This thesis presents a search for resonant and standard model non-resonant di-Higgs production in theb¯bτ +τ− decay channel, with the semi-leptonic ditau state. The search uses data collected by the ATLAS experiment at the LHC of pp-collisions at √s=13 TeV during 2015 and 2016, corresponding to an instantaneous luminosity of 36:1fb-1. No deviation from the Standard Model prediction is observed. For the non-resonant standard model di-Higgs production an upper limit is set on the cross-section compared to the Standard Model prediction,σ/σSM< 24:44 @ 95%CL. Upper limits are set on the cross-section times branching ratio for the resonant searches, where the results are interpreted in terms of constraints on a 2HDM heavy scalar Higgs model and a Randall-Sundrum Kaluza-Klein graviton model. The use of Support Vector Machines is investigated and benchmarked against the nominal results of the Boosted Decision Tree multivariate analysis, in pursuit of potential future improvements.

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Colanero Klaus John Charles. / Thesis (Ph.D.)--Chinese University of Hong Kong, 2001. / Includes bibliographical references (p. 109-112). / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Mode of access: World Wide Web. / Abstracts in English and Chinese.

This thesis presents searches for supersymmetry in √s=7 TeV and 8 TeV proton-proton collisions at the LHC using data collected by the ATLAS detector in 2011 and 2012. Events with exactly three electrons or muons and missing transverse energy were selected, since these can be a striking signature of supersymmetry with low Standard Model background. Additional kinematic criteria were investigated and applied in order to increase the sensitivity to supersymmetric signals. Standard Model backgrounds were estimated by a combination of Monte Carlo simulations and data-driven techniques. The agreement between data and background predictions was validated before analysing data in the signal regions. No significant excess was recorded in any of the analyses performed: the results were therefore interpreted as lower bounds on the masses of supersymmetric particles in a variety of different model scenarios.

The Sudbury Neutrino Observatory solved the solar neutrino problem, confirming neutri- nos have non-zero masses. Massive neutrinos raise questions about the nature of neutrinos, implying physics beyond the standard model and potentially a solution to the observed matter-antimatter asymmetry in the universe. The Sudbury Neutrino Observatory is be- ing upgraded, with the goal of probing the nature of neutrino masses. The experiment will also study reactor, geo, supernovae and solar neutrinos. The upgrade is characterised by changing the target mass from heavy water to scintillator. Using scintillator allows for the lowering of the energy threshold, but this increases sensitivity to backgrounds. To meet the requirements of the physics on detector performance, a detailed optical calibration is needed. Due to increased background sensitivity, a new external LED-based calibration system has been developed and the existing laser calibration system has been modified to meet radiopurity requirements. This thesis describes the the development and imple- mentation of both of these calibration systems. With a study of the potential use of the LED system to monitor the detector's structure, enabling a better definition of the fiducial volume by reducing the effects of external backgrounds. An assessment of the impact of these systems on the detector performance will be presented.

Two searches for events containing τ-leptons, jets, large missing transverse momentum, and zero or one light leptons (ℓ = e,μ) in the final state have been performed using proton-proton collision data at √s = 7 TeV, recorded with the ATLAS detector at the Large Hadron Collider. The first search was performed using 2:05fb-1 of the ATLAS data collected in 2011, and focused on the final state containing two hadronically decaying τ-leptons. For the second search the dataset was extended to 4:7fb-1, and four separate channels (1τ, 2τ, τ+e, τ+μ) were combined for the final result. No excess above the Standard Model background expectation is observed and 95% CL visible cross-section upper limits for new phenomena are set. In the framework of gauge-mediated SUSY-breaking models (GMSB), exclusion limits on the GMSB mass scale Λ are set at 54 TeV in the regions where the lightest ~τ is the next-to-lightest SUSY particle (tan β > 20). These limits provide the most stringent tests to date of GMSB models in a large part of the considered parameter space.

