Searches for the Higgs boson decaying into a meson and an associated photon are presented using √s = 13 TeV proton-proton collision data collected by the ATLAS experiment during LHC Run II in 2015 and 2016. No significant excess of events is observed above the expected backgrounds. 95% confidence-level upper limits on the branching fractions of the Higgs boson decays to J/ψγ, ψ(2S)γ, and ϒ(nS)γ of 3.5×\(10^{−4}\), 2.0×\(10^{−3}\), and (4.9, 5.9, 5.7)×\(10^{−4}\) (n = 1, 2, 3), respectively, are obtained assuming Standard Model Higgs production with 36.1 \(fb^{−1}\) of data. Analogous limits on the branching fractions of the Higgs boson decays to φγ and ργ of 4.8×\(10^{−4}\) and 8.8×\(10^{−4}\), respectively, are also found using up to 35.6 \(fb^{−1}\) of data at √s = 13 TeV. In the next decade, the LHC will undergo an upgrade to increase the delivered instantaneous luminosity, which will aim to collect 3000 \(fb^{−1}\) of integrated luminosity over its lifetime. With this improvement the searches for Higgs boson decays into a meson and an associated photon will have a sensitivity closer to the Standard Model predicted branching fractions. The ATLAS detector will be upgraded to sustain performance with the higher rate of collisions. The University of Birmingham is among a consortium of facilities that irradiate and test prototype silicon sensors intended for the upgrade of the ATLAS detector. This thesis also presents early results during the commissioning of the sensor test system.

Part I: An attempt was made to measure the impedance of an r.f. discharge by standing-wave methods. However, this was unsuccessful because the r.f. oscillator which was built for this purpose proved to have insufficient frequency stability. Part II: An r.f. discharge in helium was examined with a quartz prism spectrograph in order to deduce the temperature of its excited atoms and ions from the Doppler breadth of their spectrum lines. A new 150 Mc/s. oscillator supplied a 20 microsecond pulse of about 25 K.W. into the discharge fifty times per second, and a constricted discharge tube was used in order to concentrate the power in to a small volume of gas. The discharge was run at low pressure in order to reduce Stark broadening and was made part of a gas circulating system in order to avoid losses from 'clean-up'. The temperature drift of the spectrograph caused considerable difficulty, but the line breadths were corrected for this effect to a first approximation. At 0.03 mm. Hg. the breadth of the He I lines at 2723, 2764, 2829 A. corresponded to about 2,000°K. and that of the He II line at 2733 A. corresponded to 20,000°K.; this difference is attributed to Stark effect , but besides this it is possible that some contribution to the He I spectrum may have come from the relatively cool after glow. The relative intensities of the lines gave an excitation temperature of the order of 1,500°K. for He I and 4000°K. for He II (lines at 2733 and 3203 A .). Besides the possible contribution of the afterglow to He I, the most likely reason for these low excitation temperatures is that the electrons' velocity distribution was not Maxwellian but contained a greater proportion of low energy electrons because of inelastic collisions with the walls and secondary electron emission from the walls of the constriction. The relative intensity of the He I and He II spectra gave an equilibrium temperature of the order of 16,000°K.

This research describes a two-dimensional modelling effort of heat and mass transport in simplified intrusive models of sills and their feeder dykes. These simplified models resembled a complex intrusive system such as the Great Dyke of Zimbabwe. This study investigated the impact of variable geometry to transport processes in two ways. First the time evolution of heat and mass transport during cooling was investigated. Then emphasis was placed on the application of convective scavenging as a mechanism that leads to the formation of minerals of economic interest, in particular the Platinum Group Elements (PGEs). The Navier-Stokes equations employed generated regions of high shear within the magma where we expected enhanced collisions between the immiscible sulphide liquid particles and PGEs. These collisions scavenge PGEs from the primary melt, aggregate and concentrate it to form PGEs enrichment in zero shear zones. The PGEs scavenge; concentrate and ‘glue’ in zero shear zones in the early history of of viscosity and dispersive pressure (Bagnold effect). The effect of increasing the geometry size enhances scavenging, creates bigger zero shear zones with dilute concentrate of PGEs but you get high shear near the roots of the dyke/sill where the concentration will not be dilute. The time evolution calculations show that increasing the size of the magma chamber results in stronger initial convection currents for large magma models than for small ones. However, convection takes, approximately the same time to cease for both models. The research concludes that the time evolution for convective heat transfer is dependent on the viscosity rather than on geometry size. However, conductive heat transfer to the e-folding temperature was almost six times as long for the large model (M4) than the small one (M2).Variable viscosity as a physical property was applied to models 2 and 4 only. Video animations that simulate the cooling process for these models are enclosed in a CD at the back of this thesis. These simulations provide information with regard to the emplacement history and distribution of PGEs ore bodies. This will assist the reserve estimation and the location of economic minerals.

