School of Physics (ETDs)

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Development of TileCoM firmware and software for the off-detector electronics of the ATLAS Tile Calorimeter at the HL-LHC
(University of the Witwatersrand, Johannesburg, 2023-08) Gololo, Mpho Gift Doctor; Argos, Fernando Carrio; Mellado, Bruce
In 2010 the LHC started to operate as the energy frontier particle accelerator in the world, situated close to Geneva and 100 m below the French and Swiss border in a circular tunnel of 27 km. The HL-LHC which is an upgrade of the LHC is envisioned to maximize the instantaneous luminosity of L = 1 × 1034 cm−2s −1 by a factor of 5 to fully exploit the physics potential at the energy frontier. During 10 years of operation, an improved TDAQ system architecture will have the capability to accommodate the trigger rates and the amount of data generated from the HL-LHC. TileCal is the ATLAS central hadronic calorimeter, a sampling calorimeter with iron as passive medium and plastic scintillator tiles as active medium. The ATLAS TileCal Phase-II upgrades will prepare the ATLAS experiment for the HL-LHC and includes new requirements in terms of radiation levels, an increase in data bandwidth, and clock distribution. To meet the requirements of the HL-LHC, a completely new readout electronics is designed to support the data acquisition system of TileCal. As part of the new readout electronics, this thesis is focused on the design of the TileCoM and Tile GbE Switch. The Tile GbE Switch PCB is manufactured by two South African companies, Trax Interconnect and Jemstech. The PCBs are fully func tional and have been integrated with new readout electronics. Three main function alities are implemented on the TileCoM in software and firmware implementation as key elements of the TDAQ and DCS of the ATLAS TileCal at the HL-LHC. The TileCoM and Tile GbE Switch are successfully integrated with the ATLAS Phase II TileCal upgrade electronics. This is achieved by successful remote control and monitoring of the ATLAS TileCal Phase-II upgrade electronics. This thesis shows monitoring results based on voltage, current and other parameters.
Hunting dark matter with faint radio halos
(University of the Witwatersrand, Johannesburg, 2023-10) Sarkis, Michael David; Beck, Geoff
The nature of Dark Matter (DM), the elusive substance that constitutes a significant amount of the total matter in the universe, remains an unsolved problem in modern physics despite a decades-long search effort. One approach to this problem has been to search for faint emission signatures that are produced indirectly from the DM present in large astrophysical structures, and thus infer properties about theoretical DM models from observational data. In recent years, the results from studies that use this type of indirect search have produced stringent constraints on the most popular DM particle candidate parameter spaces, ruling out swathes of viable DM models. These compelling results have been enabled by the arrival of sophisticated interferometric radio telescopes, which are excellent DM hunters due to their high sensitivity and resolution. In this thesis, we focus on the use of the latest data from the MeerKAT radio interferometry telescope, through the first public release of the MeerKAT Galaxy Cluster Legacy Survey, to search for DM emissions in a set of nearby galaxy clusters. Each step of this process, from the creation of theoretical DM emission models to the statistical analysis of the observational data, has been described in detail in this thesis. With this data, we find an almost universal improvement to results found with corresponding modelling scenarios in the literature. Since this work is among the first to use MeerKAT data in astrophysical DM searches, these results present a strong argument for continued work in this field. Another central focus of this thesis is the accurate modelling of the physical processes involved in the production of the DM-induced radio emissions, as the quality of current radio data requires theoretical models that are sufficiently accurate to describe the emission at such high resolutions. One aspect of the modelling that has lacked this accuracy has been the solution to the diffusion-loss equation, which is a crucial factor in determining indirect DM emissions. A new algorithm for solving this equation, which provides higher accuracy and computational efficiency than previous public methods, has thus been developed and presented in this thesis. These aspects of DM indirect detection study will become ever more important as we approach the inauguration of the Square Kilometre Array (SKA), which will provide data with unprecedented potential with which to continue the hunt for DM.
Implementation of the DAQ software for the ALTI module in the ATLAS TileCal and the search for new physics in the four lepton final state
(University of the Witwatersrand, Johannesburg, 2023-06) Tlou, Humphry Sijiye; Wilkens, Henric; Ruan, Xifeng; Mellado, Bruce
The discovery of the Standard Model (SM) Higgs boson in 2012 presents new challenges and opportunities for the Large Hadron Collider (LHC) experiments. After a long period of operation, the LHC experiments needed to maintain and upgrade their detectors in order to continue and conduct research beyond the SM. As part of the upgrades, the Tile Calorimeter (TileCal) participated in Phase-I of the upgrades (December 2018 - March 2022). TileCal, the central hadronic calorimeter (|η| < 1.7) of the ATLAS experiment uses a set of Trigger and Data Acquisition (TDAQ) software to readout, transport and store physics data resulting from collisions at the LHC. In preparation for the Phase-I upgrades, the ATLAS Local Trigger Interface (ALTI) module was designed for the ATLAS experiment at CERN for TDAQ purposes. It is a 6U VME electronics board, which is a part of the Timing, Trigger and Control (TTC) system. It integrates the functionalities of four legacy modules, currently used in the experiment: Local Trigger Processor, Local Trigger Processor interface, TTC VME bus interface and the TTC emitter. The ALTI module provides the interface between the Level-1 Central Trigger Processor and the TTC optical broadcasting network to the front-end electronics of each of the ATLAS sub-detectors. This thesis discusses the development, validation and integration of the TileCal specific ALTI software in the TileCal online software by the author. A set of ALTI boards were installed in the back-end electronics of the sub-detector and fully validated for the ATLAS detector at CERN. Performance testing and maintenance of the ALTI modules and software were performed during the second half of the upgrade period, in preparation for Run 3 (2022–2025) data-taking period. The thesis also discusses the search for the presence of a new heavy resonance produced via gluon-gluon fusion and decaying to the four-lepton (4ℓ) final state, in association with missing transverse energy (EmissT), with ℓ = e, µ (where ℓ is the lepton, e is the electron and µ is the muon). The search uses 2015–2018 proton-proton collision data at √s = 13 TeV, corresponding to an integrated luminosity of 139 fb−1, collected by the ATLAS detector. The data are interpreted in terms of two models, firstly the R → SH → 4ℓ + EmissT , where R is a scalar boson, which decays to two lighter scalar bosons (S and H). The S decays to a pair of neutrinos and the H decays into 4ℓ, through ZZ bosons. The second model is the A → Z(νν)H(ZZ) → 4ℓ + X, where A is considered to be a CP-odd scalar which decays to a CP-even scalar H and the Z boson. The Z boson decays to X, which can be a pair of neutrinos or jets, and the H decays to the 4ℓ final state.
