Wing Sheung Chan

Chapter 2. The Large Hadron Collider and the ATLAS detector Experiment is the only means of knowledge at our disposal. Everything else is poetry, imagination. — Max K. E. L. Planck To solve the mysteries in fundamental physics, we need to carefully test our current knowledge of Nature in order to find clues of New Physics. Our current knowledge of Nature is best summarised by the two theoretical frameworks, the general theory of relativity and the Standard Model. To test the former, one must look to astronomical- scale objects and events, where the effect of gravity becomes prominent. For that, the most recent breakthrough is the detection of gravitational waves by the LIGO and Virgo experiments [63] , which has found no evidence against the predictions of GR so far [64] . And to test the latter, one must instead look to events at the smallest scale. However, that by no means implies that the experiments for such tests are small in scale as well. On the contrary, due to the fundamental quantum-mechanical relation of L = ~ c E , (2.1) where L and E are the length and energy scales respectively, these experiments tend to be physically huge to pursue high energy. In fact, the largest single machine in the world, the Large Hadron Collider, is exactly built for such a purpose. The high precision and performance required to observe such high-energy events also push the size and complexity of the detectors used in these experiments. The ATLAS detector, designed to test multiple aspects of the SM and search for evidence of various BSM phenomena, is once again the largest particle detector in the world. In this chapter, the design and workings of the Large Hadron Collider and the ATLAS detector will be described. These state-of-the-art apparatus are what provided the data that make the work in this thesis possible. 27