Electronic Theses and Dissertations (Masters)

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Leveraging Machine Learning in the Search for New Bosons at the LHC and Other Resulting Applications
(University of the Witwatersrand, Johannesburg, 2023-09) Stevenson, Finn David; Mellado, Bruce
This dissertation focuses on the use of semi-supervised machine learning for data generation in high-energy physics, specifically to aid in the search for new bosons at the Large Hadron Collider. The overarching physics analysis for this work involves the development of a generative machine learning model to assist in the search for resonances in the Zγ final state background data. A number of Variational Auto-encoder (VAE) derivatives are developed and trained to be able to generate a chosen Monte Carlo fast simulated dataset. These VAE derivatives are then evaluated using chosen metrics and plots to assess their performance in data generation. Overall, this work aims to demonstrate the utility of semi-supervised machine learning techniques in the search for new resonances in high-energy physics. Additionally, a resulting application of the use of machine learning in COVID-19 crisis management was also documented.
Modelling the Sensitivity of the KM3NeT/ARCA and KM3NeT/ORCA to Neutrinos from Quiescent Blazars
(University of the Witwatersrand, Johannesburg, 2023-09) Nkosi, Bhuti Linda; Chen, Andrew
Blazar jet emission has been modelled using two families of models, leptonic and hadronic, to explain the double-peaked SED. In hadronic models, the higher energy peak is explained by proton interactions with the jet material and external fields. Lepto-hadronic models are a type of blazar emission model that accounts for both leptonic and hadronic processes in the jet. In these models, the low-energy bump of the spectral energy distribution SED is produced by synchrotron radiation from primary electrons, while the high-energy bump is produced by a combination of radiation from protons and secondary particles. Lepto-hadronic models can explain the variability and spectral features of blazars in different states, such as flaring or quiescent. In this project, we used a one-zone lepto-hadronic model to simulate the blazar jet and calculate the neutrino emission and detection prospects with KM3NeT.
Multi-messenger Indirect Dark Matter Searches in Milky Way Satellites
(University of the Witwatersrand, Johannesburg, 2023-09) Noorbhai, Raees Mubeen; Beck, Geoffrey
First suggested 90 years ago, the Dark Matter (DM) mystery has been deepened by a range of astronomical observations, from the galactic to the cosmological scale, demonstrating anomalous gravitational phenomena which necessitate the existence of some unknown DM. In the 1970s, particle DM models, including the WIMP hypothesis considered in this work, were proposed and have subsequently been subjected to empirical scrutiny. Over the past 2 decades, all DM direct detection experiments, collider searches and indirect detection searches have failed to detect a DM signal, placing stringent constraints upon WIMP parameters and ruling out WIMP-Hadron interactions. Following the detection of an excess e−/e + flux at approximately 1.4 TeV by DAMPE in 2017, a number of Massive Leptophilc Majorana Particle (MLMP) WIMP hypotheses were proposed to explain the flux. To conduct a model-independent test of these hypotheses, Leptophilic WIMPs in the 1-2 TeV mass-energy range are considered, accounting for self-Annihilation along all leptonic channels, as well as the 3l democratic case. The dwarf spheroidal galaxies orbiting the Milky Way (MW), particularly the Ultrafaints, are DM-dominated and are thus strong candidates for indirect DM searches using next-generation telescopes - such as CTA in gamma, KM3NeT in neutrinos and MeerKAT in radio, with sensitivities that dwarf those of prior telescopes like LHAASO. Accounting for the respective fields-of-view of these telescopes, 6 dwarf spheroidals, 4 Ultrafaints and 2 Classicals, are chosen as potential target environments for the multi-messenger analysis. Equations are also derived for the Mean Free Path (MFP) and Mean Annihilation Period (MAP) of the WIMPs in the respective DM Halos, for the case of both an arbitrary Halo boundary and at the virial radius boundary. Utilising conservative estimates of telescope sensitivities, non-detection upper bounds are placed upon the Annihilation cross-section ⟨σv⟩ψ and Decay rate Γψ. These bounds are taken in comparison to the bounds imposed by the Super-amiokande neutrino search in the MW Halo and centre, the ATCA radio search in Reticulum II and the ASKAP/EMUradio search in the LMC. In all cases, the non-detection bounds imposed by observations of the Ultra faints are more stringent, but with greater error margins than is the case with the Classicals. For CTA, non-detection bounds in the case of all Ultrafaints are competitive with those imposed by the ASKAP/EMU search and stronger than those imposed by both