ATLAS experiment

Coordinates: 46°14′8″N 6°3′19″E / 46.23556°N 6.05528°E / 46.23556; 6.05528ATLAS (A Toroidal LHC ApparatuS) is one of the seven particle detector experiments constructed at the Large Hadron Collider (LHC), a particle accelerator at CERN (the European Organization for Nuclear Research) in Switzerland. The experiment is designed to take advantage of the unprecedented energy available at the LHC and observe phenomena that involve highly massive particles which were not observable using earlier lower-energy accelerators. ATLAS was one of the two LHC experiments involved in the discovery of the Higgs boson in July 2012. It was also designed to search for evidence of theories of particle physics beyond the Standard Model. Coordinates: 46°14′8″N 6°3′19″E / 46.23556°N 6.05528°E / 46.23556; 6.05528ATLAS (A Toroidal LHC ApparatuS) is one of the seven particle detector experiments constructed at the Large Hadron Collider (LHC), a particle accelerator at CERN (the European Organization for Nuclear Research) in Switzerland. The experiment is designed to take advantage of the unprecedented energy available at the LHC and observe phenomena that involve highly massive particles which were not observable using earlier lower-energy accelerators. ATLAS was one of the two LHC experiments involved in the discovery of the Higgs boson in July 2012. It was also designed to search for evidence of theories of particle physics beyond the Standard Model. The ATLAS detector is 46 metres long, 25 metres in diameter, and weighs about 7,000 tonnes; it contains some 3000 km of cable. The experiment is a collaboration involving roughly 3,000 physicists from over 175 institutions in 38 countries. The project was led for the first 15 years by Peter Jenni, between 2009 and 2013 was headed by Fabiola Gianotti, from 2013 to 2017 by David Charlton, and afterwards by Karl Jakobs. The ATLAS Collaboration, the group of physicists who built and run the detector, was formed in 1992 when the proposed EAGLE (Experiment for Accurate Gamma, Lepton and Energy Measurements) and ASCOT (Apparatus with Super Conducting Toroids) collaborations merged their efforts to build a single, general-purpose particle detector for the Large Hadron Collider. The design was a combination of the two previous experiments, and also benefitted from the detector research and development that had been done for the Superconducting Supercollider. The ATLAS experiment was proposed in its current form in 1994, and officially funded by the CERN member countries in 1995. Additional countries, universities, and laboratories have joined in subsequent years. Construction work began at individual institutions, with detector components then being shipped to CERN and assembled in the ATLAS experiment pit starting in 2003. Construction was completed in 2008 and the experiment detected its first single beam events on 10 September of that year. Data taking was then interrupted for over a year due to an LHC magnet quench incident. On 23 November 2009, the first proton-proton collisions occurred at the LHC and were recorded by ATLAS, at a relatively low injection energy of 450 GeV per beam. Since then, the LHC energy has been increasing: 900 GeV per beam at the end of 2009, 3,500 GeV for the whole of 2010 and 2011, then 4,000 GeV per beam in 2012. After a long shutdown in 2013 and 2014, in 2015 ATLAS saw 6,500 GeV per beam. The first cyclotron, an early type of particle accelerator, was built by Ernest O. Lawrence in 1931, with a radius of just a few centimetres and a particle energy of 1 megaelectronvolt (MeV). Since then, accelerators have grown enormously in the quest to produce new particles of greater and greater mass. As accelerators have grown, so too has the list of known particles that they might be used to investigate. The most comprehensive model of particle interactions available today is known as the Standard Model of Particle Physics. With the important exception of the Higgs boson, now detected by the ATLAS and the CMS experiments,all of the particles predicted by the model had been observed by previous experiments. While the Standard Model predicts that quarks, electrons, and neutrinos should exist, it does not explain why the masses of these particles differ by orders of magnitude. Due to this, many particle physicists believe it is possible that the Standard Model will break down at energies at the teraelectronvolt (TeV) scale or higher. If such beyond-the-Standard-Model physics is observed, a new model, which is identical to the Standard Model at energies thus far probed, can be developed to describe particle physics at higher energies. Most of the currently proposed theories predict new higher-mass particles, some of which may be light enough to be observed by ATLAS. ATLAS is designed to be a general-purpose detector. When the proton beams produced by the Large Hadron Collider interact in the center of the detector, a variety of different particles with a broad range of energies are produced. Rather than focusing on a particular physical process, ATLAS is designed to measure the broadest possible range of signals. This is intended to ensure that whatever form any new physical processes or particles might take, ATLAS will be able to detect them and measure their properties. Experiments at earlier colliders, such as the Tevatron and Large Electron-Positron Collider, were designed based on a similar philosophy. However, the unique challenges of the Large Hadron Collider – its unprecedented energy and extremely high rate of collisions – require ATLAS to be significantly larger and more complex than previous experiments. At 27 kilometres in circumference, the Large Hadron Collider (LHC) collides two beams of protons together, with each proton carrying up to 6.5 TeV of energy – enough to produce particles with masses significantly greater than any particles currently known, if these particles exist. ATLAS is designed to detect these particles, namely their masses, momentum, energies, lifetime, charges, and nuclear spins. In order to identify all particles produced at the interaction point where the particle beams collide, the detector is designed in layers made up of detectors of different types, each of which is designed to observe specific types of particles. The different traces that particles leave in each layer of the detector allow for effective particle identification and accurate measurements of energy and momentum. (The role of each layer in the detector is discussed below.) As the energy of the particles produced by the accelerator increases, the detectors attached to it must grow to effectively measure and stop higher-energy particles. As of 2017, ATLAS is the largest detector ever built at a particle collider. ATLAS investigates many different types of physics that might become detectable in the energetic collisions of the LHC. Some of these are confirmations or improved measurements of the Standard Model, while many others are possible clues for new physical theories.