School of Physics - Theses
Permanent URI for this collection
Search Results
1 - 10 of 12
-
ItemNon-Equilibrium Processes in Neutron Stars and Ultracold Gases( 2023-06)From the booms and busts of the economy to the schooling of fish, non-equilibrium phenomena are ubiquitous and appear at all scales. However, non-equilibrium systems have proven infamously difficult to model and understand. In this thesis we present two different of non-equilibrium systems, one classical and one quantum mechanical, and thoroughly investigate their behaviour: (i) the repeated localised mechanical failure of the crust of a spinning down neutron star, and (ii) the dynamics of quenched few-body quantum systems. As an isolated neutron star spins down the centrifugal force weakens but the gravitational force doesn't change. This results in the crust changing shape and accruing mechanical strain to the point of failure. Mechanical failure locally deforms the crust and dissipates and redistributes strain. This can result in avalanches of further failures as one region of the crust failing may prompt a neighbouring region to fail. The evolving crust is a classical far-from-equilibrium system capable of avalanche behaviour like the classic sandpile model. The statistics of crustal failure events are of much interest due to their suggested relevance to transient phenomenon such as glitches or fast radio bursts. We present a cellular automaton designed to describe the evolution of the crust over spin down and the effects of local failure. This automaton describes when and where crustal failures occur and how large they are. Additionally this automaton describes the failure-induced change in the shape of the crust. Using this automaton we find that the star needs to be born spinning over \approx 750 Hz to accumulate sufficient strain to fail at all, that the waiting-times between subsequent events are distributed as a power-law spanning seven orders of magnitude, and that the ellipticities of isolated neutron stars are in the range 10^{-13} to 10^{-12}, among many other results. It has been suggested that the mechanical failure of the crust is the cause (or result) of a variety of transient phenomena such as glitches or gamma ray bursts. This model provides predictions of the statistical behaviour of crustal failure which can be compared to the observed behaviour of these transients. Additionally, the model describes the shape of the crust and the rotational frequency at all times which allows for the wave strain of emitted gravitational waves to be calculated with implications for searches for continuous gravitational wave sources. Cold quantum gases have attracted a great deal of experimental and theoretical interest thanks to the high degree of experimental control possible over them which makes them excellent testing grounds of quantum theory. Additionally, they are excellent tools for the study of quantum thermalisation. We consider a few interacting particles initially in some equilibrium state and suddenly change (quench) the interaction strength which kicks the system away from equilibrium. Specifically, we consider systems of two and three bodies of arbitrary mass and various particle symmetries interacting via a contact interaction in an isotropic three-dimensional harmonic trap. We take particular interest in quenching between the weakly and strongly interacting regimes and the following far-from-equilibrium post-quench evolution. We describe the non-equilibrium post-quench evolution of the system by analytically and semi-analytically calculating two observables: the Ramsey signal and the particle separation. We are able to calculate these quantities for the two-body system with arbitrary particle masses for any quench in interaction strength. Additionally, we extend these calculations to three-body systems of two identical fermions and a distinct particle or three identical bosons where the quench is between the strongly and weakly interacting regimes. In the two-body case we find when quenching from weak to strong interactions the particle separation oscillates periodically between \approx0.85a_{\mu} and \approx1.15a_{\mu}, where a_{\mu} is the simple harmonic oscillator length-scale. For the same quench in the three-body case the particle separation varies depending on the specifics of the system. For the fermionic case the particle separation oscillates periodically, peaking at \approx 2.18a_{\mu} with the mass ratio of the two species determining the minimum separation. For the bosonic case the oscillation is aperiodic. Both the maximum and minimum particle separation are determined by a quantity called the three-body parameter, but particle separation generally oscillates between \approx a_{\mu} and \approx 2a_{\mu}. However, in all cases when quenching from strong to weak interactions the calculations of the particle separation do not converge. This divergence is present whatever the initial state, mass ratio, particle symmetry, etc. and is present only for this particular quench from strong to weak interactions. We investigate possible sources of this divergence and future avenues of research into its causes. Finally, we note that these theoretical predictions of Ramsey signal and particle separation are experimentally testable with current techniques.
-
ItemEstimation of the cosmogenic activation and measurement of the quenching factor of NaI(Tl) crystal with spectrum-fitting for the SABRE experiment( 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.