the ATCA and the Super-Kamiokande searches. For KM3NeT, no novel non-detection bounds are imposed for observations of all 6 dwarf spheroidals. For MeerKAT, in the case of the µ −/µ + channel, observations of Reticulum II are competitive with the ASKAP/EMU bounds. From the multi-messenger analysis, it is concluded that the strongest non-detection bounds are imposed by CTA observations of Segue 1 and MeerKAT observations of Reticulum II. In the Decay case, the bounds are compared to those imposed by the Fermi indirect search in the IGRB. In the case of all next-generation telescopes, no novel non-detection constraints can be imposed upon Γψ . In the case of the MFP and MAP results, the non-detection lower limits are often many orders of magnitude greater the Hubble time. At the relic density limit, the Halo-independent MAP at the virial limit is 14 orders of magnitude greater than the age of the Universe. This illustrates the severe extent to which the Annihilation channel for WIMPs has been suppressed, since successive instances of non-detection have placed tight bounds on ⟨σv⟩ψ . In light of this, proposed astrophysical explanations for the DAMPE flux are favourable, as they do not require the presupposition of WIMP Dark Matter.
Properties of composite nanomaterials for gas sensor applications
(University of the Witwatersrand, Johannesburg, 2023-09) Diantantu, Aime Diakanwa; Usman, Ibrahim
Sensors are- important devices nowadays that have been instrumental towards the development of the Internet of Things (IoT) amongst other recent technological innovations. They are used to detect and respond to some form of input or stimulus from the environment we are living in. There are different types of sensors in the market nowadays, depending on the materials used for their manufacture and their applications, namely position sensors, pressure sensors, gas sensors, etc. Gas sensors use semiconductors as materials. Metal oxides, conducting polymers, carbon nanotubes, graphene, and transition metal chalcogenides are some semiconductors materials used in gas sensors. Metal oxides are very good gas sensors materials due to their low cost, high stability, and sensitivity but their high operating temperature disqualify them. Conducting polymers are also good sensors materials due to their flexibility and low operating temperature but they are altered by humidity. To counteract humidity problem, conducting polymers need to be modified or doped with selected elements or molecules. In this project, cellulose was drugged with carbon nanotube (CNT) to create a mechanically and chemically stable structure, which can interact and sense many gases. The chemical and physical properties of cellulose make it a potential material for the development of conductive and potential sensing stuff. This led to the focus of this investigation, which is the development of mixed cellulose nanocrystal (CNC) – CNT materials for sensor application. The CNC was synthesized through the Tempo oxidation method, and various amounts of CNT were added into the CNC below the aggregation threshold of 2.5% using ultrasonication to form a CNC – CNT rectangular sheet. The developed mixed materials were characterized using Scanning Electron Microscopes (SEM) and Transmission Electron Microscope (TEM) to determine the morphology. Fourier Transform Infrared (FTIR), Raman Spectroscopy and X-ray Diffraction (XRD) were employed to investigate the structure of the final material, while TGA has shown similar degradation temperatures of CNC and CNC – CNT. SEM images showed an interconnected network-like structure with a porous architecture assembled by curved thin sheets, and the increase in CNT resulted in aggregate formation within the CNC. TEM micrographs confirmed the structure of CNC, which was rod-like and artefactual dendrites particles, and the presence of CNT in the matrix, while FTIR confirmed the main functional groups of the mixed matrix sheet. The degree of graphitization and presence of disordered cellulose in the mixed materials were determined by Raman spectroscopy to vary between 0.98 and 1.2. The XRD pattern has shown that the crystallinity index of the CNC – CNT composite is correlated to the increase in the concentration of CNT. However, the TGA data has shown that the CNC – CNT materials exhibited similar thermal behaviour, this is expected, since the concentrations of the composites have similar bonding structure and configuration compared to the pristine CNC. It is also evident that the increase in CNT content reduces the thermal degradation (reduced slope) of the CNC. The research work has developed CNC – CNT materials for sensor applications. The composite has exhibited sensor response and thereby detected H2, CO2, NO2 and Ar gases at room temperature through the changes in their electrical conductivities. The ability of CNC-CNT to respond to these gases at room temperature opens-up the possibility for its easy use in indoor and outdoor monitoring.
