Ultra-high energy cosmic rays (UHECRs) are cosmic rays with energy exceeding 10¹⁸ electronvolts. The sources of these particles remain unknown despite decades of research. This thesis presents a series of studies aimed at constraining the sources of UHECRs both directly by studying their observed arrival directions and indirectly through their expected secondary gamma-ray signatures. An analysis of the arrival direction distribution of the highest energy cosmic rays detected at the Pierre Auger Observatory is presented. The aim of the study was to determine whether the arrival directions of observed UHECRs follow the distribution of nearby extragalactic sources, which is expected if UHECRs are light nuclei of extragalactic origin. A departure from isotropy at the 95% level is observed but no clear correlation with the extragalactic matter distribution is found. The sensitivity of upcoming UHECR experiments, with an order of magnitude higher annual exposure than current experiments, to the expected UHECR anisotropy has been investigated through simulations. It is shown, that with five years of data from such a detector an anisotropy should be detectable at the 99% level as long as the composition is proton dominated. In a scenario where the UHECR source distribution is strongly clustered, similar to the distribution of galaxy clusters, an anisotropy at the 99.9% level is expected even if the fraction of protons at the highest energies is as low as 30%. Constraints on the sources of UHECRs may also come from the secondary particles that UHECRs produce during their propagation. A study of the expected secondary gamma-ray signatures of UHECR accelerators embedded in magnetised environments is presented. The secondary gamma-ray emission expected in this model is shown to be consistent with the spectra of a number of extreme blazars. It is shown that this model is more robust to variations of the overall extragalactic magnetic eld strength than other proposed scenarios, which is appealing in view of the large uncertainty surrounding the strength and con guration of extragalactic magnetic fi elds.

This thesis reports on the fabrication and characterisation of self-assembled nanofibres of poly(3-hexylthiophene) (P3HT), and demonstrates how these nanofibres can be used in applications like thin-film transistors and solar cells. The first results chapter describes a preparation method of P3HT nanofibres in a solution of chlorobenzene by using di-tert butyl peroxide (DTBP) as an additive. This method allows the fabrication of films of P3HT with high molecular order and gives control over the film retention. The films are characterised using a range of experimental techniques, including optical absorption, X-ray diffraction and atomic force microscopy, which also allows to determine the dimensions of individual, separated nanofibres. A more detailed investigation into the temperature dependence of the photoluminescence (PL) of nanofibre films in comparison to P3HT thin-films is presented in the second results chapter. The line-shape of the measured PL of the nanofibres shows significant differences to the thin-film, which is most distinctive at a temperature of around 150K. At this temperature, the measurements show a change of the emission characteristics for the nanofibres which is absent in the thin-film. The cause for the observed transition can be related to the increased planarisation of the polymer backbone inside the nanofibres with increasing temperature. This gives rise to more dominant intrachain coupling for the fibres, in contrast to predominantly interchain coupling in P3HT thin-films. The third results chapter demonstrates the application of the nanofibre films in thin-film transistors and solar cells. It is shown how the high molecular order of the nanofibres in combination with the formation of fibre networks can be used to control the field-effect mobility of P3HT films. Solar cells are fabricated by successive deposition of a nanofibre film and an electron acceptor layer, resulting in power conversion efficiencies comparable to bulk heterojunction solar cells.

The double-beta decay of Ca-48 was analysed with data taken by the NEMO-3 experiment with 6.99 grams of isotope and 1918.5 days of live time. The half-life of the two-neutrino double-beta decay of Ca-48 was measured to be 6.4 +0.6-0.7 (stat.) +1.2-0.9 (syst.) x 1019 years, with the corresponding nuclear matrix element measured to be 0.02 +- 0.002. A lower limit of 2.0 x 1022 years was placed on the half-life of the neutrinoless mode, yielding an upper limit on the effective Majorana neutrino mass of 6.1 - 38 eV. Limits were also placed on the R-parity violating supersymmetry trilinear coupling λ'111, on right-handed current couplings λ and η, and on the coupling of the Majoron to the electron neutrino. Work in preparation for the commissioning of the tracking detector for the SuperNEMO double-beta decay experiment is described. The data acquisition and high voltage supply electronics were prepared to read 504 SuperNEMO tracker cells and successfully tested with a smaller 2 x 9 array of tracker cells. A system to supply the SuperNEMO Demonstrator with the tracking gas mixture was designed and built with stringent radon emanation requirements. The system was shown to perform well on tests with a single-cell tracker prototype.

