The Large Hadron Collider (LHC) is the world's largest machine, and the highest-energy particle accelerator.   It works by colliding opposing particle beams of either protons or lead nuclei.  These experiments are expected  to address the most fundamental questions of physics, to increase our understanding of the deepest laws of nature, including the existence of the hypothesized Higgs boson particle, which is often referred to as "the God particle" in the media.

The LHC was built by the European Organization for Nuclear Research (CERN).  It crosses the border between Switzerland and France at four points, with most of it in France. Surface buildings hold ancillary equipment such as compressors, ventilation equipment, control electronics and refrigeration plants.  The accelerator itself lies in a tunnel as much as 175 metres (570 feet) beneath the Franco-Swiss border near Geneva, Switzerland, it's 27 kilometres (17 miles) in circumference.  Although it is located in Europe it is funded by and built in collaboration with over 100 countries around the world and around 10,000 scientists and engineers from universities and laboratories.

LHC Time Line

7 January 1995 - Representatives of the 19 member nations of the European Laboratory for Particle Physics (CERN, finally approved construction of the Large Hadron Collider (LHC).  It was due to be completed in 2008.

7 March 2005 - The first of the approximately 5,000 magnets were put in place.

10 September 2008 - Proton beams were successfully circulated in the main ring of the LHC for the first time.

19 September 2008 - Operations were halted due to a serious fault between two superconducting bending magnets.  Repairing the resulting damage and installing additional safety features took over a year.

8 June 2009 - Microsoft founder Bill Gates visited CERN.

9 October 2009 - A researcher at CERN was arrested for suspected links with al-Qaeda.

20 November 2009 - Proton beams were successfully circulated again.

23 November 2009 - First proton-proton collisions at the injection energy of 450 GeV per particle were recorded.

The first high-energy collisions are expected to be attempted in early 2010.

The Experiments

The collider tunnel contains two adjacent parallel beam pipes that intersect at four points, each containing a proton beam, which travel in opposite directions around the ring. Some 1,232 dipole magnets keep the beams on their circular path, while an additional 392 quadrupole magnets are used to keep the beams focused, in order to maximize the chances of interaction between the particles in the four intersection points, where the two beams will cross. In total, over 1,600 superconducting magnets are installed, with most weighing over 27 tonnes. Approximately 96 tonnes of liquid helium is needed to keep the magnets at their operating temperature of 1.9 K (−271.25 °C), making the LHC the largest cryogenic facility in the world at liquid helium temperature.

At full power, trillions of protons can race around its ring at 11,245 times a second -- some 99.99 percent the speed of light.Prior to being injected into the main accelerator, the particles are prepared by a series of systems that successively increase their energy. The first system is the linear particle accelerator LINAC 2 generating 50-MeV protons, which feeds the Proton Synchrotron Booster (PSB). There the protons are accelerated to 1.4 GeV and injected into the Proton Synchrotron (PS), where they are accelerated to 26 GeV. Finally the Super Proton Synchrotron (SPS) is used to further increase their energy to 450 GeV before they are at last injected (over a period of 20 minutes) into the main ring. Here the proton bunches are accumulated, accelerated (over a period of 20 minutes) to their peak 7-TeV energy, and finally circulated for 10 to 24 hours while collisions occur at the four intersection points.

Once or twice a day, as the protons are accelerated from 450 GeV to 7 TeV, the field of the superconducting dipole magnets will be increased from 0.54 to 8.3 teslas (T). The protons will each have an energy of 7 TeV, giving a total collision energy of 14 TeV. At this energy the protons have a Lorentz factor of about 7,500 and move at about 99.9999991% of the speed of light. It will take less than 90 microseconds (μs) for a proton to travel once around the main ring – a speed of about 11,000 revolutions per second. Rather than continuous beams, the protons will be bunched together, into 2,808 bunches, so that interactions between the two beams will take place at discrete intervals never shorter than 25 nanoseconds (ns) apart. However it will be operated with fewer bunches when it is first commissioned, giving it a bunch crossing interval of 75 ns.

The LHC physics program is mainly based on proton–proton collisions. However, shorter running periods, typically one month per year, with heavy-ion collisions are included in the program. While lighter ions are considered as well, the baseline scheme deals with lead ions (see A Large Ion Collider Experiment). The lead ions will be first accelerated by the linear accelerator LINAC 3, and the Low-Energy Ion Ring (LEIR) will be used as an ion storage and cooler unit. The ions then will be further accelerated by the PS and SPS before being injected into LHC ring, where they will reach an energy of 2.76 TeV per nucleon (or 575 TeV per ion), higher than the energies reached by the Relativistic Heavy Ion Collider.

Why do they do it?

Physicists hope that the LHC will help answer the most fundamental questions in physics, questions concerning the basic laws governing the interactions and forces among the elementary objects, the deep structure of space and time, especially regarding the intersection of quantum mechanics and general relativity, where current theories and knowledge are unclear or break down altogether.

They hope to answer the following questions:-

  • Is the Higgs mechanism for generating elementary particle masses via electroweak symmetry breaking indeed realised in nature?  It is anticipated that the collider will either demonstrate or rule out the existence of the elusive Higgs boson(s), completing the Standard Model.
  • Is supersymmetry, an extension of the Standard Model and Poincaré symmetry, realised in nature, implying that all known particles have supersymmetric partners?  These may clear up the mystery of dark matter.
  • Are there extra dimensions, as predicted by various models inspired by string theory, and can we detect them?

Other questions they hope to answer are:

  • Are electromagnetism, the strong nuclear force and the weak nuclear force just different manifestations of a single unified force, as predicted by various Grand Unification Theories?
  • Why is gravity so many orders of magnitude weaker than the other three fundamental forces?
  • Are there additional sources of quark flavour violation beyond those already predicted within the Standard Model?
  • Why are there apparent violations of the symmetry between matter and antimatter?
  • What is the nature of dark energy?
  • What was the nature of the quark-gluon plasma in the early universe?

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