School of Physics - Theses

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    Improved hidden Markov models for continuous gravitational wave searches
    Clearwater, Patrick Winston ( 2022-11)
    The direct detection of gravitational waves in 2015 has ushered in a new way of making astronomical observations and provided a rich stream of data for making astrophysical inferences. The detections reported by the Advanced Laser Interferometer Gravitational-Wave Observatory (Advanced LIGO) and the Virgo detector during their first three observing runs have so far all been compact binary coalescences, which are short duration signals from the late stages of compact object mergers. There is much left to be discovered, and this thesis advances the state of the art in searches for continuous wave signals: persistent, relatively-weak signals from sources such as neutron stars. The thesis describes two significant improvements to the hidden Markov model (HMM) scheme often used for continuous wave searches, applies the HMM to a search of LIGO Observing Run 2 (O2) data, and describes two ancillary improvements (graphics processing unit optimisation and few-bit digitisation) that improve the performance and memory-efficiency of the implementation. HMMs are used in continuous wave searches to account for spin wandering: small stochastic variations in signal frequency. They work by splitting detector data into short time segments, calculating a detection statistic as a function of frequency at each segment, and then tracking the most likely path for the signal frequency based on a user-specified transition model (an unbiased random walk in this thesis). We introduce a detection statistic called the J-statistic which is sensitive to sources that are part of a binary system. The J-statistic reliably detects signals weaker by a factor of four compared to the Bessel-weighted F-statistic, the previous detection statistic used in HMM searches for binary sources. This improved HMM scheme allows searches for binary sources to be as sensitive as searches for isolated sources. We use the J -statistic HMM pipeline, called "version 2", to search LIGO O2 data for gravitational radiation from the low-mass x-ray binary Scorpius X-1 over a 60-650 Hz frequency band. While no detection is claimed, three candidates survive our follow-up veto procedure. Assuming a non-detection, the search sets a 95 per cent confidence upper limit on strain h_0 of 3.47e-25 at 194.6 Hz when marginalising over the inclination angle of the source. One drawback of the HMM is that each time segment is combined incoherently: version 2 of the HMM does not enforce a consistent signal phase in the transition between blocks. We introduce version 3 of the HMM, which does track inter-block phase. The result is a detection pipeline, applicable to either isolated or binary sources, that is a factor of ~1.5 more sensitive than version 2, and closes much of the gap between the HMM and a fully-coherent search while retaining the computational efficiency of earlier HMM versions. We describe an implementation of the J -statistic and HMM on graphics processing units (GPUs), which provides an order-of-magnitude improvement in processing speed and was essential for covering the wide parameter range used in the O2 Scorpius X-1 search. Running that search using the GPU implementation of the pipeline required approx < 3e5 GPU-hours. We further describe the first application of few-bit digitisation techniques to continuous gravitational wave search methods, finding a decrease in sensitivity of only 6 per cent (two-bit digitisation) or 25 per cent (one-bit) in return for a factor of 32 or 64, respectively, reduction in memory use.
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    Finding Quasars in the Southern Hemisphere Sky Using Random Forest Machine Learning
    Alonzi, Noura Mohammad H ( 2022-04)
    Quasars are the most luminous persistent sources in the universe. The main goal of this thesis is to search for quasars at different redshifts using an efficient machine learning algorithm: the Random Forest classifier. A technique was developed and tested on a small photometric sample, the Early Data Release (EDR) from the SkyMapper Southern Survey (SMSS). After classification by Random Forest, candidates were prioritised for confirmation using observations from the ANU 2.3m telescope. SkyMapper is the first digital optical survey in southern hemisphere and has been used to build the required training subsets and the dataset. SMSS has been matched with other available surveys in the southern hemisphere to provide a broader range of colours for selection algorithms. The predictions were greatly improved by combining photometric colours in the optical from SMSS with mid-infrared data from AllWISE. Random Forest Machine Learning techniques provided classifications with probabilities of up to 81%. The EDR pilot study, predicted 119 QSO-candidates. Of these 78 have been confirmed as quasars, either previously or by new observations, and the remainder still need to be observed. So far, only one galaxy and one star were found amongst the candidate list. In addition, the classifier has been trained and applied to a much larger dataset, the Third Data Release of SMSS. This provides a preliminary study of the techniques that will be required to study extremely large samples of quasars identified in the Legacy Survey of Space and Time which will commence in a few years time on the Vera C. Rubin Observatory. Other observations in the thesis explored different AGN types, such as narrow line objects, using the BPT diagnostic diagram to determine the source of excitation energy. Future machine learning algorithms may be able to determine finer AGN classifications, as the range and quality of the input non-spectroscopic datasets improves. The conclusion from these studies is that candidate quasars can be identified with high confidence using machine learning, if a sufficiently large spectroscopic test sample is available.
