The production of a light Standard Model Higgs boson in association with a top-quark pair at the Large Hadron Collider (LHC) is studied in a simulation of the multipurpose ATLAS experiment. The Higgs boson is assumed to decay into a b\bar{b} pair, and the top-quark pair to decay semi-leptonically. The main background process for this channel is the production of t\bar{t} events, which can be divided into reducible and irreducible components. In the process of generating these components separately, an overlap of events is created through the addition of b-quarks to t\bar{t} events via parton showering in the t\bar{t}X sample. These events are already included in the matrix-element cross-section calculation for the t\bar{t}b\bar{b} sample. A new procedure for the overlap removal is presented in this thesis. Two analyses are studied, where one aims at the full reconstruction of the nal state using a cut-based approach. Recently, this way of reconstructing the t\bar{t}H^{o}(H^{o}\rightarrow b\bar{b}) process has been found to be extremely challenging. The other analysis is based on a new method employing state-of-the-art jet reconstruction and decomposition techniques where the t\bar{t} pair and the Higgs boson are required to have large transverse momenta and can therefore be reconstructed as massive Higgs and top jets. A recent phenomenological study has shown that the t\bar{t}H^{o} process can be recovered as a promising search channel for a low mass Standard Model Higgs boson around 120 GeV using this approach. Finally, to enhance the sensitivity of the t\bar{t}H^{o} channel, a combination of the two analyses is presented.

Uranium is the only element in the periodic table to exhibit a charge-density wave and superconductivity at ambient pressure. The competition between these effects in technologically important high-temperature superconducting systems has come under increasing scrutiny, and uranium offers a model system in which to study the CDW. However, the element is difficult to grow in single-crystals in the bulk. We describe the growth by magnetron sputtering and characterisation of single-crystal epitaxial thin- films of alpha-uranium in the (110) orientation on the Nb(110)/A-plane sapphire buffer layer/substrate system. We use X-ray scattering methods to determine the influence of the thickness of the component layers of the samples on the microstructure, and find that there is a non-trivial dependence of the microstructural state of the uranium layers on both the thickness of the uranium layers themselves and the thickness of the niobium buffer layers upon which they are grown. In particular, the widths of the uranium peaks decrease when the uranium layer thickness is increased, but increase when the buffer layer thickness is increased. An extensive review of the methods for characterisation of thin- film microstructures using X-ray diffraction is given, and several widely used models and interpretations are critically discussed, in particular those given in the many instances in which two-component line shapes are seen in transverse scans from thin- films. We also use X-ray diffraction from high-intensity synchrotron sources to characterise the charge-density wave state in these samples, and discuss the effect of uranium-layer thickness on its characteristics as a function of temperature. Important differences between the CDW seen in bulk uranium and in thin- films are seen and discussed in terms of the microstructure of the films. In particular, no incommensurate{commensurate transition is seen in the films, and a large intensity asymmetry is seen between the 2+2+1± and 2+2-1± CDW satellites. Furthermore, the correlation-length of the CDW is limited in the plane of the film, and dependent on the thickness of the uranium layers.

The world’s most precise half-life measurement of T^2nu_1/2 = [7.02 ± 0.01(stat) ± 0.46(syst)] × 10^18 years has been made for the 2\nu\beta\beta decay of ^100Mo using data from 6.9kg collected with the NEMO3 detector over 1471 days. The 2\nu nuclear matrix element has been extracted using T^2nu_1/2 and is M^2\nu = 0.126 ± 0.004. The 0\nu\beta\beta search yielded a limit on the half-life of T^0\nu_1/2 1/1 > 1.1 × 10^24 years at the 90% CL, corresponding to a limit on the effective Majorona mass of \langle M\nu e\rangle < 0.3 - 1.0eV, one of the most stringent constraints on \langle M\nu e\rangle in the world. Limits on the right-handed currents and Majoron 0\nu\beta\beta modes have also been set. The world’s most stringent bound has been set on the Majoron to neutrino coupling constant of \langle g \chi o \rangle < (0.2 − 0.7) × 10^−4. SuperNEMO is a next generation \beta\beta decay experiment, based on the design and experience of NEMO3. Due to start demonstrator operation in 2013, SuperNEMO aims to achieve a sensitivity of 10^26 years, corresponding to \langle M\nu e\rangle < 50-100meV using ^82Se. An alternative to the baseline calorimeter design was considered, using 2m x 10cm x 2.54cm scintillator bars. An energy resolution of 10% FWHM at 1 MeV and a time resolution of ~ 450ps was achieved for the alternative design. This is an unprecedented energy resolution for a scintillator of this size.

