This thesis is an investigation into the current bounds on the trilinear R–parity–violating couplings in the Minimal Supersymmetric Standard Model without R–parity conservation. The model is described, and its implications are discussed. Bounds on the couplings are obtained from leptonic and mesonic decay data, approximating mediating sfermions as much heavier than the decaying particles and assuming that only one set of couplings is non–zero for each decay. Those bounds from the purely leptonic decay data are compared to bounds from the LEP–II data, over a large range of sfermion masses. A potential signal of R–parity–violation at existing lepton colliders is calculated assuming that certain couplings are close to their bounds. The signal is found to be feasible and the backgrounds to the process are found to be negligible.

The switching on of the Large Hadron Collider (LHC) in March 2010 and its successful operation thereafter has opened doors in the search for new physics beyond the Standard Model. Supersymmetry (SUSY) is one of the leading theories that extends the Standard Model of particle physics. A search for new SUSY particles is presented requiring large numbers of hadronic jets and missing transverse momentum. Novel background estimation techniques were developed specifically for this final state, allowing the control of the backgrounds where the missing transverse momentum is dominated by jet miss-measurement. Other backgrounds are estimated from Monte-Carlo simulations validated and normalised in dedicated control regions. No significant excess was observed in the search. Model independent upper limits on the number of signal events passing the selection are given and the results are also interpreted in two planes of the parameter space. This is the most recent and sensitive incarnation of several searches developed by the author in this channel.

The SNO+ experiment will use a liquid scintillator based detector to study solar, geo, and reactor neutrinos and double beta decay. This thesis discusses the effect of backgrounds on the measurement of neutrinoless double beta decay and describes analysis techniques developed to reduce their impact. Details of the modeling of the photomultiplier tubes in the SNO+ Monte Carlo RAT are first described and comparisons are made with the SNO Monte Carlo SNOMAN. SNOMAN has been extensively verified with calibration sources and RAT is shown to be in good agreement. The event reconstruction techniques are then presented and predict an achievable 15cm position and 7% energy resolution. The backgrounds are discussed and pileup backgrounds identified, including many previous unknown pileup backgrounds. Techniques to reject the pileup background are presented and shown to give over 99% rejection in the region of the double beta decay end point (3-4MeV), below the irreducible background from solar neutrinos. Finally the resulting limit on the effective Majorana neutrino mass SNO+ could achieve in 2015 is predicted to be 270meV and this is compared with other experiments that are underway.

MINOS is a long-baseline neutrino oscillation experiment. It consists of two large steel-scintillator tracking calorimeters. The near detector is situated at Fermilab, close to the production point of the NuMI muon-neutrino beam. The far detector is 735 km away, 716 m underground in the Soudan mine, Northern Minnesota. The primary purpose of the MINOS experiment is to make precise measurements of the "atmospheric" neutrino oscillation parameters (Δm²_atm and sin²2θ_atm). The oscillation signal consists of an energy-dependent deficit of v_μ interactions in the far detector. The near detector is used to characterize the properties of the beam before oscillations develop. The two-detector design allows many potential sources of systematic error in the far detector to be mitigated by the near detector observations. This thesis describes the details of the v_μ-disappearance analysis, and presents a new technique to estimate the hadronic energy of neutrino interactions. This estimator achieves a significant improvement in the energy resolution of the neutrino spectrum, and in the sensitivity of the neutrino oscillation fit. The systematic uncertainty on the hadronic energy scale was re-evaluated and found to be comparable to that of the energy estimator previously in use. The best-fit oscillation parameters of the v_μ-disappearance analysis, incorporating this new estimator were: Δm² = 2.32^+0.12_-0.08 x 10^-3eV² sin²2θ > 0.90 (90% C.L.). A similar analysis, using data from a period of running where the NuMI beam was operated in a configuration producing a predominantly V‾_μ beam, yielded somewhat different best-fit parameters Δ‾m² = (3.36^+0.46_-0.40 (stat.}) ± 0.06(syst.)) x 10^-3eV², sin²2‾θ =0.86^+0.11_-0.12(stat.) ± 0.01(syst.). The tension between these results is intriguing, and additional antineutrino data is currently being taken in order to further investigate this apparent discrepancy.

We present original calculations of standard model processes involving two weak bosons at NLO in QCD, and study related phenomenology with reference to Higgs boson and new physics searches at the LHC. We employ a new theoretical technique, D-dimensional generalised unitarity, to obtain the multi-particle, one-loop scattering amplitudes for the processes pp → W⁺W^- + 1j, pp → W⁺W^- + 2j, pp → W⁺W⁺ + 2j, and gg → W⁺W^-g. We consider the LHC phenomenology of: the W⁺W^- + n jets background to Higgs searches, for n = 0,1,2; the effects of anomalous tri-linear gauge couplings in WW and WZ production; the background W⁺W⁺ + 2j to new physics searches involving like-sign leptons; and a method of spin determination of new states in a scenario where conventional methods fail.