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    Designing and assessing model independent tests of the DAMA modulation
    Zurowski, Madeleine Jane ( 2022-12)
    Particulate dark matter is a long hypothesised solution to various astrophysical observations seemingly at odds with a completely luminous universe. Despite the success of dark matter in explaining these observations, to date physicists have been unable to conclusively observe its interactions with Standard Model matter directly. This thesis will focus on trying to understand the results from the DAMA collaboration, which for the past two decades has reported a modulation signal consistent with dark matter, but in tension with other null experimental results under the usual dark matter assumptions. This study demonstrates the need for a model independent test of this signal to understand its origin, the requirements of such a test, and how different dark matter experiments can be compared or assessed to understand how sensitive they are to this elusive signal. This thesis examines such a study through the lens of a dark matter detector currently under construction in Australia: SABRE South. In particular, it will focus on purification techniques that can be used to produce benchmark low background equipment, detailed simulation studies that can guide the design of SABRE South, and the detailed analysis that must take place to understand how sensitive and or competitive SABRE South will ultimately be. It will also touch on interesting phenomenology studies that can be conducted with such a detector; examining non-standard or unusual dark matter models and signatures that are produced by relaxing the assumptions typically made about its fundamental nature, and distribution with the galaxy.
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    Semi-leptonic decays at the Belle and Belle II experiments
    Ferlewicz, Daniel Adam ( 2022)
    This thesis explores semi-leptonic decays of the type $B \to V L$ along two avenues; their contribution to high-precision tests and metrology of the Standard Model, and their potential for probing physics beyond the Standard Model. The magnitude of the Cabbibo-Kobayashi-Maskawa matrix element corresponding to $b\to c$ quark transitions, $|V_{cb}|$, has a long-standing discrepancy between its measurements from exclusive and inclusive decays, which presents a puzzling gap in our knowledge of the Standard Model. In this thesis, recent Lattice QCD calculations of form factors at nonzero hadronic recoil are incorporated into novel analysis methods to examine the phenomenology and analysis techniques behind the exclusive $|V_{cb}|$ measurement in $B \to D^{*}\ell^+ \nu_\ell$ decays. This results in a measurement of $|V_{cb}|=(38.49 \pm 0.54 \pm 0.92 \pm 0.91) \times 10^{-3}$, the least model-dependent result to date. The results of this analysis of the $B \to D^{*}\ell^+ \nu_\ell$ branching fraction and decay rate are used in a test of Standard Model QCD predictions of hadronic factorisation, with the first measurement of the parameter $|a_1|$ within a single experiment, based on a ratio that cancels most experimental systematic uncertainties, resulting in $|a_1| = 0.884 \pm 0.004 \pm 0.003 \pm 0.016$. This corresponds to an $8.9\sigma$ discrepancy from the Standard Model, suggesting the existence of large non-factorisable contributions in hadronic $B$-decays, or potential contributions from new physics amplitudes. A probe for new physics in photon-dipole interactions, described by the $\mathcal{O}_7$ term in an operator product expansion, is then performed to constrain non-Standard Model right-handed currents in flavour-changing neutral current semi-leptonic decays. New lepton identification techniques in the Belle experiment are developed for use in the first study of $B \to K^* e^+ e^-$ decays at the very low dilepton invariant mass region, $q^2 < 1.12 \gevccsq$, to constrain the Wilson coefficient $\mathcal{C}_7$, and its right-handed counterpart $\mathcal{C}_7^{\prime}$, expected to be zero in the Standard Model.