Here the ab-initio R-Matrix method has been used to carry out electron-molecule collision calculations on the the molecules of interstellar interest C3N, C2H & CN, and molecules found in industrial plasma applications SiBr, SiBr2 and NaI. These were carried out using the UK Molecular R-Matrix codes, along with the Quantemol expert system for running these codes. Calculations have also been carried out on electron collisions with atomic oxygen using these codes, with details included about the problems faced in running an atomic calculation with the molecular codes. Calculations on each species include comparison of different models, including staticexchange and close-coupling models (with different size CAS tried), various basis sets, and for some species different initial orbitals. These different initial orbitals were either taken from the codes themselves (for SCF orbitals), or the quantum chemistry program MOLPRO (for natural orbitals), for the latter numerous state averaged orbitals were tried with different weightings in order to produce good target energies for carrying into the scattering calculation. Results for all calculations include scattering observables such as eigenphase sums, elastic and excitation cross-sections, bound anionic states and resonance positions and widths. Also a new theory has been developed for calculating rotational cross-sections which includes the spin angular momentum of the incoming scattering electron, this has been implemented into the already existing code ROTLIN, which can calculate rotational cross-sections using the scattering data from an R-Matrix calculation.

Domain structure in materials is important for their physical properties, technological uses and response to external perturbations. Domains are small regions within a material with a consistent atomic structure that may have different ordering origins or different orientations. Domain structure is present in any material which exhibits super-structure ordering with correlation lengths shorter than the extent of the sample. The domain size is controlled by the strength of the ordering interactions and growth conditions. Domains are present in a huge range of materials from con¬densed matter to biological samples with a structure unique to the individual sample. For domains in crystalline samples with sizes of Angstroms to nanometres X-rays are an ideal probe. Many domain systems exhibit no X-ray amplitude contrast, i.e. all domains attenuate the X-ray beam uniformly, the domain structure is apparent only in the deviation of the phase of the incident X-ray beam. An imaging method is required which is sensitive to these phase differences. Coherent X-ray Diffraction Imaging (CXDI) is a method which exploits the Fourier transform relationship between the sample and its far field diffraction pattern collected at a pixellated detector to iteratively solve the phase problem and reconstruct an amplitude and phase image of the sample. Support based coherent X-ray diffraction methods have been successfully applied to the three dimensional imaging of crystalline structures by collecting the scattering around the sample Bragg peaks in reflection geometry. In general, phase retrieval algorithm constraints require that the sample is isolated within the X-ray beam and as a result these methods have not been successful at imaging extended domain systems. Ptychography is a combined experimental and analysis procedure that can overcome the requirement for the sample to be isolated by collecting a series of diffraction patterns from overlapping regions of the sample. This thesis develops the ptychography algorithms and experimental methods for use in Bragg geometry with the goal of imaging phase domain structures in extended crystalline samples. Bragg coherent diffraction imaging and ptychography methods are reviewed before the adaptations of the experimental method and algorithm for the application of ptychography in Bragg geometry are discussed and detailed. Simulations of ptychography on phase domain structures and a Bragg geometry ptychography X-ray experiment with a specifically designed phase domain test sample confirm that the method is capable of providing accurate quantitative phase information on the domain structure of extended samples. A coherent diffraction experimental setup for ptychography is developed at Diamond beamline I16. Bragg ptychography is applied to the investigation of domain structure in a niobium thin film and anti-phase domain structure in the binary alloy Fe65Al35 and the results of the reconstructions are presented.

