Future Colliders

Abstract:

A summary of the constraints from searches performed by the ATLAS Collaboration for the electroweak production of charginos and neutralinos is presented. Results from eight separate ATLAS searches are considered, each using 140 fb⁻¹ of proton–proton data at a centre-of-mass energy of √𝑠 = 13 TeV collected at the Large Hadron Collider during its second data-taking run. The results are interpreted in the context of the 19-parameter phenomenological minimal supersymmetric standard model, where 𝑅-parity conservation is assumed and the lightest supersymmetric particle is assumed to be the lightest neutralino. Constraints from previous electroweak, flavour and dark matter related measurements are also considered. The results are presented in terms of constraints on supersymmetric particle masses and are compared with limits from simplified models. Also shown is the impact of ATLAS searches on parameters such as the dark matter relic density and the spin-dependent and spin-independent scattering cross-sections targeted by direct dark matter detection experiments. The Higgs boson and 𝑍 boson ‘funnel regions’, where a low-mass neutralino would not oversaturate the dark matter relic abundance, are almost completely excluded by the considered constraints. Example spectra for non-excluded supersymmetric models with light charginos and neutralinos are also presented.

Abstract:

A liquid scintillator consisting of linear alkylbenzene as the solvent and 2,5-diphenyloxazole as the fluor was developed for the SNO+ experiment. This mixture was chosen as it is compatible with acrylic and has a competitive light yield to pre-existing liquid scintillators while conferring other advantages including longer attenuation lengths, superior safety characteristics, chemical simplicity, ease of handling, and logistical availability. Its properties have been extensively characterized and are presented here. This liquid scintillator is now used in several neutrino physics experiments in addition to SNO+.

Abstract:

A liquid scintillator consisting of linear alkylbenzene as the solvent and 2,5-diphenyloxazole as the fluor was developed for the SNO+ experiment. This mixture was chosen as it is compatible with acrylic and has a competitive light yield to pre-existing liquid scintillators while conferring other advantages including longer attenuation lengths, superior safety characteristics, chemical simplicity, ease of handling, and logistical availability. Its properties have been extensively characterized and are presented here. This liquid scintillator is now used in several neutrino physics experiments in addition to SNO+.

Abstract:

Designing high-intensity accelerators has traditionally relied on using computer simulations to study the beam dynamics. As intense beams are comprised of large numbers of particles, all interacting via Coulomb forces, such simulations require significant computational power in order to numerically predict these interactions. The Intense Beams Experiment (IBEX) is a linear Paul trap that can replicate the transverse beam dynamics in accelerators by trapping low-energy ions using RF electric fields that emulate the magnetic focusing elements of particle accelerators. IBEX’s flexibility allows different lattice designs and beam intensities to be tested with ease, which means that it can be used to test novel lattice configurations for high-intensity accelerators. Examples of such lattices arise from the theory of Nonlinear Integrable Optics, and, as discussed in this thesis, the related theory of Quasi-Integrable Optics (QIO). These theories suggest techniques for introducing nonlinear elements such as octupoles into an accelerator lattice, while keeping the system integrable and hence maintaining stable particle motion.

In this work, an upgrade to the original IBEX trap was designed, manufactured, and commissioned with the aim of experimentally testing the principles of QIO. Simulations were used to test the ability of a quasi-integrable lattice to damp a space-charge-driven coherent resonance without exciting the 4th order incoherent resonance in the vicinity. This lattice was then compared to a lattice which broke the integrability conditions, which was shown to excite the 4th order resonance. Using the newly-commissioned IBEX-2 trap, we were then able to test the quasi-integrable lattice experimentally and verify the results from simulations. This thesis demonstrates the first ions successfully trapped in a quasi-integrable lattice in a Paul trap, and discusses the benefits of introducing octupole elements according to the method prescribed by the theory of QIO. The experimental results presented here show the potential value of QIO to research on high-intensity beams in accelerators.

Abstract:

Recent studies have highlighted the potential of jet substructure techniques to identify the hadronic decays of boosted heavy particles. These studies all rely upon the assumption that the internal substructure of jets generated by QCD radiation is well understood. In this article, this assumption is tested on an inclusive sample of jets recorded with the ATLAS detector in 2010, which corresponds to 35 pb^-1 of pp collisions delivered by the LHC at sqrt(s) = 7 TeV. In a subsample of events with single pp collisions, measurementes corrected for detector efficiency and resolution are presented with full systematic uncertainties. Jet invariant mass, kt splitting scales and n-subjettiness variables are presented for anti-kt R = 1.0 jets and Cambridge-Aachen R = 1.2 jets. Jet invariant-mass spectra for Cambridge-Aachen R = 1.2 jets after a splitting and filtering procedure are also presented. Leading-order parton-shower Monte Carlo predictions for these variables are found to be broadly in agreement with data. The dependence of mean jet mass on additional pp interactions is also explored.