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    Investigating a Third Parameter in the Tully-Fisher Relation
    Ozbilgen, Sinem ( 2022)
    The scaling relations of galaxies play an important role in constraining the formation and evolution of galaxies. One of the key relations, the Tully-Fisher relation (TFR) also has a crucial place in determining galaxy distances. The relation states that absolute magnitudes of spiral galaxies are correlated with their maximum rotational velocities. This thesis focuses on how the TFR can be improved by investigating the use of an additional parameter. Following the Fundamental Plane, central velocity dispersion is studied as this parameter. Additionally, with the help of hydrodynamical simulations, possible biases are explored in the determination of the TFR. The optical data of the thesis is obtained using the WiFeS (Wide Field Spectrograph) on the ANU (Australian National University) 2.3m Telescope at Siding Spring Observatory. The sample was chosen from the HICat (HIPASS (HI Parkes All-Sky Survey) Catalogue). As velocity dispersion is the main focus of the thesis, early type spiral galaxies (S0 to Sc types) are preferred because of their velocity dispersion supported bulges. In total 137 HIPASS galaxies were observed during total of 32 nights of observing over the course of 2.5 years. The related HI data is taken from the HICat. Obtaining both HI and optical rotational velocities allowed comparison of the TFR with K band magnitudes at both wavelengths. The results are in agreement with the literature where HI data gives less scatter and a tighter correlation in the TFR. Furthermore, morphology and inclination are also studied. It is found that morphological type is not a cause for the scatter in the relation whereas there is a clear indication that higher inclination angles correlate better than the smaller inclinations. With the velocity dispersion that is obtained from the optical data, the $\sigma/V_c$ parameter is investigated as a possible third parameter. Even though adding this parameter does not result in different TFRs, there is a notably greater scatter for the earlier type of spirals, where $\sigma/V_c > 0.6$. Lastly, the $S_K$ parameter, which also consists of $\sigma$ and $V_c$, is studied to determine whether the correlation has less scatter. It is found that introducing $S_{0.5}$ reduces the scatter as in the earlier studies. Observational data is then compared with the simulated data of the EAGLE (Evolution and Assembly of Galaxies and their Environments) project. The Recal-L025N0752 model at $z=0$ is used to obtain the data. All the galaxies with $M_{\star} > 10^{10} M_{\odot}$ within a radius of 30kpc are extracted and total of 81 galaxy data is obtained to study the TFR. As the simulation has the velocity and mass of the star and gas particles, velocity dispersion and rotational velocity widths are also calculated for each galaxy. However, to mimic the observations, velocity dispersions are calculated randomly from a similar ratio of apertures as the optical sample. It is found that instead of aperture of the galaxy from which velocity dispersion is calculated, the selection of the individual galaxies affects the slope of the TFR. Using the $S_{0.5}$ parameter does not make a difference to the result.
