School of Physics - Theses
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ItemDetecting and characterising extrasolar planets in reflected light(University of Melbourne, 2009)
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ItemFault-tolerant quantum computation with local interactions(University of Melbourne, 2009)
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ItemMonte Carlo characterisation of narrow photon fields for intensity modulated radiotherapy(University of Melbourne, 2006)
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ItemFocusing of an atomic beam using a TEM01 mode lens(University of Melbourne, 2006)
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ItemImproved hidden Markov models for continuous gravitational wave searches( 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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ItemFinding Quasars in the Southern Hemisphere Sky Using Random Forest Machine Learning( 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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ItemDesigning and assessing model independent tests of the DAMA modulation( 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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ItemSemi-leptonic decays at the Belle and Belle II experiments( 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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ItemInvestigating a Third Parameter in the Tully-Fisher Relation( 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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ItemThe Panoramic Deep Fields( 2001)The Panoramic Deep Fields are a deep multicolour survey of two ~ 25 ° fields at high galactic latitude. The survey images have been constructed by digitally stacking scans UK Schmidt plates. Deep images (Bj ~23.5) with low contamination have been obtained by subtracting the background from the individual plates scans and using bad pixel rejection during the stacking. The size and depth of the fields allow the accurate statistical measurement of the environments and evolution of galaxies and AGN. The clustering of galaxies and galaxy clusters has been measured from z ~0.4 until the current epoch. The clustering properties of galaxies are strongly correlated with colour and blue U – Bj selected galaxies exhibit weaker clustering than any morphologically selected sample. The weak clustering (ro ≤ 3h -1 Mpc) of blue galaxies implies galaxy colour and stellar population are more strongly correlated with environment than galaxy morphology. Despite the large fields-of-view, the clustering of red galaxies and clusters varies significantly between the two fields and the distribution of clusters is consistent with this being due to large-scale-structure at z ~0.4. The evolution and environments of AGN have been measured at intermediate redshifts with the Panoramic Deep Fields. Photometric redshifts, colour selection and the NVSS have been used to compile a catalogue of ~ 180 0.10 < z< 0.55 radio galaxies. The evolution of the radio galaxy luminosity function is consistent with luminosity evolution parameterised by L (z) ~ L(0) (1+z)3.4. The environments of UBR selected AGN and radio galaxies have been measured at z~0.5 using the Panoramic Deep Field galaxy catalogue. By applying photometric red-shifts and colour selection criteria to the galaxy catalogue, it has been possible to increase the signal-to-noise of the angular correlation function and measure the cross-correlation with specific galaxy types. Most AGN host environments are comparable to the environments of galaxies with the same morphology. However, ~6% of UBR selected AGN are in significantly richer environments. No significant correlation between AGN luminosity and environment was detected in the Panoramic Deep Fields. The richness of AGN environments is not strongly correlated with redshift and the rapid evolution of the AGN luminosity function is not caused by evolution of AGN host environments.