1. New Dimensions Collider Physics with the ATLAS Detector The LHC is designed to accelerate protons to near light speeds, then guide clockwise and counter-clockwise proton beams to produce collisions. Several particle detectors are poised to record the products of the proton collisions. The products of high energy collisions are fascinating to physicists because they include particles which are not encountered at lower energies. The ATLAS experiment is one of several particle detectors situated at collision points on the LHC ring. The experiments are designed to observe and record the products of proton collisions. Students and physicists from UC Santa Cruz have played a leading role in the design and construction of the innermost component of the detector. In particular, they have contributed to development and testing of silicon sensors and readout electronics which are resistant to high doses of radiation. The Standard Model of Particle Physics describes the particles which constitute matter, as well as those which mediate communication between matter particles. The matter particles include the up and down quarks, which collaborate to form protons and neutrons and thus to form the building blocks of the material world which surrounds us. They also include more massive siblings of the down quark called strange and beauty, and more massive siblings of the up quark called charm and truth. The Standard Model is well tested but incomplete. For example, the familiar electron has siblings called the muon and the tau, which are more massive than the electron but are otherwise identical. Physicists do not understand why there are three generations of matter particles. Supersymmetry postulates that every known particle has an undiscovered partner. This hypothesis more than doubles the number of particle species. It also results in a mirror world, where every familiar face is accompanied by an unknown visage. Andrea Bangert, Dan Damiani, Peter Manning, Santa Cruz Institute for Particle Physics Humans perceive four spacetime dimensions. The model called Universal Extra Dimensions postulates that elementary particles inhabit not only these four familiar dimensions but also additional, compact dimensions. One promising version of the theory predicts exactly two additional dimensions. This model successfully predicts the three generations of matter particles. The Large Hadron Collider (LHC) is situated at the European Organization for Nuclear Research (CERN) in Switzerland, near Lake Geneva. The accelerator ring is 16.8 miles in circumference and is housed in a subterranean tunnel 300 feet below the surface of the earth. The machine will begin accelerating protons in September 2009. No supersymmetric particle has ever been observed. It is therefore not yet known whether predictions made by the theory are correct. One of the most important goals of the ATLAS collaboration is to determine whether supersymmetry is realized in nature. Students and faculty at UCSC are engaged in the search for supersymmetric particles using data delivered by the Large Hadron Collider. If supersymmetry is observed, the discovery will profoundly change our understanding of the universe. Large Hadron Collider ATLAS Detector Supersymmetry Extra Dimension s The Standard Model By observing the behavior of very energetic particles produced at the LHC, physicists hope to obtain information about compact dimensions. The discovery of additional dimensions would transform our knowledge of the universe.