-
ItemSpin-down signatures of young neutron stars( 2022)The spin down of neutron stars has been invoked to explain a wide variety of electromagnetic and gravitational-wave signals. This thesis explores two different signals associated with the spin down of neutron stars, one electromagnetic signal and one gravitational-wave signal. Binary neutron star coalescences, confirmed as the progenitor of at least some short Gamma-ray bursts (sGRBs) in 2017, are predicted to form either a black hole or a highly magnetized neutron star. Up to 20% of sGRBs observed by the Neil Gehrels Swift telescope display prolonged X-ray emission, sometimes called a ``canonical'' afterglow, consisting of three phases: an initial power-law luminosity decay; a 'plateau', lasting between 10 s and 105 s, during which the X-ray luminosity is approximately constant; and a final power-law decay. Previous authors have noted that the evolution of the canonical light curve is broadly consistent with the expected spin-down luminosity of a neutron star. Key ideas from analytic, one-zone models of plerions (also called pulsar wind nebulae) can be used to model the evolution of a synchtrotron nebula fuelled by the the spin-down luminosity of a neutron star formed in an sGRB. An analytic expression for time-dependent, spatially-averaged electron energy distribution in the nebula is found and used to calculate the light curve and the point-in-time spectra. The light curves predicted by the plerionic model are consistent with the shape and luminosity of the X-ray light curves and reproduce the observed correlation between plateau duration and luminosity (i.e. brighter plateaux end sooner). Furthermore, Bayesian parameter estimation comparing the point-in-time spectra to time-averaged spectra of six Swift sGRBs with canonical X-ray afterglows and of known redshift allows estimation of the parameters of the neutron-star central engine, including its poloidal field strength Bp and its rotation period P0 at birth, and injection parameters within the shock, including the energy range of the relativistic electrons and their power-law index. All six sGRBs favour a neutron star with Bp ~ 1011 T and P0 ~ s, consistent with the prediction the neutron star should be highly magnetized and rapidly spinning. We also apply the point-in-time spectra to four time-averaged spectra taken at four separate epochs in the X-ray afterglow of GRB130603B and infer the evolution of the magnetic field in the synchrotron bubble B. We find the evolution of B is slower than the expected evolution of the far-field limit of the stellar magnetic field. Rotating, non-axisymmetric neutron stars spin down via the emission of continuous gravitational waves which may be detectable by current terrestrial interferometers such as the advanced Laser Interferometric Gravitational-wave Observatory (LIGO) and advanced Virgo. Young core-collapse supernova remnants are likely hosts of young neutron stars and are common targets for wide-band directed searches for continuous gravitational waves targeting non-pulsating neutron stars. In this work, we present the results for two searches for continuous waves from neutron stars in young supernova remnants using a hidden Markov model (HMM). The HMM tracking scheme models the frequency evolution as a random walk with secular spin down and remains sensitive in the presence of stochastic spin wandering similar to that observed in pulsar timing observations. A search targeting twelve neutron stars in young supernova remnants in the second observing run (O2) of advanced LIGO using an HMM tracking scheme identifies 1012 potential candidates, 18 of which survive a series of standard vetoes. Further assessment of the 18 survivors based on their dependence on sky position and Doppler modulation confirms they are all consistent with terrestrial noise. A second search, conducted with the the LIGO-Virgo-KAGRA (LVK) collaboration, targets fifteen neutron stars in young supernova remnants in the first half of the third observing run (O3a) of advanced LIGO and advanced Virgo using three search pipelines, including an HMM tracking scheme, and reports no candidates consistent with an astrophysical origin after a rigorous veto and follow-up process. The HMM tracking scheme sets the first 95% confidence limits on gravitational-wave strain, h095%, for these targets with a random-walk signal model, reaching a sensitivity of h095% = 2.64 x 10-25 at 172 Hz for G353.6-0.7. The constraints on h095% are converted to upper limits on neutron-star ellipticity below 10^-5 above 150 Hz and constrain the maximum amplitude of internal r-mode oscillations below 10^-3 above 150 Hz.
-
ItemThe origin of matter and dark matter( 2018)Why is the mass density of dark matter throughout the universe similar to that of ordinary matter? Asymmetric symmetry breaking models can explain this apparent coincidence by exploring how dark matter could naturally have a similar mass to the proton and how the number density of dark matter particles could be almost the same as ordinary nucleons. In this work models of high energy scale mirror symmetry connecting the standard model to an exact copy are spontaneously broken to produce models of asymmetric dark matter, with composite dark matter candidates, that naturally solve the dark matter mystery.