This thesis presents lifetime measurements of charmless two-body decays of b hadrons, specifically the decay modes known as B->hh', where B refers to meson or baryon containing a b quark and h' refers to a proton p, pion pi or kaon K. Using the large data samples collected by the LHCb detector, the \BTohh channels with the largest branching fractions provide an opportunity to perform high-precision measurements of the properties of the decays. The leading-order processes in B->hh' decays are tree and penguin topologies, where the loop-dominated channels could be sensitive to non-standard model physics. The Bs->KK mode is particularly interesting as it has a CP-even final state, as well as being dominated by penguin decay processes. The Bs->KK effective lifetime can be used to calculate the Bs decay-rate asymmetry A_{\Delta \Gamma}, which quantifies the amount of CP violation in the decay. Using LHCb data with an integrated luminosity of 3 fb^{-1} collected at centre of mass energies of 7 TeV and 8 TeV, the lifetimes of the decays Bs->KK, Bd->Kpi, Bs->piK, Bd->pipi, Lambda_b->ppi and Lambda_b->pK are measured to be tau_{Bs->KK} = 1.410 +\- 0.009 ps (stat) +\- 0.011 ps (syst), tau_{Bd->Kpi} = 1.504 +\- 0.006 ps (stat) +\- 0.023 ps (syst), tau_{Bs->piK} = 1.548 +\- 0.028 ps (stat) +\- 0.023 ps (syst), tau_{Bd->pipi} = 1.495 +\- 0.012 ps (stat) +\- 0.007 ps (syst), tau_{Lambda_b->ppi} = 1.511 +\- 0.028 ps (stat) +\- 0.012 ps (syst), tau_{Lambda_b->pK} = 1.477 +\- 0.022 ps (stat) +\- 0.022 ps (syst). All these lifetime measurements are compatible with the current world averages. The decay-rate asymmetry is calculated to be A_{\Delta \Gamma}= -0.975 +\- 0.092 (stat) +\- 0.113 (syst) \pm 0.082 (ext), which agrees with the standard model prediction. A performance study measuring the photoelectron yield of the Ring Imaging Cherenkov (RICH) subdetector during 2012, 2015 and 2016 is also presented in this thesis. The RICH is used for particle identification and it distinguishes between pion, kaon and proton tracks. During the long shutdown between the 2012 and 2015 data taking periods, the aerogel in the RICH 1 detector was removed and the centre-of-mass energies of the proton-proton collisions increased from 7-8 TeV to 13 TeV. The photoelectron yield of RICH 1 was found to increase by ~3 photoelectrons in 2015 and 2016 and the photoelectron yield of RICH2 remained the same. This increase in RICH1 was expected due to the increased path length of the tracks through the Cherenkov medium once the aerogel was removed. Overall the contribution of photoelectron yield to the performance of the RICH remains acceptable for continued data taking.

The Bern-Carrasco-Johansson (BCJ) double copy, which relates the scattering amplitudes of gauge and gravity theories has been an active area of research for a few years now. In this thesis, we extend the formalism of BCJ to consider classical solutions to the field equations of motion, rather than scattering amplitudes. One first approach relies on a family of solutions to the Einstein equations, namely Kerr-Schild metrics, which linearise the Ricci tensor. Using them we propose a simple ansatz to construct a gauge theory vector field which, in a stationary limit, satisfies linearised Yang-Mills equations. Using such ansatz, that we call the Kerr-Schild double copy, we are able to relate, for example, colour charges in Yang-Mills with the Schwarzschild and Kerr black holes. We extend this formalism to describe the Taub-NUT solution (which is dual to an electromagnetic dyon), perturbations over curved backgrounds and accelerating particles, both in gauge and gravity theories. A second, more utilitarian approach consists on using the relative simplicity of gauge theory to efficiently compute relevant quantities in a theory of perturbative gravity. Working along this lines, we review an exercise by Duff to obtain a spacetime metric using tree-level graphs of a quantum theory of perturbative gravity, and repeat it using a BCJ inspired gravity Lagrangian. We find that the computation is notably simplified, but a new formalism must be developed to remove the unwanted dilaton information, that naturally appears in the double copy.