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.
Search for new resonances in the four-lepton channel and implementation of the LED integrator panel for the PROMETEO system in the ATLAS Tile Calorimeter
(University of the Witwatersrand, Johannesburg, 2024) Mtintsilana, Onesimo; Kumar, Mukesh; Mellado, Bruce
The Large Hadron Collider (LHC) has transformed our understanding of fundamental particles and forces, notably with the seminal discovery of the Higgs boson in 2012, which completed the Standard Model (SM) of particle physics. Despite its success, the SM leaves numerous unanswered questions, motivating a quest for new physics. This thesis explores three main avenues: Firstly, it investigates the possibility of an extended Higgs sector or alternative SM extensions, focusing on heavy ZZ resonances that decay into four leptons. Using a dataset of 139 fb−1 from proton-proton collisions at the LHC, this study explores both gluon-gluon fusion and vector-boson fusion production mechanisms. Although no significant signal for a new resonance is observed, upper limits on the production cross section of spin-0 or spin-2 particles are established. These limits provide constraints on specific theoretical models, such as Type-I and Type-II two-Higgs doublet models for spin-0 resonances, and the Randall-Sundrum model for spin-2 resonances. Intriguingly, the combined results of ATLAS and CMS for Run 2 and Run 3 data in the final state of 4 leptons exhibit an excess around 250 GeV, reaching a significance of 2.4σ which is in the region of interest of the multi-lepton anomalies.. In the second part, the analysis extends to heavy boson decays resulting in a final state with four leptons, specifically focusing on the R boson or the A boson decays into a combination of the SM Higgs boson and another boson, denoted S, which further decays into dark-matter candidates. No evidence contradicting SM predictions is found, yielding stringent upper limits on the production cross-sections of these hypothesised bosons and their branching ratios at a 95% confidence level. Lastly, the thesis highlights advancements in Higgs boson studies and new particle discovery potential in the upcoming High-Luminosity LHC era starting in 2029, emphasising improvements to the ATLAS detector electronics, particularly the integration of a new LED Integrator Panel within the Prometeo portable readout module system, enabling precise calibration and monitoring of individual detector components
Study on the influence of Nuclear Deformation on the Pygmy Dipole Resonance in Samarium isotopes
(University of the Witwatersrand, Johannesburg, 2023) Jivan, Harshna; Sideras-Haddad, Elias; Pellegri, Luna
The past decade has seen an increase in studies dedicated to understanding the low-lying electric dipole (E1) response, commonly referred to as the Pygmy Dipole Resonance (PDR). These studies revealed that the PDR has a mixed isospin nature, and that the use of complimentary probes is needed to fully understand this response. Since majority of studies on the PDR focused on spherical nuclei, the influence that deformation has on the PDR response is yet to be understood. Preliminary relativistic proton scattering studies on 154Sm performed at RCNP (Japan), showed potential evidence for a splitting in the PDR responses similar to that of the Giant Dipole Resonance with deformation. A tentative interpretation suggested that this splitting could be connected to the splitting of the resonance structure with respect to the K quantum number. Theoretical studies considering the deformed HFB+QRPA model however, suggest that this splitting is connected to the isospin mixed character of these states as observed in spherical nuclei. The isoscalar responses of the spherical 144Sm and axially deformed 154Sm isotopes were investigated for the first time using the inelastic scattering of alpha particles at 120 MeV. The comparative experiments were performed at iThemba LABS in South Africa, coupling together for the first time, the K600 magnetic spectrometer in zero-degree mode with the BaGeL (Ball of Germanium and LaBr3:Ce detectors) array. The particle-gamma coincidence measurement was used to obtain the cross section for the population of the pygmy states. In both nuclei, the region of the PDR was excited and the E1 multipolarity of the transitions was supported by the angular correlation between the α-particles and the co-incident γ-rays measured. The total exclusive cross section measured for 144Sm amounted to 24.3 ± 3.8 mb/sr while for 154Sm to 18.8 ± 2mb/sr. The experimental results were compared with the prediction of the RQTBA and the deformed HFB+QRPA theories, respectively. The theoretical cross sections were extracted within a semiclassical coupled-channel approach. The fragmentation observed in the experiment for the 144Sm was underestimated by the calculations, although good agreement with the total cross section measured was found. In the case of the deformed 154Sm however, the experimental cross section accounted for only 52% of the predicted cross section in the same excitation region. The isoscalar response extracted in this thesis was compared with the isovector strength obtain from an experiment performed at RCNP using the relativistic proton scattering at forward angles. The double hump observed in the isovector channel was not found in the case of the isoscalar strength. This implies that the difference obtained between these two experiments is related to the “isospin splitting” of the PDR rather than a splitting of thestrength connected with the K quantum number.
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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