Sunday, August 7, 2011


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 bosonand 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 2013.

The LHC is the world's largest and highest-energy particle accelerator. The collider is contained in a circular tunnel, with a circumference of 27 kilometres (17 mi), at a depth ranging from 50 to 175 metres (160 to 574 ft) underground.
The 3.8-metre (12 ft) wide concrete-lined tunnel, constructed between 1983 and 1988, was formerly used to house the Large Electron–Positron Collider. 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 collider tunnel contains two adjacent parallel beam pipes that intersect at four points, each containing a proton beam, which travels 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, made of copper-clad niobium-titanium, 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.

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 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 0.999999991 c, or about 3 metres per second slower than the speed of light (c).  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 design luminosity of the LHC is 1034 cm−2s−1, providing a bunch collision rate of 40 MHz.
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.
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 will then 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. The aim of the heavy-ion program is to investigate quark–gluon plasma, which existed in the early universe.

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