The discovery of the Higgs boson by the ATLAS and CMS experiments is one of the main results of Run 1 of the Large Hadron Collider. However, clear evidence for the Higgs boson decay to a pair of b-quarks has not been observed and is crucial to establish the nature of the new found particle. The work presented in this thesis focuses on the search for the Higgs boson in the VH(bb) channel, where it is produced in association with a leptonically decaying vector boson (W, Z), and decays to a pair of b-quarks. Prior to the start of LHC operations, the challenges posed by a pp collider to a H→bb search motivated the development of jet substructure techniques. The boosted regime plays a vital role in the sensitivity of a VH(bb) search and the topologies where the decay products merge can be recovered by implementing a substructure-based selection. The sensitivity of such an approach in a VH(bb) search is studied using ATLAS pp collision data, at a centre-of-mass energy of √s = 7 TeV. It was found that the sensitivity in the boosted region of the VH(bb) channel in Run 1 is already fully exploited by the resolved approach. The mass of the Higgs boson, the energies and luminosities delivered and the good performance of anti-kt jets resulted in little or no gain, at this stage, from performing a jet substructure analysis. The final ATLAS VH(bb) Run 1 result is presented. The systematic uncertainties related to the W+bb process are estimated and discussed. As an irreducible background to this search, the description of W+bb events plays an important role on the final obtained sensitivity. Finally, in preparation for Run 2 and future colliders, the potential benefits from jet substructure techniques are reviewed at different centre-of-mass energies in the context of a boosted WH(bb) search. A detailed study of the signal significance as a function of the boost of the system reveals that the region of highest sensitivity is already fully exploited by the resolved reconstruction. A substructure approach is only beneficial in events with boosts greater than 600 GeV, outside the phase-space region of maximum significance.

The latest jet data from the Large Hadron Collider is studied in the context of the MSTW Parton Distribution Functions (PDFs). The effect of recent ATLAS inclusive- and dijet results on the current standard PDF sets is investigated, compared and contrasted to similar results from the Tevatron. A parton reweighting technique is used to gain information on the optimum PDFs for each new data set. New PDF sets are produced and studied using the new LHC data. These jet results provide a new central value of the PDF and reduce the uncertainty on the distributions. Additionally, a new method of parametrising the quark PDFs using Chebyshev Polynomials is tested in relation to the ATLAS W/Z rapidity data, which is described poorly using the standard PDFs. The effect of parton showering on jet physics is studied using various shower Monte Carlo generators within the context of jets produced in deep inelastic scattering, and the possibility of using charged current jet production in PDF fits is tested.

Accretion onto compact objects plays a central role in high-energy astrophysics. The presence of a compact object considerably alters the structure and dynamics of the accreting plasma, as well as its radiative emissions. For accreting black holes in Active Galactic Nuclei (AGN) this is especially true. A significant fraction of the emission may originate or pass near the event horizon. Strong gravity modifies the radiation from an emission source. Photons no longer propagate in straight lines and experience frequency shifts. Gravitational lensing creates multiple images of an emission source, further modifying its temporal and spectral properties. Addressing these effects, the first part of this thesis formulates the equations of radiative transfer for particles with and without mass in a manifestly covariant form. Using ray-tracing, the observed images and line emission from accretion disks and tori are calculated. The effects of absorption, emission and optical depth gradients are investigated. The second part of this thesis examines scattering in general relativity. The general relativistic Compton scattering kernel and its angular moments are expressed in closed-form for the first time, in terms of hypergeometric functions. This has the advantage of being fast, accurate and not restricted by specific energy ranges. The results are in perfect agreement with semi-analytic calculations and Monte-Carlo simulations of Compton scattering of monochromatic emission lines. Finally, I investigate the effects of variability in the accretion flow. Two models are considered: a plasmoid on a Keplerian orbit around a black hole and a magnetically-driven plasmoid ejection from the disk corona. Deriving a new time-dependent radiative transfer formulation, I calculate this variable emission, presenting the results in the form of spectrograms and lightcurves.

