independent of the source class, the distance or the ambient target material. These predictions are used to create an empirical model for the neutrino emission of the Milky Way. This model can be used to search for neutrino emission from individual gamma-ray sources as well as testing for neutrino emission from potential source populations in the Milky Way. This model was then compared to the possible neutrino sources detected by ANTARES where 56 of the possible neutrinos were associated with the predicted neutrino emission but there is no significant correlation to the ANTARES neutrino candidates and the predicted neutrino emissions.
Synthesis and the investigation of structural, electronic and magnetic properties of 𝐑𝐮𝐅𝐞𝟑𝐒𝐢 and 𝐑𝐡𝐅𝐞𝟑c
(2022) Magoda, Nyawasedza
Iron-based novel materials have been generating enormous interest within the scientific community for a number of decades. The efforts on such materials have intensified after the discovery of the fascinating iron-pnictide superconductors. This intriguing discovery also provides added evidence to the significance of the structure-property relationship in establishing certain functional properties. This study is expected to lead to potential exciting and tuneable properties and might open a new direction for future investigations. The focus of this research is on the synthesis and physical property characterization of novel metallic compounds, RuFe3Si and RhFe3C. In this study, the compounds under investigation were synthesized using arc-melting, the properties of these materials were explored using various experimental techniques such as X-ray diffraction, Mössbauer Spectroscopy both performed at room temperature, magnetic susceptibility (𝜒), electrical resistivity (𝜌) and heat capacity (𝐶P) measurements. The results from wavelength-dispersive spectroscopy confirmed the ratio of each of the elements relative to each other of Ru:Fe:Si and Rh:Fe:C as 1:3:1. No impurities were detected for both compounds, but patches of unreacted carbon were identified in RhFe3C. Rietveld refinement of data collected from powder X-ray diffraction experiments established that both compounds crystallized in a cubic structure with 𝐹𝑚3̅𝑚 (225) space group symmetry. RuFe3Si is a single phase compound whereas RhFe3C had a secondary phase of unreacted carbon. Transmission Mössbauer spectroscopy performed at room temperature show that RuFe3Si has four different Fe sites within its structure where one of the sites has similar environment as -Fe. RhFe3C has three distinct Fe sites which are characterized by high magnetic internal fields. The (T) data of RuFe3Si measured at H = 1000 Oe between 2 to 950 K was analyzed and revealed a high transition temperature Tc = 773(5) K. Subsequent measurements performed at H = 100 Oe resulted in an increase in the magnitude of the Curie temperature, Tc = 873(3) K. The  1 (T) data was found to obey the Curie-Wiess law which resulted in 𝜃 = 828(17) K and an effective magnetic moment eff = 3.4(2) B/Fe . The positive sign of the Weiss temperature, 𝜃 demonstrates the presence of ferromagnetic (FM) interactions within the sample. Isothermal magnetization measurements as a function of the applied field were performed at various temperatures ranging from 2 K to 900 K for the compound RuFe3Si. A very narrow hysteresis loop is observed with small values of the coercive field and the remnant magnetization, which is indicative of a soft ferromagnet. The (T) data of RhFe3C measured at H = 1000 Oe between 300 to 1000 K showed no magnetic phase transitions within this temperature range. The 𝜌(𝑇) measurements performed on both samples revealed a positive temperature coefficient indicating that they are metallic. Experimental 𝐶𝑃 (𝑇) and 𝜌(𝑇) data for RuFe3Si was analyzed using Padé approximants representing the Debye model and Bloch-Grüneisen model which resulted in the following Debye temperatures 𝛩D (all T) = 430(1) K. and 𝛩𝑅 = 404(1)K. The 𝐶𝑉 (𝑇) data for RhFe3C was fitted with the Debye model with an added Einstein lattice contribution which resulted in 𝛩D(all T) = 784(14) and 𝛩E(all T) = 237(3) K . Fitting the low temperature data with a linear equation yielded a calculated value of 𝛩D = 453(2) K. The 𝜌(𝑇) data followed a quadratic trend which obeyed 𝜌(𝑇) = 𝜌0 + 𝐴𝑇 2 . This study confirmed the stoichiometric ratio of the constituent elements Ru:Fe:Si and Rh:Fe:C as 1:3:1 as expected. Secondary electron images collected over the polished surfaces of RuFe3Si revealed a homogeneous structure with no detectable impurities whereas in the case of RhFe3C traces of unreacted carbon along with the main phase were observed. Both are ferromagnetic compounds that crystalize within the 𝐹𝑚3̅𝑚 (225) space group symmetry and have a very high Curie ordering temperature.