The improvement of optical mode loss in semiconductor lasers is of significant commercial value, contributing to lower threshold current densities and higher above-threshold slope efficiencies. While it has improved significantly with successive developments in semiconductor laser design resulting from improvements to the fabrication process, characterisation methods for the measurement of optical mode loss have not kept pace. Today values of optical mode losses as low as 1cm-1 are frequently reported, and yet the precision and accuracy of its measurement are rarely better than 1cm-1 itself. To address this, I demonstrate that a modulated approach to the segmented contact method for the measurement of modal absorption and loss can measure optical mode losses with precisions as low as 0.1cm-1. I also demonstrate the removal of non-linearity from detection system using a novel approach to amplitude calibration by flux superposition. I apply this improved methodology in the comparison of InAs QD samples with differing waveguide core dimensions (2440Ǻ and 3740Ǻ), finding a reduction in optical mode loss between these samples from (4.8±0.4)cm-1 to (2.25±0.10)cm-1. In addition to measuring the optical mode loss at the lasing wavelength I investigated the origins of the observed wavelength dependence of optical mode loss below the material band-edge, comparing experimental values of modal absorption and loss spectra with a waveguide model. I show the wavelength dependent optical mode loss of wide waveguide core samples agrees well with losses predicted from model incorporating free carrier absorption, intervalence band absorption and waveguide dispersion. I also show that in both samples optical mode losses and their respective wavelength dependences are insensitive to changes in temperature from 298-370K. I also investigate the causes of oscillating modal absorption and loss seen in narrow waveguide core samples.

Nonlinear ultrasonic techniques have the potential to allow for earlier detection of material degradation. Recent advances in the area show promising results, but the methods used are often impractical outside laboratory conditions. Non-contact methods have the potential to address some of the current measurement limitations. Electromagnetic acoustic transducers (EMAT) are one potential candidate for noncontact, nonlinear measurements in metals. The EMAT ultrasonic wave generation process is complex and requires an in-depth understanding of the electro-mechanical coupling phenomena. The potential of EMAT transducers as generators and detectors has been evaluated for multiple nonlinear measurement techniques. The ultrasonic wave, generated by the interaction of the dynamically generated magnetic field and the eddy currents within the sample surface, called the self-field Lorentz force, have a significant impact on generated nonlinearity and must always be considered. For longitudinal wave measurements, the ultrasonic wave generated by the self-field Lorentz force has a similar amplitude and behaviour to the second harmonic wave generated by the material nonlinearity, thus making the measurements impractical. For shear waves, the third harmonic nonlinear ultrasonic generation shows inconclusive results with regards to microstructural change. Additionally, shear wave, nonclassical nonlinear ultrasonics show no significant change during the fatigue lifetime for aluminium alloy. Ultrasonic measurements of the temperature dependence of the sound velocity and of the coefficient of temperature expansion show a promising new field of study where EMAT application is well suited. Meanderline EMATs have a great potential as receivers, but not as ultrasonic transmitters for nonlinear Rayleigh waves. The ultrasonic Rayleigh waves generated by the self-field Lorentz force, have similar amplitude and behaviour to the nonlinear ultrasonic Rayleigh waves. Optimised PPM (periodic permanent magnet) EMATs are used for entirely non-contact, nonlinear measurement of third harmonic shear horizontal waves and show great potential. A new method to study and minimise experimental system nonlinearity is proposed. Measuring the ultrasonic waves generated from an artificially added harmonic distortion to the generation signal can provide a method to separate the effects of the system and the material nonlinearities.