The theoretical and experimental study of low-dimensional systems has dominated much of modern condensed matter physics. Such systems present a range of different phenomena which are not observed in more isotropic, three-dimensional materials. This thesis constitutes the study of the excitations of two types of low-dimensional system: the magnetic excitations in the one-dimensional S = 1/2 antiferromagnetic chain compound Sr2CuO3 as studied using inelastic neutron scattering, and the phonons in quasi-two-dimensional graphite intercalation compounds CaC6 and BaC6 studied using inelastic X-ray scattering. Initially an introduction to the vast field of low-dimensional systems is provided, followed by a description of the neutron and X-ray scattering technique. Spinon excitations in Sr2CuO3 have been measured up to ≈ 600 meV, and are found to be well-described by the Heisenberg model. A new version of the magnetic form factor for Cu2+ in Sr2CuO3 provides an explanation for the considerable reduction in the measured inelastic neutron scattering intensity, which may have significant consequences for past and future work on other related materials. The phonons in superconducting CaC6 (Tc = 11.5 K) are found to be in reasonable agreement with Density Functional Perturbation Theory calculations, but the measured dispersions in non-superconducting BaC6 are significantly different to those predicted. No direct evidence for electron-phonon coupling is found in either compound. It is concluded that the X-ray data supports the validity of the theoretical description of CaC6, but the use of significantly inaccurate lattice parameters for BaC6 in the calculation may provide an explanation of the poor agreement in this case.

The production of beauty quarks in ep collisions should be accurately calculable in perturbative Quantum Chromodynamics (QCD) since the large mass of the b quark provides a hard scale. Therefore it is interesting to compare such predictions to results using photoproduction events where a low-virtuality photon, emitted by the incoming lepton, collides with a parton from the incoming proton. A measurement of beauty in photoproduction has been made at HERA with the ZEUS detector using an integrated luminosity of 126 pb−1. Beauty was identified in events with a muon in the final state by using the transverse momentum of the muon relative to the closest jet. Lifetime information from the silicon vertex detector was also used; the impact parameter of the muon with respect to the primary vertex was exploited to discriminate between signal and background. Cross sections for beauty production as a function of the muon and the jet variables were measured and compared to QCD predictions and to previous measurements. The data were found to be well described by the predictions from next-to-leading-order QCD. The dijet sample of beauty photoproduction events was also used to study higher-order QCD topologies. At leading order, the two jets in the event are produced back-to-back in azimuthal angle, such that \Delta\phi^j^j=\phi^j^1-\phi^j^2=\pi. Additional soft radiation causes small azimuthal decorrelations, whilst \Delta\phi^j^j significantly lower than \pi is evidence of additional hard radiation. In this thesis, the cross section versus \Delta\phi^j^j for beauty photoproduction and the comparison to NLO QCD predictions and Monte Carlo models are presented.