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    Characteristic X-ray Spectroscopy
    Melia, Hamish Alexander ( 2022)
    X-ray spectroscopy is the study of radiation emitted as a result of electronic transitions inside an atom. X-ray spectroscopy provides otherwise unobtainable information regarding the structure of the atom and has been vital in our understanding of the quantum mechanical laws that are now fundamental to modern science. The accuracy of recent Cu Kalpha measurements have reached the one part per million level. However, discrepancies between measurements have been revealed. A new measurement of the Cu Kalpha spectrum, using a novel detection system, allows the investigation of these discrepancies as well as satellite contributions and the relativistic quantum mechanics that underpin them. Our new characterisation is one of only three in the literature that allows the relative intensity of the Kalpha3,4 satellite to be measured. Our characterisation implies a 2p shake probability of 0.5%. In addition, this research highlights critical questions regarding the portability and transferability of x-ray spectra and helps consolidate the Cu Kalpha peak energy as a standard laboratory measurement. Contributions from the radiative Auger effect (RAE) have often been unmodelled in characterisations of x-ray spectra, resulting in incorrect conclusions being drawn from empirical fits. For the first time, the Cu Kbeta spectrum has been characterised including a component to model the RAE. The results show our characterisation to be superior to the traditional 5-Lorentzian model and also explains long-standing residuals of empirical fits around the low energy Kbeta1,3 tail. An observed and imputed fitted Auger contribution of 2% demonstrates the need for Auger components to always be considered when characterising x-ray spectra and highlights the importance of developing new atomic theory to model the radiative Auger effect using an ab initio method. Theoretical predictions of atomic spectra have been pivotal in continuing our understanding of fundamental physics. The multiconfiguration Dirac-Hartree- Fock (MCDF) method represents our best avenue for approximating the wave function of multi-electron atoms and molecules. However, the open shells of the 3d transitions metals provide significant challenges for convergence. Using the MCDF method, we present the most comprehensive calculation of transition probabilities and energy eigenvalues relating to Cu Kalpha and Cu Kbeta. Initial and final state wavefunctions show convergence, with the expansion of the active set, to 0.05 eV or 0.0006 %. The methods presented in this thesis for obtaining complete, well-converged wavefunctions for multi-hole initial and final states overcame numerous challenges and provide an approach for other researchers in computing wavefunctions for any complex system. Although we have shown that the multiconfigurational framework is necessary and capable of obtaining well-defined wavefunctions of open-shell systems, the calculation of shake probabilities using these multiconfiguration wavefunctions have continued to struggle to reproduce experimental results. This thesis provides the most complete calculation of shake probabilities using MCDF wavefunctions and the sudden approximation, while also addressing incorrect assumptions about their implementation when matching experiment. These calculations bring us a step closer to predicting x-ray emission spectra using a fully ab initio method. The significant discrepancy between satellite intensities measured experimentally and ab initio calculations of the shake processes that lead to these satellites, represents a significant challenge in the field. We test our MCDF calculations and ab initio shake probabilities by using them to simulate x-ray spectra and fitting to the best available experimental data. For the first time the Kalpha1,2 spectrum is modelled using the diagram lines and four spectator vacancy satellites: the 3d9, the 4s0, the 3d94s0 and the 3d8. We show that fixing satellite intensities to ab initio shake probabilities can simulate peak energies to within 0.1 eV of experiment. Furthermore, our MCDF calculations are used to model the Kalpha3,4 feature with agreement between theory and experiment far greater than ever before.
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    Constraining cosmology with SPT-3G
    Balkenhol, Lennart ( 2022)
    Cosmic Microwave Background (CMB) observations are rich in cosmological information and a key pillar of modern cosmology. The South Pole Telescope offers a particularly clear view of the millimetre sky and its latest receiver, SPT-3G, facilitates unprecedented measurements of CMB anisotropies on small angular scales. In this thesis, we present measurements of the CMB temperature and polarisation power spectra using SPT-3G data recorded in 2018 covering a region of approximately 1500deg^2 in the southern sky. We also present the first comparison of covariance conditioning schemes for CMB data at the matrix- and the parameter-level. For empirical covariance estimators using less than or approximately 100 independent data realisations, moderate conditioning schemes can lead to an underestimation of the parameter error by up to a factor of 1.3 compared to the uncertainty inferred from the likelihood. These results inform the analysis SPT-3G data. We demonstrate the internal consistency of the SPT-3G 2018 temperature and polarisation band powers across frequency bands and spectra, and find excellent agreement. We present cosmological constraints using the polarisation spectra and report consistency with the standard model. We find H0 = 68.8 km/s/Mpc, which is considerably lower than the most precise local determination of the expansion rate. Cosmological constraints from SPT-3G and Planck data are in good agreement. We produce constraints using SPT-3G 2018 data alone and in combination with other data on a series of model extensions drawn from the following parameters: the effective number of neutrino species; the primordial helium abundance; the sum of neutrino masses; the mass of a sterile neutrino; the mean spatial curvature; the baryon clumping induced by primordial magnetic fields; the initial field value, critical redshift, and contribution to the energy density at the critical redshift of an early dark energy component. We discuss the constraints on each model in detail and report no statistically significant and robust preference for any of these extensions over the standard model.
