This thesis describes searches for new particles predicted by the super symmetry (SUSY) theory, a theory extending beyond the current Standard Model of particle physics, using the ATLAS detector at the CERN Large Hadron Collider. The thesis focuses on searches for stop and sbottom squarks, the SUSY partners of the top and bottom quarks, which are expected to be lighter than the partners of the first and second generation quarks and therefore good candidates for the first evidence of SUSY. It describes novel techniques for estimating and rejecting the Standard-Model backgrounds to searches for these particles. It also includes an independent analysis seeking to constrain the Standard Model ttZ background process, which also represents the first ATLAS search for this rare process at the LHC. The stop squark analysis described, with substantial leading contributions from the author, is the first search for these particles at the LHC to use the jets plus missing transverse energy plus 0-lepton signature and provides the world's best limits on the stop mass for light neutralino LSPs. All in all, the thesis describes three different world-leading analyses in both Standard Model and SUSY physics and therefore represents a major contribution to the field.

This thesis presents a search for long-lived particles decaying into displaced electrons and/or muons with large impact parameters. This signature provides unique sensitivity to the production of theoretical lepton-partners, sleptons. These particles are a feature of supersymmetric theories, which seek to address unanswered questions in nature. The signature searched for in this thesis is difficult to identify, and in fact, this is the first time it has been probed at the Large Hadron Collider (LHC). It covers a long-standing gap in coverage of possible new physics signatures. This thesis describes the special reconstruction and identification algorithms used to select leptons with large impact parameters and the details of the background estimation. The results are consistent with background, so limits on slepton masses and lifetimes in this model are calculated at 95% CL, drastically improving on the previous best limits from the Large Electron Positron Collider (LEP).

This thesis describes the search for Dark Matter at the LHC in the mono-jet plus missing transverse momentum final state, using the full dataset recorded in 2012 by the ATLAS Experiment. It is the first time that the number of jets is not explicitly restricted to one or two, thus increasing the sensitivity to new signals. Instead, a balance between the most energetic jet and the missing transverse momentum is required, thus selecting mono-jet-like final states. Collider searches for Dark Matter have typically used signal models employing effective field theories (EFTs), even when comparing to results from direct and indirect detection experiments, where the difference in energy scale renders many such comparisons invalid. The thesis features the first robust and comprehensive treatment of the validity of EFTs in collider searches, and provides a means by which the different classifications of Dark Matter experiments can be compared on a sound and fair basis.

This annual SUSY conference has become the world's largest international meeting devolted to new ideas in high energy physics. The main subject of the conference is theoretical and phenomenological aspects of supersymmetric theories, and dark matter and dark energy, and other comological connections. New, interesting results from various experimental groups are increasingly presented at the conference as well. With roughly 200 plenary and parallel presentations, SUSY08 will likely deliver energy and enthusiasm of both theorists and experimentalists who are searching the frontier of high energy physics.

This thesis focuses on searches for squarks with the ATLAS detector in "compressed" scenarios where the scalar top is very close in mass to the lightest supersymmetric particle. These models are theoretically appealing because the presence of a quasi-degenerate scalar top enhances the self-annihilation cross-section of the lightest supersymmetric particle, acting therefore as a regulator of the dark matter relic density. Two main analyses are presented: the first is a search for scalar tops decaying to charm quarks. The identification of jets originating from the charm quark is very challenging due to its short lifetime. The calibration of tools for charm-tagging has paved the way to measuring the decay of the Higgs boson to pairs of charm quarks. The second analysis presented is the development of a novel technique for reconstructing low momentum b-hadrons. This tool has enabled the ATLAS collaboration to explore topologies that were previously inaccessible.

This OA text develops the basic concepts of supersymmetry for experimental and phenomenological particle physicists and graduate students.

This first open access volume of the handbook series contains articles on the standard model of particle physics, both from the theoretical and experimental perspective. It also covers related topics, such as heavy-ion physics, neutrino physics and searches for new physics beyond the standard model. A joint CERN-Springer initiative, the "Particle Physics Reference Library" provides revised and updated contributions based on previously published material in the well-known Landolt-Boernstein series on particle physics, accelerators and detectors (volumes 21A, B1,B2,C), which took stock of the field approximately one decade ago. Central to this new initiative is publication under full open access