-
ItemB0→K0π0 and direct CP violation at Belle( 2017)Rare B-meson decays such as the B0 → Ksπ0 which proceed without a charm quark provide a probe for physics beyond the standard model. This decay proceeds mainly via the b → s penguin transition, with the b → u transition being colour suppressed, allowing CP-violating effects to be observable. The asymmetric e+e− KEKB collider and the Belle detector provide the large luminosity and data collection required to observe these rare B decays. Methods to reduce the large qq backgrounds are investigated. The use of optimised neural networks using TensorFlow shows a significant improvement compared to the commonly used NeuroBayes software. Techniques for reducing correlations between variables introduced by TensorFlow are also investigated, proving that the use of adversarial neural networks can provide an improved background suppression as compared to NeuroBayes, whilst minimising correlations introduced by the neural network. An improved method of measuring the direct CP violation is introduced. Using Monte Carlo data with sample sizes corresponding to the full Belle datatset of (771.581 ± 10.566) × 106 BB events, the statistical uncertainty in ACP using this method is reduced from the latest Belle result of 0.13 to 0.1035 ± 0.0032. This method would also provide an up to date measurement on B(B0 → K0π0).
-
ItemNovel plasmonic elements for the generation, manipulation and detection of polarised light( 2017)Plasmonics provides an opportunity to develop nanoscale optical devices, where the spectral, angular and polarisation response can be tailored. The aim of this research is to determine which designs prove suitable as polarisation sensitive features, how novel nanofabriction techniques can be employed to scale up the production and in what settings can we use these scalable plasmonic polarising devices to address current nanophotonics challenges. Presented here is a study of potential plasmonics based methods of manipulating polarisation, including converting the polarisation state of a beam from linear to circular with carefully designed cross nanoapertures in a metallic film, strong filtering of left and right circular polarised light using 2D chiral geometries as well as creating a compact nanoantenna-enabled metal--semiconductor--metal photodetector to determine the polarisation state of a beam. To ensure the economic feasibility of the devices, special attention is also paid to novel scalable nanofabrication techniques. The cost of a feature is of great concern, as it is of little use to have an expensive feature for applications in consumer photonics. To that end a direct imprinting technique for the low cost production of plasmonic metasurfaces is investigated, including a study of the optical phenomena achievable and some potential applications are discussed. Finally, altering the quantum properties of emitters coupled to these scalable plasmonic features is investigated. Particular attention is paid to increasing the emission rate and polarising or focussing light from quantum sources using plasmonic nanocavities. These plasmon-exciton devices could see a reduction in the energy requirements of LED displays.
-
ItemMYTHEN CdTe: a new generation state-of-the-art X-ray imaging detector( 2016)MYTHEN is a single photon counting hybrid strip X-ray detector that has found application in X-ray powder diffraction (XRPD) experiments at synchrotrons worldwide. Originally designed to operate with hole collecting silicon sensors, MYTHEN is suited for detecting X-rays above 5 keV. However many PD beamlines have been designed for energies above 50 keV where silicon sensors have an efficiency of only a few percent. In order to adapt MYTHEN to meet these energies, the absorption efficiency of the sensor must be substantially increased. Cadmium-telluride (CdTe) has an absorption efficiency approximately 30 times that of silicon at 50 keV, and is therefore a very promising replacement sensor material candidate. Furthermore, the large dynamic range of the pre-amplifier of MYTHEN and its capability to process charge carriers of either polarity has enabled the characterization of both electron and hole collecting CdTe sensors. A selection of Schottky and ohmic type CdTe MYTHEN test structures have undergone a series of characterization experiments including bias and settings optimization, energy calibration, count rate capability as well as stability tests of bias and radiation induced polarizations. The performance of those systems will be presented and discussed in this thesis. Both, the radiation and bias induced polarization effects remained manageable. The MYTHEN system combined with CdTe sensors has proven to be reliable and stable despite high stress experiments. When biased over an extended period of time, the results of the studies have demonstrated that overdepletion of the sensors allowed the system to remain functional for a period of time 6 fold longer. During the high radiation studies, a count rate loss as well as a shift in threshold were observed, leading to the conclusion that individual charge carriers are been trapped. When applying a high bias as well as high flux, the detector system remained functional for 30 minutes. It was also demonstrated that a brief power cycle resumed normal performance after the system had shown symptoms of either polarization effect. Overall, the polarization effects observed on MYTHEN CdTe strip detector are temporary and show a slower impact than reported in the literature. Generally, a higher bias improved the stability of the detector.
