A search for the electroweak production of charginos and neutralinos decaying into final states involving three electrons or muons is presented. The analysis is based on 36.1 fb^-1 of √s = 13TeV proton–proton collisions recorded by the ATLAS detector at the Large Hadron Collider. Scenarios considered are based on simplified models with the associated production of the next-to-lightest neutralino and the lightest chargino, followed by their decays into final states with leptons and the lightest neutralino via either sleptons or Standard Model gauge bosons. No significant deviations from Standard Model expectations are observed and stringent limits at 95% confidence level are placed on the masses of relevant supersymmetric particles. For a massless lightest neutralino, masses up to 1.13TeV are excluded for the associated production of the next-to-lightest neutralino and the lightest chargino, assuming slepton mediated decays, whereas for gauge-boson-mediated decays, masses up to 380 GeV are excluded.

Phase transitions and critical phenomena are of central importance in quantum field theory and statistical physics. We investigate the low energy properties of O(N) symmetric scalar field theories using functional renormalisation group methods for all N. This modern formulation of Wilson's renormalisation group allows a continuous interpolation between short and long distance physics without resorting to a weak coupling expansion. To leading order in the derivative expansion, we study the phase transition and the approach to convexity in the deep infrared limit. In the limit of infinite N, the fluctuations of the Goldstone modes dominate allowing for a complete analytical discussion of the effective potential. For finite N, the radial fluctuations become important and we resort to systematic series expansions. In both cases a systematic and thorough analysis of the diverse fixed point solutions is carried out. This leads to a comprehensive picture of the scaling potential for a large number of universality classes. We also study the dependence of our results on the regularisation scheme. Finally, we establish that the infrared completion of the effective potential in the broken phase is driven by a fixed point that leads to the flattening of the non-convex part of the potential.

The work in this thesis extends the applications of the Electric Potential Sensor (EPS) designed by the Sensor Research Technology Centre. Combined is work undertaken in two areas related by their application in security systems: low-field nuclear magnetic resonance with electric-field acquisition, and particle detection for alpha, beta and neutron radiation. In both these areas the EPS is used as to acquire signals. The first half of the thesis consists of the work undertaken to design a low-field Nuclear Magnetic Resonance spectrometer to detect drugs and explosives. In doing so, the use of the electric field detection technique - patented by Sussex University - is extended to low-field NMR work. The eventual negative results in this field lead first to the design of a simpler proton magnetometer apparatus, a design which would confirm the use of the EPS at low frequencies, and eventually to a change in direction of the research: particle detection. Detailed in this first section are a theoretical explanation of NMR in chapter 2, and a chapter covering the design and testing of the equipment in chapter 3. The particle detection part of the thesis covers modifications made to the EPS in order to detect particles and experiments conducted to confirm their operation. As in the NMR section, the work is split into a theory chapter which underpins the work, providing context for the experiments chapter. Chapter 5 covers the detection of alpha, beta and neutron radiation and the use of feedback to control the RC time constant of the front end of the sensor. The work in this thesis concludes negative results in the NMR area, but proves the EPS particle detector as a viable, cost effective alternative to conventional detectors.

This thesis presents a search for the electroweak production of supersymmetry using the dataset taken by the ATLAS detector at the Large Hadron Collider with √8 = 8 TeV during 2012. Events with three leptons are selected and required to satisfy additional kinematic criteria that define optimised signal regions. In these signal regions, Standard Model processes are discriminated against, whilst retaining a large fraction of events produced by specified compressed supersymmetry scenarios. Compressed refers to near massdegeneracy between the decaying gauginos and the final state gauginos. The expected number of Standard Model events are estimated using a combination of Monte Carlo and data-driven methods, where the predictions are tested against data in specifically designed validation regions. Exclusion limits are then set at 95% confidence level (CL) on via ℓ`L- and via WZ-decay scenarios for the decaying charginos and neutralinos. With the ℓ`L halfway between the decaying charginos and neutralinos and the final state neutralinos, there is a new sensitivity up to 250 GeV. In scenarios with large mass splitting, the decaying chargino and neutralino masses are excluded up to 740 GeV. Looking forward to the 13 TeV data-taking, a search for the electroweak production of supersymmetry with threelepton final states is presented, with a first look at an optimisation strategy to improve sensitivity to charginos and neutralinos.