An understanding of Blue Compact Dwarf galaxies (BCDs) and the processes occurring within their chemically un-evolved environments is fundamental in our understanding of the early universe. This thesis presents an investigation into their physical conditions, kinematics, chemical abundances and dust compositions. An optical integral field spectroscopy investigation of two perturbed BCDs, UM420 and UM462, is presented. Emission line maps show that both galaxies display signs of ongoing perturbation and/or interaction. Electron temperatures, densities and chemical abundances are computed from spectra integrated over the whole galaxies and for each area of star formation. A similar yet more complicated analysis is undertaken of the BCD Mrk996, which displays multi-component emission lines. The high excitation energy [O III] \lambda4363 and [N II] \lambda5755 lines are detected only in the inner regions and purely in broad component form, implying unusual excitation conditions. A separate physical analysis of the broad and narrow emission line regions is undertaken, yielding a revised metallicity and N/O ratio typical for the galaxy’s metallicity. The mid-IR properties of 19 BCDs are studied through Spitzer spectral and imaging data. The depeletion of PAH emission in BCDs is investigated and found to be due to formation and destruction effects. The [S III] flux ratio is used as a density diagnostic, showing typically low-densities. Maps of PAH emission and radiation field hardness are derived from IRS spectral mapping data. Blackbody fits to IR photometric SEDs typically reveal two dust components. The observed physical and chemical properties of Mrk 996 are successfully reproduced using the photoionisation code MOCASSIN. The best-fit model involved the inclusion of a filling factor and an amorphous carbon dust component with a two-zone dust distribution. A STARBURST99 input spectrum was used, yielding ages consistent with the known young WR stars and old super star clusters within Mrk 996.

Place cells in the hippocampal formation form the cornerstone of the rat’s navigational system and together with head direction cells in the postsubiculum and grid cells in the entorhinal cortex are the key elements of what O’Keefe and Nadel (1978) postulate to be a “cognitive map”. The hippocampal formation is ideally positioned anatomically to receive highly processed inputs from almost all brain regions. Previous research has focused on the cues that determine and contribute to place cell selectivity. Such cues include information about the external world that the rat perceives through its senses (“exteroceptive cues”) as well as cues internal to the body such as proprioception or somatosensation (“interoceptive cues”). This thesis uses a novel experimental paradigm in which the rat runs on a moving-treadmill linear track to investigate the relative contribution of interoceptive and exteroceptive cues for determining place cell spatial selectivity. The major finding is that place fields shift in the direction of the moving treadmill, both when the animal runs along with or against the motion of the treadmill, indicating that self-motion information is a key input to place cells. Furthermore, place fields in the middle of the track shift more than fields closer to the end walls suggesting that exteroceptive information interacts with interoceptive information to assist in accurate navigation. This conclusion is further supported by experiments performed in complete darkness where two populations of cells are observed: the first are cells which become quiescent or remap, presumably under strong exteroceptive control, while the second are cells that maintain similar firing characteristics under both lighting conditions, putatively under the influence of interoceptive inputs.

This thesis describes the study of the channel t \overline{t} H^{0}(H^{0}\rightarrowb\overline{b} with the ATLAS detector with 30fb^-1 of data and a center of mass energy of 10 TeV. Chapter 1 provides a description of the ATLAS detector, followed by a theoretical background in Chapter 2 and a discussion of phenomenology and event generation in Chapter 3. Issues associated with leptons and missing energy are presented in Chapter 4, with focus on optimising the preselection cuts to reduce the rate of background processes, including those previously unconsidered for this channel but found to be important as a consequence of this study. In addition, the reconstruction of the leptonically decaying W Boson from lepton and missing energy is described. The treatment of jets is introduced in Chapter 5, with the focus being again on the optimisation of preselection cuts. Studies presented here are on corrections for energy lost via both muons and neutrinos in semi-leptonic bdecays and preselection cuts based on the transverse momenta and b-weights of individual jets. The issues associated with combinatorial background and the use of jet charge to reduce it is also introduced here. The choice of jet algorithm is considered of great importance for this channel, thus is presented in detail in Chapter 6. Chapter 7 explores the reconstruction of the Higgs Boson from jet pairs, focusing on the segregation of jets by b-weight. The reconstruction of the t \overline{t} H^{0} system is studied with various techniques; an investigation of the use of jet charge to discriminate between b and \overline{b} jets is presented as a novel likelihood variable.Chapter 8 summarises the results obtained using the optimised preselection, jet algorithm and jet charge method. Systematic uncertainties are discussed throughout the thesis where relevant and also summarised.