-
ItemSearch for the Higgs Boson produced in association with a Vector Boson at ATLAS( 2015)On the 4th of July, 2012, the ATLAS and CMS experiments at CERN announced the discovery of a particle consistent with the Standard Model Higgs boson through its decay to bosons. Subsequent measurements have established that the particle’s spin, parity and coupling strengths are in line with Standard Model expectation, making this the strongest evidence of the Standard Model electroweak symmetry breaking mechanism. This thesis concerns the search for Higgs boson decays to two tau leptons, one of the leading Standard Model Higgs decay modes, with particular focus on events in which the Higgs is produced in association with a weak vector boson. Evidence for the existence of such events constitutes a crucial test of the Higgs’ coupling to leptons. The search is conducted using the 20.3 fb−1 of data collected by the ATLAS detector in 2012 from proton-proton collisions in the Large Hadron Collider at a centre-of-mass √ energy of s = 8 TeV. The search is optimised for events in which the associated vector boson decays to an electron or muon and one or both tau leptons from the Higgs decay subsequently decay to hadrons. The dominant background comes from events in which a jet is misidentified as the hadronic decay products of a tau decay, which is estimated from data samples in a signal-free control region using the rate of misidentification. The trigger and identification efficiencies are measured using dedicated calibration data samples with a tag-and-probe technique. No significant excess above the expected background yield is observed in any signal region. The signal strength of best fit, for a Higgs boson mass of 125 GeV, is 2.3 ± 1.6. Instead, limits are placed on the rate at which the event rate might exceed the Standard Model expectation. The event rate is found to be no greater than 5.6 times that expected by the Standard Model at a 95% confidence interval. This observed limit is less than the expected limit of 3.5 times the SM event rate, under the no signal hypothesis, or 3.7 times the SM event rate with signal injected, but is consistent within uncertainties.
-
ItemMeasurement of the branching fraction of B0 → π0 π0 decays using the final Belle dataset( 2015)This thesis presents a measurement of the branching fraction of B0 → π0 π0 using data collected by the Belle experiment based at the KEKB accelerator in Tsukuba, Japan. This study is performed using the final and complete Belle dataset of 711 fb^−1, comprising about 771 × 10^6 BB pairs collected at the Υ(4S) resonance. The measurement of B0 → π0 π0 is a vital component of the study into the B → ππ system, through which a measurement of the CP violation parameter φ2 (also called α) can be made. This analysis makes use of an artificial neural network to improve continuum suppression as well as making use of timing information pertaining to photons detected in the Belle electromagnetic calorimeter to help distinguish between signal and background originating from QED backgrounds. Photon reconstruction was enhanced through the recovery of photons that converted within the inner detector. The timing information for 20% of the Belle dataset was recovered specifically for the use of this analysis. This is the first time a branching fraction for B → π0 π0 has been presented using an artificial neural network and such QED suppression techniques. The branching fraction of B → π0 π0 is measured to be Br(B → π0 π0 ) = (1.22 +0.28 −0.27,+0.10 −0.12 ) × 10^−6.
-
ItemDiagnostics and control of transverse coupled-bunch instabilities in third generation electron storage rings( 2011)The Australian Synchrotron is a newly commissioned third-generation light source situated in Melbourne, Australia. Synchrotron radiation is produced from the 216 metre circumference storage ring where 3 GeV electrons are trapped within a lattice formed by dipole bending magnets and multipole focussing magnets. The appearance of coupled-bunch instabilities form the primary limitation of modern storage rings. Instabilities enforce an upper limit on stored current and can reduce the utility of radiation production by increasing the effective emittance of the ring. Stored current limitations due to beam instabilities were discovered early in the commissioning phase of the Australian Synchrotron storage ring and were initially controlled by substantially increasing the chromaticity of the lattice from (ξx; ξy) = (2; 2) to (ξx; ξy) = (3:5; 13). Subsequent additions to the ring have resulted in an increase of the strength of destructive instabilities to the point where detrimental side-effects from chromatic corrections reduce the ability of the ring to damp instabilities. This increase in instability strength has lead to the shift from purely passive methods of instability control to the design and construction of an active transverse feedback system. This thesis describes the commissioning of a bunch-by-bunch transverse feedback system designed to combat coupled-bunch instabilities, allowing for the reduction of chromaticity within the storage ring lattice back to the initial design values (ξx; ξy) = (2; 2). Reducing the chromaticity also removes detrimental effects such as the reduction of the dynamic aperture and an increase in the lifetime of the beam. Novel methods for tuning the system and maximising the damping rate of the beam are introduced. Using these methods, the feedback system was successfully commissioned and was shown to have the stability required for user-mode storage ring operations. The bunch-by-bunch transverse feedback system can also be leveraged as a powerful diagnostic tool. New data acquisition techniques have been designed to allow for the study of different instability mechanisms as well as parameters present in the equations of motion for stored particles. These techniques and the suite of results achieved are presented.