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    Searching for Dark Matter
    McNamara, Peter Charles ( 2022)
    The nature of Dark Matter (DM) is one of the most prominent unanswered questions in particle physics. The Standard Model (SM) has been remarkably successful in describing subatomic phenomena, however not all observations can be explained using this model including DM. The existence of DM is supported by a number of independent astrophysical observations, which when taken together, indicate DM is an elementary particle or particles, however their nature remains largely unknown. The focus of this thesis is on work towards experimental searches for particle DM under the Weakly Interacting Massive Particle (WIMP) paradigm using alter- native but complementary methods to the astrophysical observations in order to test the particle nature of DM. The first approach used is collider searches which test for DM production from the incident SM particles in particle colliders. The second approach is Direct Detection (DD) which aims to observe DM scattering off a SM particle. Using the motion of the Earth and Sun, some more unique features of the expected DM signal may be used to enhance experimental sensitivity. The orbit of the Earth around the Sun results in a time dependent signal with period of a year. The large dataset collected by A Toroidal LHC ApparatuS (ATLAS) allows searches for DM in many areas of phase space. These searches are limited by the ability to identify and discriminate the hypothesised DM signals from back- ground. As such the reconstruction and proper identification of objects in the detector over the largest possible range of momenta plays a key role in what is experimentally accessible. The use of track-jets to allow the identification of low momentum b-hadrons as well as the extension of this identification to lower momentum ranges will be described. The pioneering use of these detector objects to search for DM in regions of phase space previously thought to be inaccessible or too difficult will be described. Many experiments have failed to find DM using direct detection but only one (DArk MAtter (DAMA)) still maintains they have found it, appearing as a time dependent signal. This result is somewhat at odds with other results, however due to experimental differences, it is not completely incompatible. To properly test this an independent experiment using the same experimental approach as DAMA is needed to verify the results. This is the aim of the Sodium-iodide with Active Background REjection (SABRE) experiment, the creation of data acquisition and management systems will be described as well as simulation results used to inform the design and understand detector backgrounds.
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    Optical bilirubin sensing: Towards the point-of-care monitoring
    Ndabakuranye, Jean Pierre ( 2022)
    Bilirubin has been clinically confirmed as a liver function biomarker and used in a number of prognostic scoring systems for patients with cirrhosis and neonatal hyperbilirubinemia. Within clinical settings, optical and chemical methods are used to determine the blood bilirubin concentration, using complex, costly, and bulky instruments. This, combined with the expertise needed to operate these instruments, means that existing bilirubinometric technologies do not lend themselves to applications outside the clinics within the community settings. Consequently, blood bilirubin concentration is typically assessed intermittently, which results in missed episodes that may benefit from timely clinical interventions. This thesis outlines the development of a point-of-care (PoC) system to measure bilirubin in the blood. The system is developed using commercially available optoelectronic and microelectronic components, with an emphasis on low cost and ease of use. Two possible bilirubin level measurement approaches have been investigated: 1) the multi-wavelength method and 2) the photodegradation kinetics approach. The multi-wavelength method uses distinct optical signatures of bilirubin to implement a system based on dual-wavelength absorption measurement. A benchtop optical system was designed and implemented for bilirubin measurement using 470 nm (analytical wavelength) and 525 nm (reference wavelength). In contrast, the photodegradation kinetics approach uses bilirubin degradation under blue light to estimate its levels by analysing the variation in its spectrophotometric characteristics. A new kinetic model for degradation reaction is introduced to model the photodegradation kinetic results. Results show that bilirubin degrades exponentially with residual absorbance at long degradation