School of Physics - Theses
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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.
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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.
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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.
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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.
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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.
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ItemSearching for the Light Supersymmetric Top Quark with the ATLAS experiment( 2011)The nature of dark matter and the source of the matter-antimatter asymmetry in the universe are two of the most important questions in particle physics and cosmology. The current Standard Model of particle physics, while being a very successful description of the observed fundamental particles and their interactions, cannot fully account for either of these phenomena. Theoretical extensions of the Standard Model, however, possibly can. One such extension is the Minimal Supersymmetric Standard Model (MSSM). This thesis begins by exploring the MSSM parameter space in which the matter- antimatter asymmetry of the universe is dynamically generated through electroweak baryogenesis. In this scenario, one of the supersymmetric partners to the top quark, the light stop quark, must be lighter than the top quark. It is found that this parameter space region is highly constrained by experimental limits on the electric dipole moment of the electron and the branching ratio of a bottom quark into a strange quark and a photon. If the additional requirement of matching the observed dark matter abundance by the relic density of the lightest supersymmetry particle is necessitated, the allowed MSSM parameter space is further constrained. The focus of the thesis then moves to the investigation of the collider phenomenology of supersymmetric electroweak baryogenesis, in particular, the evaluation of the discovery potential of light stop quark pair production at the LHC using the ATLAS experiment. This study assumes a light stop decay topology involving the lightest chargino and neutralino where the visible final state products mimic those from top quark pair production. Feasibility studies are performed for proton-proton collisions at centre of mass energies of 10 TeV and an integrated luminosity of 1/fb, concentrating on the dileptonic and semileptonic decay channels where there are two or one charged leptons in the final state. It is found that signal points with stop masses less then 120 GeV and stop-neutralino mass differences greater than 60 GeV have the greatest discovery potential in the dileptonic decay channel, while the semileptonic decay channel is swamped by backgrounds and requires detailed understanding of the detector and backgrounds in order to extract a signal. Finally, a preliminary study is conducted on 41.4/pb of data collected at collisions with centre of mass energies of 7 TeV in the dielectron decay channel, focusing on the understanding of selection variables and backgrounds.
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ItemDiscovery potential of neutral MSSM Higgs bosons decaying to tau-lepton pairs in the ATLAS experiment( 2011)The Large Hadron Collider (LHC) is the highest energy particle collider ever built. It recently began operation at CERN and will probe physics at unprecedented scales. ATLAS is a particle detector located at one of the collision points on the LHC ring and is designed to be sensitive to the wide range of physics that could be produced. The primary objective of the LHC experiments is to determine the mechanism of electroweak symmetry breaking, of which many theoretical models exist. In the Minimal Supersymmetric Standard Model (MSSM), electroweak symmetry breaking is achieved through the Higgs mechanism, however, the Higgs sector must be extended with respect to the Standard Model and contains five physical Higgs bosons. The discovery potential of the MSSM Higgs bosons in ATLAS has been evaluated in previous studies, demonstrating adequate sensitivity for discovery or exclusion over a large region of the parameter space. However, these studies were performed using now outdated software, without an estimation of the expected systematic uncertainties or the inclusion of data-driven background estimation procedures. In this thesis, the discovery potential of the neutral MSSM Higgs bosons when decaying to tau-lepton pairs in the ATLAS experiment is evaluated. One tau is required to decay leptonically while the other is required to decay hadronically. Higgs boson mass hypotheses in the range 150 GeV - 800 GeV are considered. The study assumes a proton-proton collision energy of 14 TeV and an integrated luminosity of 30/fb. The expected systematic uncertainty on the background measurement is evaluated and included in the calculation of the discovery potential. Data-driven estimation techniques are developed for the W+jets and QCD di-jet backgrounds. The contributions of all signal and background processes are estimated using Monte Carlo simulated event samples. The discovery potential is interpreted in the mh-max benchmark scenario, and is presented in the mA-tan(beta) plane. A small degradation in performance with respect to the previous studies is found for Higgs boson masses below 450 GeV due to the inclusion of systematic uncertainties. It is confirmed that a large fraction of the parameter space will be accessible to the ATLAS experiment, which will be able to probe far beyond the regions already excluded by the LEP and Tevatron experiments. Two separate studies are also included, describing contributions to the modelling of hadronic tau reconstruction in the ATLAS fast simulation packages ATLFAST-I and ATLFAST-II. Firstly, a complete parameterisation of the calorimeter-based hadronic tau reconstruction for use in ATLFAST-I is presented. Secondly, the validation of the track-based hadronic tau reconstruction in ATLFAST-II is presented, including the extraction of correction terms to match the performance in ATLFAST-II to the standard ATLAS simulation.