School of Physics - Theses

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Subject: physics × Start date: 2017 × End date: 2017 ×

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    B0→K0π0 and direct CP violation at Belle
    Hawthorne-Gonzalvez, Anton ( 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).
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    Novel plasmonic elements for the generation, manipulation and detection of polarised light
    CADUSCH, JASPER ( 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.