times. The variations in the spectrophotometric characteristics of bilirubin were then used to estimate bilirubin levels. Building on the two approaches, the feasibility of combining both methods was investigated using a miniature system-on-chip platform. Experiments were performed using pure bilirubin, blood phantoms and whole animal blood within the pathophysiological bilirubin concentration range of 1.2 mg/dL to 50 mg/dL corresponding to a healthy person to a cirrhotic patient. Experiments have shown that both approaches (dual-wavelength and photodegradation kinetics) show high bilirubin sensitivity (over 95 %). Additionally, the techniques exhibit the average accuracies of +/- 6.64 % (for multi-wavelength) and +/- 8.03 % (for photodegradation). Importantly for cirrhotic patients, the techniques exhibit a more significant predictive accuracy of 4.83 % and 7.41 %, respectively, at higher bilirubin concentrations of 10 mg/dL and above. This work demonstrates the viability of an optical PoC system for bilirubin measurement, using a low-cost technique that is also accessible to the general population. This approach provides an opportunity for extending the reach of bilirubin measurement into community settings such as general practitioner clinics, pharmacies and even at home. Moreover, given that the measurement is performed using whole blood samples, the need for reagents is mitigated, thus making this a valuable approach for use in remote locations. Future clinical studies are needed to benchmark the method in terms of its specificity, sensitivity, and accuracy. Nevertheless, the preliminary results obtained here suggest that the technique may supplement the more accurate clinical prognostic scoring to provide an opportunity for more timely intervention.
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    Estimation of the cosmogenic activation and measurement of the quenching factor of NaI(Tl) crystal with spectrum-fitting for the SABRE experiment
    Mahmood, Ibtihal ( 2022)
    Though evidence suggests that 84% of all matter consists of dark matter, its lack of substantial interaction with ordinary matter continues to obscure its exact physical qualities. With the tantalizing prospect of uncovering a rich amount of information about a seemingly fundamental aspect of our Universe, the physics community has attempted to mine this ore of knowledge for the past century. Among these attempts, the use of direct detection experiments to probe the weak interactions between dark and baryonic matter has since mostly yielded null results. An exception to these is the annual modulation signal detected by the DAMA/LIBRA experiment, whose dark matter interpretation remains inconclusive. The Sodium-iodide with Active Background Rejection Experiment (SABRE) will conduct low energy (keV) measurements of dark matter direct detection events using radiopure NaI(Tl) crystals as a model independent test of DAMA's annual modulation signal. In order to so successfully, it is vital that the radioactive background of SABRE's crystals are low enough so that they are more sensitive to WIMP-like events than DAMA/LIBRA and that the crystals' response is properly understood by measuring their scintillating properties beforehand. In this thesis, the radioactive background of SABRE's crystal, due to cosmogenic activation while stored on the surface and during transport to its laboratory site, is estimated. This estimation takes into account the amount of cosmic ray flux and geomagnetic shielding for two possible freight travel scenarios, either by air or sea. The subsequent decay of each considered isotope at their underground site is also considered in order to determine how significantly they would contribute to the background over the lifetime of the experiment. In light of these calculations, recommendations for the storage time and method of travel of SABRE's crystals can be motivated with knowledge on whether the cosmogenic background produced will be sufficiently low for SABRE's purposes. Additionally, the quenching factors of SABRE's NaI(Tl) crystal must also be known to low uncertainty in order to determine the energies of the nuclear scattering interactions. A novel spectrum-fitting methodology was developed and tested to extract the quenching factor from sodium nuclear recoil measurements in NaI(Tl). The method employs Monte Carlo simulated recoil energy spectra to fit measured data in order to account for experiment-specific systematics. This was employed to measure the sodium quenching factors of a commercial NaI(Tl) crystal for recoil energies between 36 and 401 keV. The SABRE experiment will use this method for the measurement of their own crystal's quenching factors.