The use of machine learning in search for new physics at the ATLAS and applications to model COVID-19
(2024) Mathaha, Thuso Stephen
In this thesis, the production of a pair of top quarks in association with a heavy pseudo-scalar (A) is examined. The heavy pseudoscalar subsequently decays into another pair of top quarks, resulting in a final state of four top quarks (ttA → tttt). The ATLAS public paper Ref. [1] provided the analytical framework for this study, which aimed to investigate the four top quarks production in the multilepton final state. The study focuses on final states with two same-sign leptons of different flavours (e.g. e ±, µ±) or at most three isolated leptons (muons and electrons) without any charge requirement, as well as jets. The analysis employs a multivariate discriminant that uses twelve discriminating kinematic variables to separate the signal from the background in an effort to understand the differences between the SM and BSM production mechanisms of four top quarks. The machine learning techniques deployed for the multivariate algorithm were transferred to tackle the COVID-19 pandemic. The COVID-19 pandemic has caused significant health, social, and economic damage worldwide, with many developed countries vaccinating their citizens while African nations relied on clinical public health (CPH) strategies. Recent studies in Botswana and South Africa found age, gender, hypertension and diabetes were significant factors in disease severity, vii with the elderly population aged ≥ 60 years and those with major COVID-19 comorbidities recommended for vaccination. AI was also used to optimize vaccination roll-out strategies, targeting population groups needing
Time-efficient quantum imaging
(2024) Moodley, Chané Simone
Quantum ghost imaging offers many advantages over classical imaging, including the ability to probe an object with one wavelength and record the image with another. The low photon fluxes, which are characteristic to quantum optics, offer the ability to probe objects with fewer photons thereby avoiding photo-damage to light sensitive structures, such as biological matter. Unfortunately, quantum ghost imaging suffers from slow image reconstruction due to sparsity and the probabilistic arrival positions of photons. Progressively, quantum ghost imaging has advanced from single-pixel scanning systems to 2-dimensional (2D) digital projective masks which offer a reduction in image reconstruction times through shorter integration times. The focus of this thesis was time-efficient quantum ghost imaging, the necessary literature is presented and discussed followed by a focus on the technical details and components required for quantum ghost imaging. Several image reconstruction algorithms using two different 2D projective mask types are showcased and the utility of each is discussed. Furthermore, a notable artefact of a specific reconstruction algorithm and projective mask combination is presented and how this artefact can be used to retrieve an image signals heavily buried under artefacts is discussed. Tests to confirm the presence of quantum entanglement were conducted and discussed along with the necessary results to confirm the of quantum entanglement. The Bell inequality was successfully violated with a Bell parameter S > 2, while a full quantum state tomography was performed with an almost perfect fidelity. This thesis was aimed at time-efficient quantum ghost imaging which was achieved through implementing a series of neural network and machine learning based approaches. These approaches consisted of speeding up the image reconstruction process, enhancing images early on in the reconstruction, establishing an optimal early stopping point and achieving image resolutions that are impractical-to-measure in real time. First a two-step deep learning approach was proposed to establish an optimal early stopping point based on object recognition, even for sparsely filled images. In step one the reconstructed image was enhanced after every measurement by a deep convolutional auto-encoder, followed by step two in which a neural classifier was used to recognise the image. This approach was tested on a non-degenerate ghost imaging setup while physical parameters such as the mask type and resolution were varied. A 5-fold decrease in image acquisition time at a recognition confidence of 75% was achieved. The significant reduction in experimental running time is an important step towards real-time ghost imaging, as well as object recognition with few photons, especially in the detection of light sensitive structures. Many computationally intense deep-learning methods have been implemented in an effort to speed up image acquisition times by retrieving image information. Often over-looked, machine learning methods can offer the same, if not better, reduction up in image acquisition time by an object recognition process. Four machine learning algorithms were implemented