FIRI (Far Infrared Interferometer) is a concept for a spatial and spectral space interferometer with an operating wavelength range 25-300 μm and sub-arcsecond angular resolution, and is based on the combination of two well-known techniques, Stellar Interferometry and Fourier Transform Spectroscopy, to achieve high spectral and spatial resolution in the Far Infrared (Chapter 2). The resulting technique is called Double Fourier Spatio-Spectral Interferometry (Mariotti and Ridgway 1988). With increased spatial and spectral resolution a number of interesting science cases such as the formation and evolution of AGN and the characterization of gas, ice and dust in disks undergoing planetary formation, among others, can be investigated. In this thesis I present two approaches to study the feasibility of a FIRI system a) An experimental approach via the Cardiff University-UCL FIRI Testbed (Chapter 3), a laboratory prototype spectral-spatial interferometer to demonstrate the feasibility of the Double-Fourier technique at the Far-infrared regime, including the Wide-Filed Imaging Interferometry Testbed at the Optical and Near-infrared regime. b) The Far-Infrared Interferometer Instrument Simulator (FIInS) to assess the performance of a space-based system (Chapter 4). The main goal of this software is to simulate both the input and the output of such a system. With FIInS, once a set of modeled scientific data is available, one can compare an input sky map with the synthesized one after data reduction and processing algorithms have been applied. To validate FIInS data from the Cardiff University-UCL FIRI testbed has been used (Chapter 5). FIInS is intended to be a tool for the astronomical community to explore the limits of a space interferometer. For this reason it is also applied to simulated scientific data (Chapter 6), more specifically to simulated circumstellar disks to test the simulator in a more realistic scenario.

Observations of the cosmic microwave background have led to a golden age of cosmology, where precise measurements can be confronted with predictions from cosmological models. Ongoing and future surveys of the distribution of galaxies will continue this revolution: they will enable us to test the laws of gravity, uncover the properties of dark energy and dark matter, and reinforce the connection to high-energy physics. However, current galaxy survey analyses are already limited by our ability to identify and treat observational systematics, and this problem will be even more pronounced in future experiments. Therefore, it is essential to develop novel methods to deal with these complications when testing cosmological models and searching for new physics. This is the focus of this thesis. Firstly, I will present measurements of primordial non-Gaussianity obtained from the clustering of quasars from the Sloan Digital Sky Survey. Primordial non-Gaussianity is a powerful probe of inflation, the leading theory of the initial conditions of the universe, but its effects on the distribution of quasars are mimicked by observational systematics. I will describe a framework to deal with these systematics and robustly measure primordial non-Gaussianity from the clustering of quasars. Secondly, I will present a new set of wavelet transforms on the sphere and the ball. These approaches are highly promising for analysing cosmological and geophysical data and dealing with their systematics in novel ways. Finally, I will examine the recent claims that extra massive neutrinos can resolve the tensions between cosmic microwave background, galaxy survey and supernova observations. I will demon- strate that this conclusion is premature since it is driven by the least robust data sets. Given the growing number of cosmological observables and their varied levels of robustness, combining data sets and dealing with such tensions will become critical in the near future.

The High Luminosity Large Hadron Collider (HL-LHC) is the last planned upgrade of the LHC and it will increase the instantaneous luminosity by a factor of 10. To cope with the predicted high particle rates and the extreme radiation dosage, the ATLAS detector will require substantial modifications and in particular a new all-silicon inner tracking detector will be constructed. The thesis reviews the design of the new tracking detector and identifies the design choices that need to be made. It presents the digital design of the new ATLAS ABC130 readout chip which will be deployed in the new tracking detector and tests of the associated readout modules. Results on the electromagnetic compatibility of the readout modules are presented along with characterization and electrical modelling of the new ATLAS silicon strip sensors.

Magnetic elds are ubiquitous at all scales in the Universe and have been observed in galaxies and clusters of galaxies via observations of di use radio emission and Faraday Rotation Measures. Despite the observations, the origin and impact of the magnetic elds in these systems is poorly understood. In this thesis we develop a state of the art cosmological Smoothed Particle Magnetohydrodynamics code, GCMHD+, to enable the study of the magnetic elds of the largest bound structures in the Universe. Using a wide range of idealized test problems, we justify our choice of free parameters and demonstrate the performance of the code relative to analytical solutions and the results produced by a grid based MHD scheme. We then used the code to investigate the evolution of a seed magnetic eld due to the formation of structure. By varying the numerical scheme, we demonstrate that the growth of magnetic elds in galaxy clusters are very sensitive to the growth of numerical divergence of the magnetic eld. We nd that amplitude and topology of the cluster magnetic eld are insensitive to the mass or formation history of the cluster. Using high resolution simulations, we show that a primordial seed magnetic eld is capable of reproducing a wide range of observations of large-scale magnetic elds in galaxy clusters. Additionally, we examine the impact of the formation of spiral structure in a disc galaxy on the galactic magnetic eld. We nd that the numerical scheme can become unstable unless the divergence cleaning scheme is limited. We nd that the rotation of the galaxy produces a disc orientated magnetic eld with a spiral structure and large-scale eld reversals. The formation of spiral arms ampli es the ambient G magnetic eld to 20 G, in agreement with the observations of spiral galaxies. We conclude that additional physics is required to produce a more realistic galactic magnetic eld.