The fascinating idea that in higher-dimensional models the fundamental scale of gravity, the Planck scale, could be as low as the electroweak scale has stimulated a substantial body of work in the past decade. In addition to solving the hierarchy problem, a low quantum gravity scale also o↵ers the exciting prospect that collider experiments become sensitive to the quantum nature of gravity. Quantum gravity signatures include missing energy due to graviton emission, enhancement of standard model reference processes via virtual graviton exchange, or the production and decay of mini black holes. Dedicated searches for all of these are presently under way at the Large Hadron Collider. Previous predictions for colliders have been encumbered by the absence of a complete theory of quantum gravity. However, the recent years have also seen important progress in the understanding of gravity as an asymptotically safe quantum field theory, in which the high-energy behaviour is controlled by an interacting fixed point. The notorious divergences of perturbation theory are thus avoided, and the theory remains predictive at arbitrarily high energies. In this thesis, we investigate the effects of asymptotic safety upon predictions for graviton-mediated processes in higher-dimensions at colliders. We consider single-graviton mediated effects in the Born approximation as well as the multi-graviton exchanges which dominate the forward scattering region at transplanckian energies, as described by the eikonal approximation. Cross sections are derived and a detailed comparison with findings from effective theory is made. Using the PYTHIA event generator we find that for some regions in parameter space quantum gravity effects are enhanced over the semiclassical predictions, as well as over standard model backgrounds. The use of our results to constrain our theory parameters is discussed.

The NOvA experiment consists of two functionally identical tracking calorimeter detectors which measure the neutrino energy and flavour composition of the NuMI beam at baselines of 1 km and 810 km. Measurements of neutrino oscillation parameters are extracted by comparing the neutrino energy spectrum in the far detector with predictions of the oscillated neutrino energy spectra that are made using information extracted from the near detector.

This thesis is concerned with strongly coupled extensions to the Standard Model. The majority of the thesis is dedicated to the study of Composite Higgs models, which are a proposed solution to the hierarchy problem of the electroweak scale. In these models the Higgs is a composite pseudo-Nambu Goldstone boson which forms a part of a new strongly interacting sector. There are many different variations on the basic Composite Higgs theme { the current status of some of these variations is assessed in light of results from the Large Hadron Collider. A new kind of Composite Higgs model is presented and studied, which features an alternative mechanism for the breaking of electroweak symmetry. A mechanism for deforming one model into another is also discussed, which might find application to the UV completion of Composite Higgs models. The formalism used in the Composite Higgs literature is also applied to the study of inflation, where the inflaton is assumed to be a pseudo-Nambu Goldstone boson arising from strongly coupled dynamics. A study of the inflaton potential is performed and its cosmological implications discussed. A different extension to the Standard Model with interesting phenomenological consequences is also studied. Quirks are strongly interacting particles whose masses are significantly higher than their confining scale. If produced in colliders, they leave unusual tracks which current searches are mostly blind to. A new search strategy for these hypothetical particles is proposed.

The provision of a temporally stable and spatially uniform magnetic field is a precondition for the Cryo-nEDM experiment to conduct a successful measurement. These two aspects and some further data analysis are largely the subjects of my thesis. I propose a technique to improve the current dynamic magnetic shielding of the existing apparatus by more than 2 orders of magnitude without distorting the homogeneity of the magnetic field more than the limitations set on the proposal. By testing a 12.5 th scale model of the apparatus I have shown that the placement of a 1m long superconducting shield inside the solenoid can improve the magnetic shielding by at least a factor of 500. Magnetostatic simulations have been carried out for the full model of the experimental apparatus to investigate the effect of various parts to the magnetic field configuration over the neutron guides and the storage bottles. This model can be considered as a basis on which further additions can be made if needed. The actual response of the 21 compensation coils has been measured experimentally. This information was used to develop a systematic method to calculate the optimum currents for these coils to smooth the magnetic field inhomogeneities in the area of the storage cells of neutrons. Applying this method to the existing apparatus, it has been estimated that we can increase the T2 relaxation time from 2 seconds to more than 20 seconds. Finally, I have analysed the data taken over winter 2010 run in terms of neutrons polarisation. As a result, very useful information was extracted for the issues that have to be resolved and taken into account in future runs to improve the polarisation of neutrons and therefore the sensitivity of the experiment.