This book discusses searches for Dark Matter at the CERN’s LHC, the world’s most powerful accelerator. It introduces the relevant theoretical framework and includes an in-depth discussion of the Effective Field Theory approach to Dark Matter production and its validity, as well as an overview of the formalism of Simplified Dark Matter models. Despite overwhelming astrophysical evidence for Dark Matter and numerous experimental efforts to detect it, the nature of Dark Matter still remains a mystery and has become one of the hottest research topics in fundamental physics. Two searches for Dark Matter are presented, performed on data collected with the ATLAS experiment. They analyze missing-energy final states with a jet or with top quarks. The analyses are explained in detail, and the outcomes and their interpretations are discussed, also in view of the precedent analysis of theoretical approaches. Given its depth of coverage, the book represents an excellent reference guide for all physicists interested in understanding the theoretical and experimental considerations relevant to Dark Matter searches at the LHC.

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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 thesis describes in detail a search for weakly interacting massive particles as possible dark matter candidates, making use of so-called mono-jet events. It includes a detailed description of the run-1 system, important operational challenges, and the upgrade for run-2. The nature of dark matter, which accounts for roughly 25% of the energy-matter content of the universe, is one of the biggest open questions in fundamental science. The analysis is based on the full set of proton-proton collisions collected by the ATLAS experiment at the Large Hadron Collider at √s = 8 TeV. Special attention is given to the experimental challenges and analysis techniques, as well as the overall scientific context beyond particle physics. The results complement those of non-collider experiments and yield some of the strongest exclusion bounds on parameters of dark matter models by the end of the Large Hadron Collider run-1. Details of the upgrade of the ATLAS Central Trigger for run-2 are also included.

This book introduces the reader to the field of jet substructure, starting from the basic considerations for capturing decays of boosted particles in individual jets, to explaining state-of-the-art techniques. Jet substructure methods have become ubiquitous in data analyses at the LHC, with diverse applications stemming from the abundance of jets in proton-proton collisions, the presence of pileup and multiple interactions, and the need to reconstruct and identify decays of highly-Lorentz boosted particles. The last decade has seen a vast increase in our knowledge of all aspects of the field, with a proliferation of new jet substructure algorithms, calculations and measurements which are presented in this book. Recent developments and algorithms are described and put into the larger experimental context. Their usefulness and application are shown in many demonstrative examples and the phenomenological and experimental effects influencing their performance are discussed. A comprehensive overview is given of measurements and searches for new phenomena performed by the ATLAS and CMS Collaborations. This book shows the impressive versatility of jet substructure methods at the LHC.

This thesis describes in detail the search for new phenomena in mono-jet final states with the ATLAS experiment at the LHC. The final state is considered the golden channel in the searches for large extra dimensions (LED) but also allows access to a very rich SUSY-related phenomenology pertaining to the production of weakly interacting massive particles (WIMPS), SUSY Dark Matter candidates, GMSB SUSY models with very light gravitino masses, as well as stop an sbottom pair production in compressed scenarios (with nearly degenerated squarks and the lightest neutralino), and also invisible Higgs searches, among others. Here, a number of these scenarios are explored. The measurements presented yield new powerful constraints on the existence of extra spatial dimensions, the pair production of WIMPs, and also provide the best limit to date on the gravitino mass.

Yi-Shi Duan (1927-2016) was one of the world-renowned pioneers in the study of gauge field theory and general relativity. Trained in the former Soviet Union, Prof. Duan returned to China in 1957 to work in Lanzhou University for 60 years. In 1963, he came up with a general co-variant form of the conservation law of the energy-momentum tensor in general relativity. In 1979, he suggested that the gauge potential could be decomposed, which has important implications to gauge field theory. He trained in China a big team of talents in theoretical physics. His contributions to theoretical physics in China have earned him praise from both Professor Shiing-Shen Chern and Professor Chen-Ning Yang.