Predicted by Einstein’s General Theory of Relativity, gravitational waves are periodic fluctuations in the curvature of space-time that propagate at the speed of light and are caused by acceleration of asymmetric mass distributions. The first ever direct detection of gravitational waves was a signal originating from the final moments of a binary black hole inspiral and the subsequent merger. Four more black hole inspiral and mergers have been observed since the first detection, as well as the inspiral and merger of a binary neutron star system. These detections have provided a wealth of information about the black holes/binary stars from which the signals originate, and further detections will continue to both test General Relativity and provide ground- breaking insights into previously poorly characterised astrophysical systems. The signals were detected by ground-based interferometric gravitational wave detectors. The sensitivity of interferometric detectors is dependent on reducing numerous sources of noise. A primary limiting noise source is the motion of the front face of test mass mirrors (which reflect the laser light within the detector arms) due to vibration from latent thermal energy in the highly reflective, multilayer coating materials. The power spectral density of this noise source is proportional to operation temperature and coating mechanical loss, which is a property describing energy dissipation within a material. Upgrades to current detectors, as well as proposed next-generation detectors, include operation at cryogenic temperatures to lower this thermally-induced noise. It is therefore crucial to know the low temperature mechanical loss of coating materials of interest for improving thermal noise within gravitational wave detectors. The research presented focuses on mechanical loss and structural characterisation of coating materials with the goal of increasing the observational range of future gravitational wave detectors through reduction of coating thermal noise. An introduction to gravitational waves, overview of astronomical sources, and discussion of the current, worldwide network of detectors and the sources of noise limiting their sensitivities is given in Chapter 1, which also contains a summary of all the detected signals up to this point. A more detailed discussion of coating thermal noise is given in Chapter 2, with a breakdown of the multiple forms of thermally- induced noise within test mass mirror coatings given, along with an explanation of the direct relation between mechanical loss and thermal noise. Accurately characterising coating mechanical loss over a wide range of temperatures is valuable for both estimating thermal noise in next generation detectors and better understanding the links between coating structure and loss. The methods used for characterising coatings are discussed in Chapter 3 in addition to an overview of coating technologies. This chapter provides an explanation for the procedures behind the measurements presented throughout this thesis. The mirror coatings used in current detectors are deposited via ion-beam sputtering and are comprised of alternating layers of high refractive index titania-doped tantalum pentoxide (Ti:Ta2O5) and low refractive index amorphous SiO2. Chapter 4 contains a comprehensive study of the changes in mechanical loss of ion-beam sputtered silica with respect to post-deposition heat treatment, providing updated loss values for thermal noise estimation of current coatings and potential coating designs for future detectors. A peak in dissipation is observed at low temperatures and found to change location and shape with heat treatment, but the activation energy of this thermally activated peak remained constant. The minimum loss at low temperatures is found to occur after heat treatment at 600 C, which is in contrast to the minimum loss at room temperature after 950 C. The loss of a silica deposited by a new coating technique, reactive low voltage ion-plating, was found to be lower as-deposited than the 600 C ion-beam sputtered silica. This technique involves deposition at over twice the temperature, so this loss result could be further evidence of high energy deposition techniques producing coatings with minimal distributions of two-level systems. Chapter 5 contains measurements of Al2O3, a potential replacement for SiO2 as the low refractive index coating material in multilayer coatings. The loss is found to be lower than 600 C heat-treated SiO2 by almost 50% at the low temperature peak but about four times higher at temperatures above 100 K. The loss of Al2O3 deposited at two different thicknesses (505 nm and 2.02 μm) exhibits minimal change in magnitude at temperatures below 40 K with heat treatment up to 800 C. Heat treatment at 300 C reduces the loss above 40 K, but further heat treatment fails to produce significant reduction. In contrast, the coating stress for both thickness went from ∼475 GPa compressive stress to over 140 GPa tensile stress. This could suggest that there is no strong connection between low temperature mechanical loss and coating stress within Al2O3. The high refractive index coating layers (Ti:Ta2O5) used in the advanced detectors are the dominant source of thermal noise within the highly reflective multilayer mirror coating at room temperature and has been shown to exhibit a peak in mechanical loss at cryogenic temperatures. Chapter 6 contains the mechanical loss characterisation of a number of alternate high refractive index coating materials. Increasing the titania doping percentage to 68% results in over an order of magnitude decrease in mechanical loss at temperatures below ∼100 C after heat treatment, but the optical and structural properties require further study. Doping Ta2O5 with zirconium instead (34.5%) produces room temperature loss similar to Ti:Ta2O5 with heat treatments up to 600 C with the added bene t of increased resistance to crystallisation. A decrease in loss with further heat treatment is expected with continued measurement. Measurements of pure TiO2 show loss that decreases with heat treatment up to 300 C, which is unexpected given evidence of crystallisation in titania coatings after annealing at 200 C. The loss of reactive low voltage ion-plated silicon was measured and found to be lower than any previously measured ion-beam sputtered amorphous silicon (aSi) coating. Ta2O5 deposited by this same coating method did not display the same reduction compared to ion-beam sputtering seen with SiO2 or aSi, with loss about 25% greater than ion-beam sputtered. Ta2O5 deposited through direct current magnetron sputtered did exhibit lower loss across all measured temperatures, but, with a measured absorption of ∼85 ppm, it is not competitive from an optical standpoint. All measured Ta2O5 coatings show a low temperature loss peak, but the peaks are not aligned with each other, ranging about 25 to 55 K. Calculations of the activation energies associated with each coating’s peak are valuable for possible correlations between structural properties, mechanical loss, and deposition technique. The mechanical loss characterisation from Chapters 3 through 5 are then summarised in Chapter 7, in which the coating Brownian noise is calculated for dual and multi-material coating stacks. All proposed coating combinations are compared to the estimated thermal noise of the mirror coatings in aLIGO and Advanced Virgo. Dual-material multilayer coating thermal noise estimates confirmed the value of Al2O3 as a low index material for detectors operating at temperatures within the silica loss peak, with a 20% and 25% reduction in Brownian noise at 10 and 20 K, respectively. The greatest reductions were seen with coatings where the high index material is amorphous silicon, which currently has absorption too high to use in a dual-material coating stack. / Multi-material coatings have been proposed as a way to take advantage of these mechanical loss gains in light of the high absorption of aSi. The thermal noise estimate for the initial proposed coating design is updated based on new measurements of SiO2 after heat treatment at 450 C (the ideal annealing temperature for aSi absorption) and found to have significantly increased thermal noise. Replacing the SiO2 with Al2O3 reduces the Brownian coating noise by 35% and 39% at 10 and 20 K, respectively, and is an immediately viable option as a low noise coating for future detectors. Continued research of reactive low voltage ion-plated coating materials is important; a speculative multi-material coating deposited by this method and assumed to have loss reductions through doping and heat treatment shows great promise, with estimated Brownian noise reductions of 31%, 33%, and 21% at 10, 20, and 123 K, respectively, compared to the current advanced detector multilayer coating.