The Eigenmodes of Complex Media
(University of the Witwatersrand, Johannesburg, 2024) Peters, Cade Ribeiro; Forbes, Andrew
Structured light refers to the tailoring of light in all of its degrees of freedom. This includes amplitude, phase, wavelength and polarisation. Structuring light allows us to create complex optical fields with many interesting and useful properties. These fields have allowed us to ask deeper and more fundamental questions about Physics and have revealed new avenues for investigating aspects of the world around us. They have allowed us to significantly increase the speed at which we communicate and make information more accessible. Additionally, they allow for increased resolution and precision in imaging and measurements, both classical and quantum. One of the primary limitations when using structured light are the effects of perturbations. Many complex media, such as the atmosphere, underwater or biological specimens have a non-uniform refractive index (varying dielectric constant). This distorts most structured light beams, limiting its performance and possible uses. This works seeks to investigate this problem and offer a solution. Much attention has been given to finding which forms of structured light perform best in certain systems or scenarios. This work focuses on offering a potential solution to this problem. We begin with a discussion on common forms of structured light and models of light propagation. We then move onto methods for generating structured light experimentally. We then propose the concept of an eigenmode: modes that are perfectly invariant through such systems. They are structured light fields that are specially tailored, using our knowledge and understanding of the Physics of the system, to ensure that they propagate through the system and exit unchanged. We achieve this by modelling our system as a linear operator and then using this to find the eigenstates of this operator. We do this for two highly topical aberrations, providing approaches that can be generalised to almost any optical system. We end off this work with a discussion on important considerations when using eigenmodes for real world applications
Vacuum Arc Propulsion Systems for In Space Refuelling and Small Satellite Applications
(University of the Witwatersrand, Johannesburg, 2023-09) Stansell, Paul Robert; Lafleur, Trevor; Ferrer, Phil
In this work a compact and low power vacuum arc ion thruster and related circuitry were built utilising a vacuum arc thruster (VAT) as the plasma source. A number of VAT designs were tested before the final ion thruster design comprising of a modified coaxial VAT with a copper cathode and an extended anode grid to reduce peak ion currents was chosen. This design was chosen as it produced the best performance and prevented excessive grid arcing which was a problem encountered throughout this work. The final ion thruster required only one liter of volume, excluding electronics, and could be operated at an average power of less than one watt. Pulsed ion beams of up to 20 ± 1.4mA of copper ions with beam energy 3.23 ± 0.22keV lasting on average 320µs were produced by the system and detected by a plate downstream of the thruster. The grid system was unable to extract the large ion currents produced by the VAT which meant the overall electrical efficiency, thrust to power ratio and specific impulse were low compared to existing gas-fueled gridded ion thrusters; 2.28%, 0.474mN/kW and 33s respectively. Finally, a metallic meteorite sample was used as the cathode in a planar VAT and the ion current was measured. It was concluded that the vacuum arc ion thruster is a promising propulsion system for small satellites and has potential for refuelling from metals present in the space environment. This study adds a novel, miniature, low power and low beam voltage design to the limited literature on vacuum arc ion thrusters.

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