ATLAS experiment  ATLAS (A Toroidal LHC ApparatuS) is the largest, general-purpose particle detector experiment 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 experiment is a collaboration involving roughly 3,000 physicists from 183 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 and from 2013 to 2017 by David Charlton. The ATLAS Collaboration is currently led by Karl Jakobs. 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 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 Super Collider. 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.