and trained on a uniquely generated, noised and blurred dataset of numerical digits 1 through 9. Of the tested recognition algorithms, logistic regression showed a 10× speed up in image acquisition time with a 99% prediction accuracy. Additionally, this reduction in acquisition time was achieved without any image denoising or enhancement prior to recognition thereby reducing training and implementation time, as well as the computational intensity of the approach. This method can be implemented in real-time, requiring only 1/10th of the measurements needed for a general solution, making it ideal for quantum ghost imaging and the recognition of light sensitive structures. In quantum ghost imaging the image reconstruction time depends on the resolution of the required image which scale quadratically with the image resolution. A superresolved imaging approach was proposed based on neural networks where a low resolution image was reconstructed. The low resolution image was subsequently denoised and then super-resolved to a higher image resolution. To test the approach, both a generative adversarial network as well as a super-resolving autoencoder network were implemented in conjunction with an experimental quantum ghost imaging setup, demonstrating its efficacy across a range of object and imaging projective mask types. A super-resolving enhancement of 4× the measured resolution was achieved with a fidelity close to 90% at an acquisition time of N2 measurements, required for a complete N × N pixel image solution. This significant resolution enhancement is a step closer to a common ghost imaging goal, to reconstruct images with the highest resolution and the shortest possible acquisition time. The approaches detailed here prove valuable to the community working towards time-efficient quantum ghost imaging. Not only has the image reconstruction process been reduced by up to 10×, but general image solutions have been enhanced through denoising and super-resolving capabilities. Through the introduction of these techniques and algorithms via machine intelligence, a significant step in timeefficient quantum imaging has been achieved.
Towards teleporting quantum images
(2024) Sephton, Bereneice
Achieving higher dimensionality in quantum protocols is receiving increasing interest at the promise of a range of benefits, starting from increased information capacity to noise resilience. This also arises naturally in many quantum systems, from the multitude of photonic states in the temporal, frequency or spatial domains to many atomic levels in an atom. Generalising these to superpositions of states with unique amplitude and phases, we can call such encoded information quantum images. The ability to transfer, either the state of a system directly or information encoded as quantum images thus becomes a pressing frontier with teleportation an important building block. Bearing two distinct features, teleportation forms a fundamentally different way of communicating such that the information does not pass directly between communicating parties and when used with quantum carriers, can form the basis for a variety of quantum networks, starting from quantum repeaters to a type of quantum computing. In this thesis, the over-arching goal involves physically implementing a highdimensional system in an effort towards teleporting quantum images. To do so, Chapter 1 considers the basic language, protocol, characterising techniques and spatial states being used in this work. We then consider bringing in nonlinear optical strategies to the physical implementation of both the generation and detection aspects in Chapter 2. Particular emphasis is made on orbital angular momentum (OAM) states as they are used as a test-bed for our spatial degree of freedom. For this property, the naturally generated states in spontaneous parametric down-conversion are considered where the amplitude structure is neglected and an in-depth investigation done. Here, we look at the radial modal purities in both the generational and detection aspects of such phaseonly structuring and emphasise the general nature with experiments spanning from quantum to classical. Mitigating measures and corrective steps are also introduced which show the effects of maintaining the naturally-preserved eigenmode structures of the wave-equation. Next, the time-reversed phenomenon, sum-frequency generation, was explored as a detection mechanism. We test this classically by interacting different modal structures and looking at the generated modes. In doing so, we showed that the phase-flattening nature of conjugate states in the interaction allows for modal detection that is not confined to the same wavelength. The measurement-conditioned process could then be modelled in quantum formalism as reverse SPDC and introduced as a projector for quantum teleportation. Chapter 3 explores the full theoretical formalism of a non-linear detection-based teleportation system