Low scale supersymmetry (SUSY) is a compelling solution to the electroweak hierarchy problem. However, increasingly strong limits on the masses of superpartners, first from LEP and now the LHC, mean that the simplest models require significant fine tuning. This thesis is dedicated to the study of a possible alternative low energy superpartner spectrum, natural SUSY, in which only superparticles directly involved in stabilising the electroweak scale are light, alleviating collider limits and potentially reducing tuning. After reviewing how low scale SUSY is motivated by the hierarchy problem, we build a model of SUSY breaking and mediation that successfully generates a natural SUSY spectrum. This also suppresses the first two generation fermion Yukawas, and leads to small parameters in the hidden sector, which are required for successful SUSY breaking. A challenge in models of natural SUSY is raising the physical Higgs mass to 125 GeV, and we study the possibility that this could occur through the addition of a singlet to the theory. If stops are very light, the coupling of the singlet to the Higgs needs to be so large that it becomes nonperturbative before the scale of grand unification, raising the concern that precision gauge coupling unification may be upset. However, we find that this is not necessarily the case. Rather it is possible this could correct for the present ∽ 3% discrepancy in the two-loop minimal supersymmetric model's unification prediction. We then turn to the fine tuning in models of natural SUSY, emphasising that this should be measured with respect to the theory's ultraviolet (UV) parameters. We show that the first two generation sfermions can be made relatively heavy, beyond LHC reach, without introducing tuning. However, the gluino generates a significant tuning through the stops during the renormalisation group flow. As a result, there is no fine tuning benefit in reducing the stop masses below (50 - 75)% of the weak scale gluino mass, and we obtain strong lower bounds on the tuning of theories compatible with collider limits. We also study theories with Dirac gauginos, which have relatively low fine tuning even if the scale of mediation is high. Finally, we consider the effect of relaxing a common assumption and allowing the hidden SUSY breaking sector to modify the running of the visible sector soft masses. This may plausibly occur in realistic models and could dramatically reduce the fine tuning of SUSY theories.

The observed constraints on the variability of the proton to electron mass ratio and the fine structure constant are used to establish constraints on the variability of the Quantum Chromodynamic Scale and a combination of the Higgs Vacuum Expectation Value and the Yukawa couplings. Further model dependent assumptions provide constraints on the Higgs VEV and the Yukawa couplings separately. A primary conclusion is that limits on the variability of dimensionless fundamental constants such as and provide important constraints on the parameter space of new physics and cosmologies.

It is likely that the early Universe was pervaded by a whole host of scalar fields which are ubiquitous in particle physics models and are employed everywhere from driving periods of accelerated expansion to the spontaneous breaking of gauge symmetries. Just as these scalar fields are important from a particle physics point of view, they can also have serious implications for the evolution of the Universe. In particular in extreme cases their dynamical evolution can lead to the failure of the synthesis of light elements or to exceed the dark matter bound in contrast to observation. These scalar fields are not however isolated systems and interact with the degrees of freedom which comprise their environment. As such two interrelated effects may arise; fluctuations and dissipation. These effects, which are enhanced at finite temperature, give rise to energy transfer between the scalar field and its environment and as such should be taken into account for a complete description of their dynamical evolution. In this thesis we will look at these effects within the inflationary era in a scenario termed warm inflation where amongst other effects, thermal fluctuations can now act as a source of primordial density perturbations. In particular we will show how a model of warm inflation based on a simple quartic potential can be brought back into agreement with Planck data through renormalizable interactions, whilst it is strongly disfavoured in the absence of such effects. Moving beyond inflation, we will consider the effect of fluctuation-dissipation dynamics on other cosmological scalar fields, deriving dissipation coefficients within common particle physics models. We also investigate how dissipation can affect cosmological phase transitions, potentially leading to late time periods of accelerated expansion, as well as presenting a novel model of dissipative leptogenesis.

The heating of solar chromospheric internetwork regions is investigated by means of the absorption of electromagnetic (EM) waves that originate from the photospheric black body radiation. It is studied in the framework of a plasma slab model. The absorption is provided by the electron-neutral collisions in which electrons oscillate in the EM wave field and electron-neutral collisions damp the EM wave. It is shown that for plausible physical parameters, the absorbed heating flux is between 20% and 45% of the chromospheric radiative loss flux requirement. Further, 1.5D particle-in-cell simulations of a hot, low density electron beam injected into magnetized, Maxwellian plasma were used to further explore the alternative non-gyrotropic beam driven EM emission mechanism, which was first studied in Ref.[83]. Variation of beam injection angle and background density gradient showed that the emission is caused by the perpendicular component of the beam injection current, whereas the parallel component only produces Langmuir waves, which play no role in the generation of EM waves in our mechanism. When the beam is injected perpendicularly to the background magnetic field, any electrostatic wave generation is turned off and a purely EM signal is left. Finally, a possible solution to the unexplained high intensity hard x-ray emission observable during solar flares was investigated via 3D particle-in-cell simulations. A beam of accelerated electrons was injected into a magnetised, Maxwellian, homogeneous and inhomogeneous background plasma. The electron distribution function was unstable to the beam-plasma instability and was shown to generate Langmuir waves, while relaxing to plateau formation. Three different background plasma density gradients were investigated. The strong gradient case produced the largest fraction of electrons beyond 15vth. Further, Langmuir wave power is shown to drift to smaller wavenumbers, as found in previous quasi-linear simulations.

An angular analysis of B⁰d → K⁰(→K⁺π⁻)μ⁺μ⁻ is presented using 20.3 fb⁻¹ of pp collision data collected at √s = 8 TeV with the ATLAS experiment. The angular observables FL and S₃,₄,₅,₇,₈ were extracted in six bins of q², the invariant mass squared of the dimuon system, within the full range 0.04 < q² < 6.00 (GeV/c²)². The observables were determined from an unbinned maximum likelihood fit using four folded parameterisations of the full angular distribution. The fit results were used to obtain corresponding values for the optimised observables P₁ and P'₄,₅,₆,₈. The results presented are compatible with Standard Model predictions.