The Large Hadron Collider (LHC) is the world's largest and highest-energy particle accelerator. It is expected to address some of the most fundamental questions of physics, advancing the understanding of the deepest laws of nature.
The LHC lies in a tunnel 27 kilometres (17 mi) in circumference, as deep as 175 metres (574 ft) beneath the Franco-Swiss border near Geneva, Switzerland. This synchrotron is designed to collide opposing particle beams of either protons at an energy of 7 teraelectronvolts (7 TeV or 1.12 microjoules) per particle, or lead nuclei at an energy of 574 TeV (92.0 µJ) per nucleus. The term hadron refers to particles composed of quarks.
The Large Hadron Collider was built by the European Organization for Nuclear Research (CERN) with the intention of testing various predictions of high-energy physics, including testing for the existence of the hypothesized Higgs boson and of the large family of new particles predicted by supersymmetry. It was built in collaboration with over 10,000 scientists and engineers from over 100 countries, as well as hundreds of universities and laboratories.
On 10 September 2008, the proton beams were successfully circulated in the main ring of the LHC for the first time, but 9 days later operations were halted due to a serious fault. On 20 November 2009 they were successfully circulated again, with the first recorded proton–proton collisions occurring 3 days later at the injection energy of 450 GeV per beam. After the 2009 winter shutdown, the LHC was restarted and the beam was ramped up to 3.5 TeV per beam (half its designed energy).
On 30 March 2010, the first planned collisions took place between two 3.5 TeV beams, a new world record for the highest-energy man-made particle collisions. The LHC will continue to operate at half energy until the end of 2012; it will not run at full energy (7 TeV per beam) until 2014.
Purpose
Physicists hope that the LHC will help answer some of the fundamental open questions in physics, concerning the basic laws governing the interactions and forces among the elementary objects, the deep structure of space and time, and in particular the intersection of quantum mechanics and general relativity, where current theories and knowledge are unclear or break down altogether. Data is also needed from high energy particle experiments to indicate which versions of scientific models are more likely to be correct - in particular to choose between the Standard Model and Higgsless models and to validate their predictions and allow further theoretical development. Many theorists expect new physics beyond the Standard Model to emerge at the TeV-scale, based on unsatisfactory properties of the Standard Model. Issues possibly to be explored by LHC collisions include
Is the theoretical Higgs mechanism for generating elementary particle masses via electroweak symmetry breaking actually realised in nature? It is expected that the collider will either demonstrate or rule out the existence of the elusive Higgs boson, thereby allowing physicists to determine whether the Standard Model or its Higgsless model alternatives are more likely to be correct.
Is supersymmetry, an extension of the Standard Model and Poincaré symmetry, realised in nature, implying that all known particles have supersymmetric partners?
Are there extra dimensions, as predicted by various models based on string theory, and can we detect them?
What is the nature of the dark matter that appears to account for 23% of the mass of the universe?
Other open questions which may be explored using high energy particle collisions:
It is already known that electromagnetism and the weak nuclear force are just different manifestations of a single force called the electroweak force. The LHC may clarify whether the electroweak force and the strong nuclear force are similarly just different manifestations of one universal unified force, as predicted by various Grand Unification Theories.
Why is the fourth fundamental force (gravity) so many orders of magnitude weaker than the other three fundamental forces?
Are there additional sources of quark flavour mixing, beyond those already predicted within the Standard Model?
Why are there apparent violations of the symmetry between matter and antimatter? See also CP violation.
What are the nature and properties of quark-gluon plasma, believed to have existed in the early universe and in certain compact and strange astronomical objects today? This will be investigated by heavy ion collisions in ALICE.
Construction accidents and delaysOn 25 October 2005, José Pereira Lages, a technician, was killed in the LHC when a switchgear that was being transported fell on him.
On 27 March 2007 a cryogenic magnet support broke during a pressure test involving one of the LHC's inner triplet (focusing quadrupole) magnet assemblies, provided by Fermilab and KEK. No one was injured. Fermilab director Pier Oddone stated "In this case we are dumbfounded that we missed some very simple balance of forces". This fault had been present in the original design, and remained during four engineering reviews over the following years.
Analysis revealed that its design, made as thin as possible for better insulation, was not strong enough to withstand the forces generated during pressure testing. Details are available in a statement from Fermilab, with which CERN is in agreement. Repairing the broken magnet and reinforcing the eight identical assemblies used by LHC delayed the startup date, then planned for November 2007.
Problems occurred on 19 September 2008 during powering tests of the main dipole circuit, when an electrical fault in the bus between magnets caused a rupture and a leak of six tonnes of liquid helium. The operation was delayed for several months.
It is currently believed that a faulty electrical connection between two magnets caused an arc, which compromised the liquid-helium containment. Once the cooling layer was broken, the helium flooded the surrounding vacuum layer with sufficient force to break 10-ton magnets from their mountings. The explosion also contaminated the proton tubes with soot. This accident was more recently thoroughly discussed in a 22 February 2010 Superconductor Science and Technology article by CERN physicist Lucio Rossi.
Two vacuum leaks were identified in July 2009, and the start of operations was further postponed to mid-November 2009.
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