for spatial modes, which is numerically modelled for OAM and implemented as a test-bed. Here, the physical system was characterised and control explored by changing the generation and detection mode sizes. In doing so, we demonstrated a 15-dimensional teleportation system in OAM that exceeds the viii classical limit and extended the implementation to include a variety of states across different bases, extending from polar to Cartesian coordinates. We achieved this by encoding the information on bright coherent laser light. Despite this protocol extending to quantum carriers, present inefficiencies in the non-linear interaction require many copies to to be present, stimulating the process and yielding what we call stimulated teleportation. While the ability to develop efficiencies is both beyond the scope of this work and an active area of research, with promising advances in metasurfaces, we look towards how this system may be further applied and enhanced in Chapter 4. Notably, we numerically map out an optimisation space for pixel states and look at requirements for teleporting images of complex objects, directly. Here, we derive a technique for characterising such teleportation, based on single pixel quantum ghost imaging where signal at higher resolution is conserved, wavelength dependency is lessened and the ability to fully distinguish the complex spatial structure; the latter being is an active challenge in quantum imaging systems. Further applications are also discussed where pump engineering could lead to better fidelities, how a deterministic system could be implemented and how hybrid entanglement channels could lead to the teleportation of hybrid entangled states.
Using multi-wavelength correlations to understand blazar physics
(2024) Van Zyl, Pfesesani Victoria
Blazars are a subclass of Active Galactic Nuclei (AGN) with a jet pointing toward the observer’s line of sight. These objects are highly variable, violent emitters of non-thermal radiation from radio to gamma-rays. The Hartebeesthoek Radio Astronomy Observatory (HartRAO) has been monitoring AGNs since the 1970s and has established itself as a key contributor of radio observations in the southern hemisphere, particularly for sources south of 30 that are beyond the reach of most northern telescopes. Currently, the HartRAO 26-m radio telescope monitors over 40 continuum and spectral line sources without a dedicated data reduction and analysis software package. This has resulted in a backlog of unprocessed observations, including critical observations necessary for the observatory’s operations. In this work, I present a software solution for the HartRAO data reduction and analysis problem and test it on three blazar sources monitored at HartRAO in a multi-wavelength study. My project is arranged in two parts. The first part of my project is a scientific investigation into the potential correlated relationships in multi-wavelength light curves of three HartRAO-monitored blazar sources to establish whether multi-resolution data can be used to support the models claiming that the higher energy gamma-ray emission leads the radio. Using the discrete correlation function (DCF), I estimate the correlations and time lags between the radio and gamma-ray wavelengths of my sample and find positive correlations with the gamma-rays leading the radio wavelengths by ≥ 100 to 200 days, which supports existing literature on the topic. These time lags represent the delay between the emission of gamma-rays and radio wavelengths and provide insight into the physical processes occurring in the blazar’s jet, particularly the processes that lead to the emission of non-thermal radiation, an aspect of AGN studies that is still poorly understood The second part of my project focuses on the technical challenges of data reduction and analysis at HartRAO. I describe how data calibration is performed at HartRAO and test and evaluate the calibration processes to ensure the validity of the results. I also develop a software solution for the continuum data reduction and analysis of HartRAO data. The proposed solution includes a data reduction pipeline that utilizes an SQLite database to store the reduced and intermediate data products, as well as a user-friendly graphical user interface (GUI) for easy and quick processing of data. The new software provides the option to perform both batch and single-file processing and provides better performance compared to the old spectroscopy analysis program (LINES), previously used to process continuum data. By providing a software solution for data reduction and analysis, this thesis will enable HartRAO to make use of the critical observations that are currently backlogged and improve the observatory’s operations

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Browsing 3. Electronic Theses and Dissertations (ETDs) - All submissions by School "Physics"

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