Worldwide, zirconium alloys have long been utilised for fuel cladding elements and other structural components within several commercial designs of nuclear reactor owing to their high creep resistance, superior corrosion resistance in highly aggressive environments, and low cross section for neutron absorption. The purpose of this cladding material is to separate the uranium dioxide (UO2) fuel and the coolant water in order to prevent the escape of fission products, whilst also maintaining heat transfer to the coolant. Consequently, water corrosion of the fuel containments has become a key factor in the limitation of the lifetime of fuel rods within nuclear reactors, and maintaining containment integrity under corrosion is critical to ensuring safe operation and preventing accidental release of radionuclides into the cooling water. Therefore, developing an understanding of the mechanisms which govern the corrosion of zirconium-based alloys is vital, and is the motivation behind this study. The analysis of the corrosion of an unirradiated zirconium based alloy - Zircaloy-4 - is performed in a three-fold manner in this thesis. Firstly, an investigation of the metal:oxide interface is carried out, the results of which are set out in Chapter 4 of this thesis. Providing a clear understanding of the nanoscale structure and chemistry of this interface, alongside a thorough investigation of the morphology of any suboxide phases generated during the process of corrosion is fundamental to understanding the overall corrosion of this alloy. Secondly, systematic analysis of the corrosion and incorporation of SPPs into the oxide layer is performed in Chapter 5 of this thesis, in order to help inform the role of SPPs on the corrosion process for both autoclave, and more importantly, on irradiated oxides. In addition, spatially resolved chemical mapping, and correlation to the crystallographic structure provides an understanding not previously shown in the literature on the complex corrosion that takes place. Finally, Chapter 6 presents findings from studying oxygen content through the oxide scale, performed in order to quantitatively elucidate the details of the oxygen content from the outer porous oxide, through the stoichiometric ZrO2, and into any metastable suboxide layers present. This analysis highlights the significance of understanding such microstructural behaviour, in order to interpret the overall macro structural corrosion behaviour of zirconium alloys.

The top quark is the most massive fundamental particle of the Standard Model of particle physics and being the only quark which decays before hadronization gives a unique opportunity for studying Quantum Chromodynamics (QCD). This dissertation presents a measurement of differential cross-sections of top quark pairs decaying in a purely leptonic mode as a function of the dilepton invariant mass, transverse momentum, difference in the pseudorapidity and azimuthal angle between two leptons in the same event at 8 TeV proton-proton collision data collected by the ATLAS detector at the Large Hadron Collider. This data corresponds to an integrated luminosity of 20.3 fb−1. Events with top quark pair signatures are selected in the dilepton channel in the ATLAS acceptance (two leptons with pT < 25 GeV and |η| > 2.5). The measured cross-sections are compared to the Standard Model predictions generated using current Monte Carlo generators. All the measurements are found to be in agreement with the SM.