This thesis introduces a novel technique for resummation of a wide class of observables to next-to-next-to-leading-logarithmic accuracy in e+e− annihilation, and potentially beyond. The method is applicable to observables that exhibit recursive infrared and collinear (rIRC) safety and continuous globalness. A systematic analysis of logarithmic counting in emission phase space reveals the contributions necessary to achieve NNLL-accurate results. A detailed description of the derivation and subsequent calculation of these effects is given. A framework of computer code (called ARES) has been developed to carry out automated numerical implementation of each of the NNLL contributions. ARES (Automated Resummer of Event Shapes) provides the user with an efficient determination of the resummed result for a desired observable. New results for several observables are presented, including the first NNLL resummation of the two-jet rate in the Durham and Cambridge algorithms which is crucial for determination of the strong coupling of Quantum Chromodynamics (QCD). This work as a whole presents an important addition to phenomenological precision calculations. Validation of the obtained predictions is performed, using both matching to NNLO fixed order calculations and comparison to data from the Large Electron-Positron collider at CERN.

The thesis presents the results of two searches for the direct pair-production of third generation scalar quarks, the stop and the sbottom, in proton-proton collisions at √s = 13 TeV delivered by the Large Hadron Collider (LHC) and recorded by the ATLAS detector. Third generation squarks are studied in the context of natural supersymmetric extensions of the Standard Model, highlighting their role in the solution of the Higgs hierarchy problem and considering both R-parity conserving and violating decay scenarios. The signal models of interest produce final states characterised by the presence of two bottom quarks, and the identification of the hadronic jets generated by their fragmentation plays a crucial role in the analyses. The performance of b-jet identification algorithms is studied in detail, and a novel approach for the estimate of the associated systematic uncertainties is presented. The first analysis in the thesis is a search for a pairproduced sbottom with two-body decays into Standard Model third generation quarks and quasi-degenerate electroweakinos, while the second targets the pair-production of the stop followed by R-parity violating decays into a bottom quark and a lepton. No evidence of SUSY particles is found, and exclusion limits are set on the relevant signal models using dedicated statistical tools.

This thesis presents a search for supersymmetry using the dataset taken by ATLAS at the Large Hadron Collider with ps =8 TeV during 2012. Events with four or more leptons are selected and required to satisfy additional kinematic criteria that define optimised signal regions. These criteria are chosen to reject the majority of events produced by Standard Model processes, whilst retaining a large fraction of events produced by a variety of proposed supersymmetry scenarios. The expected number of Standard Model events are estimated using a combination of Monte Carlo and data-driven methods, the predictions of which are tested against data in specifically designed validation regions. No significant deviations from the Standard Model estimations are observed within statistical and systematic uncertainties. Exclusion limits are then set at 95% confidence level (CL) on a wide range of R-parity conserving and R-parity violating supersymmetry simplified models, as well as models of general gauge mediated supersymmetry. In R-parity violating models, 95% CL exclusion limits of 1350 GeV and 750 GeV are set on the masses of gluinos and charginos respectively. Exclusion limits are also set at 95% CL up to 620 GeV on the mass of heavy neutralinos for an R parity conserving scenario with decays via righthanded sleptons. Results are also presented for the combination of the four lepton analysis with another lepton-rich supersymmetry search.

The SNO+ experiment is primarily looking for neutrinoless double-beta decay, an unobserved, lepton number violating radioactive decay. This is achieved by loading liquid scintillator with tellurium whose isotope 130Te decays via double beta decay with a Q-value of 2527 keV. An optical calibration system, located outside the scintillator, has been developed to help meet the radiopurity requirements of the experiment. This thesis describes the hardware component of the optical calibration system which calibrates the timing and charge response of the photomultiplier tube array of SNO+. A set of quality assurance tests showed that the system was at the required standard for installation. Data taken with SNO+ and the optical calibration system showed that the system was stable enough for photomultiplier tube calibration, identified resolvable issues with the SNO+ data acquisition system and allowed measurement of single photoelectron spectra. Data quality checks have been developed to ensure data is of calibration standard. The sensitivity of SNO+ to neutrinoless double-beta decay with nearly 800 kg of 130Te and five years data taking is investigated with a comprehensive evaluation of systematic uncertainties. Two new methods for acquiring a greater sensitivity to neutrinoless double-beta decay were developed; a one dimensional fit in event energy and a multidimensional fit in event energy and position. A simple event counting analysis, developed previously by the collaboration, was shown to be sensitive to systematic uncertainties. A fit in an extended energy range was shown to constrain the systematics and achieve a half-life sensitivity of 9.30x1025 yr corresponding to a 5.6% improvement over the counting analysis which neglected systematic uncertainties. The multidimensional analysis with systematics included achieved a 20% improvement over the counting analysis with a half-life sensitivity of 1:06 x 1026 yr, corresponding to an effective Majorana mass between 52 to 125 meV.

This thesis presents an experimental investigation of the electron-phonon interaction in GaN. Bulk epilayers, grown by MBE, and AIGaN/GaN heterostructures, grown by MOCVD, have been studied. The energy relaxation rate for hot electrons has been measured over a wide range of temperatures, allowing both acoustic and optic phonon emission to be studied in GaN epilayers. Direct phonon measurements, both studying the emission and absorption processes, have been performed. Detection of phonons emitted when hot electrons relax their excess energy complements the measurements of relaxation rates. Absorption of acoustic phonons by the epilayers, using both fixed and extended metal film phonon sources, allowed investigation into the effectiveness of the 2kF cutoff in the low mobility layers. The experimental findings are compared with the predictions of theory. AIGaN/GaN heterostructures were characterised and measurements of the energy relaxation rate in the temperature range 4K-40K obtained. Excellent agreement with theory is observed. A preliminary study of phonon absorption by the 2DEG system is presented, which allowed experimental determination of the "thickness" of the 2DEG and demonstrated the applicability of the technique in the study of low dimensional systems.

The work presented in this thesis was carried out in the Physics Department at the University of Nottingham between October 1988 and October 1991. It is the original work of the author except where indicated by reference. This thesis describes the continuation of the development of Echo Volumar Imaging (EVI) to facilitate snapshot imaging of a volume within the human body. Variants of the technique which have also been investigated include a spin echo version, SE-EVI, and a zoomed version ZEVI. All formats acquired data in a modulus fashion in times ranging from 64 ms to 120 ms. Hardware limitations have restricted the image matrix size to 64 x 32 x 8 voxels and prompted the employment of more efficient gradient driver circuitry. A multi-mode resonant gradient circuit is described for use in both Echo Planar Imaging (EPI) and EVI. The circuit behaves in an overall resonant manner but at a fixed number of discrete frequencies. By choosing the number of resonant modes, the circuit can be used to generate approximations to a square wave or trapezoidal waveform. Because of the energy conserving nature of the circuit design much faster current rise times can be achieved with a given amplifier and gradient coil. The multi-mode gradient driver circuit was utilized both for planar imaging and to investigate the effect of rapidly modulated magnetic fields on the human body. A simple neural stimulation model is used to evaluate the stimulation threshold current density for a variety of magnetically induced waveforms and for sinusoidal stimulation as a function of frequency. Experimental results correlate well with the model showing that for short times, contrary to the widely held view, neural stimulation is independent of the magnetic field switching rate dB / dt, but depends on the final magnetic field value.