LHC - THE LARGE HADRON COLLIDER SourceThe Large Hadron Collider (LHC) is the world's largest and highest-energy particle accelerator complex, intended to collide opposing beams of protons charged with approximately 7 TeV of energy. Its main purpose is to explore the validity and limitations of the Standard Model, the current theoretical picture for particle physics. It is theorized that the collider will produce the Higgs boson, the observation of which could confirm the predictions and missing links in the Standard Model, and could explain how other elementary particles acquire properties such as mass. The LHC was built by the European Organization for Nuclear Research (CERN), and lies underneath the Franco-Swiss border near Geneva, Switzerland. It is funded by and built in collaboration with over eight thousand physicists from over eighty-five countries as well as hundreds of universities and laboratories. The LHC is already operational and is presently in the process of being prepared for collisions. The first beams were circulated through the collider on 10 September 2008, and the first high-energy collisions are planned to take place after the LHC is officially unveiled on 21 October 2008. Although a few individuals have questioned the safety of the planned experiments in the media and through the courts, the consensus in the scientific community is that there is no conceivable threat from the LHC particle collisions. DesignThe LHC is the world's largest and highest-energy particle accelerator.[1] 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 underground.[2] The 3.8 metre (150 inches) diameter, concrete-lined tunnel, constructed between 1983 and 1988, was formerly used to house the Large Electron-Positron Collider.[3] It crosses the border between Switzerland and France at four points, but most of it is in France. Surface buildings hold ancillary equipment such as compressors, ventilation equipment, control electronics and refrigeration plants. The collider tunnel contains two adjacent beam pipes, each containing a proton beam - a proton is one type of hadron. The two beams 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, making the LHC the largest cryogenic facility in the world at liquid helium temperature. Superconducting quadrupole electromagnets are used to direct the beams to four intersection points where interactions between protons will take place. 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 T to 8.3 T. The protons will each have an energy of 7 TeV, giving a total collision energy of 14 TeV (2.2 μJ). At this energy the protons have a lorentz factor of about 7,500 and move at about 99.999999% of light speed. It will take less than 90 microseconds 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 ns apart. When the collider is first commissioned, it will be operated with fewer bunches, to give a bunch crossing interval of 75 ns. The number of bunches will later be increased to give a final bunch crossing interval of 25 ns. 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. 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 increase their energy to 450 GeV before they are at last injected (over a period of 20 minutes) into the main ring, where proton bunches are accumulated, accelerated (over a period of 20 minutes) to their peak 7 TeV energy, and finally stored for many hours (10 to 24) while collisions occur at the four intersection points.[6][ ERROR: SPECIFIED ATTACHMENT MISSING ] The Large Hadron Collider's (LHC) CMS detectors being installed. The LHC will also be used to collide lead (Pb) heavy ions with a collision energy of 1,150 TeV. The Pb ions will be first accelerated by the linear accelerator Linac 3, and the Low-Energy Injector Ring 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.DetectorsSix detectors are being constructed at the LHC, located underground in large caverns excavated at the LHC's intersection points. Two of them, the ATLAS experiment and the Compact Muon Solenoid (CMS), are large, general purpose particle detectors.[1] "A Large Ion Collider Experiment" (ALICE) is designed to study the properties of quark-gluon plasma from the debris of heavy-ion collisions. The other three, LHCb, TOTEM, and LHCf, are smaller and more specialized. The BBC's summary of the detectors is:[7] ATLAS - one of two so-called general purpose detectors. Atlas will be used to look for signs of new physics, including the origins of mass and extra dimensions. CMS - the other general purpose detector will, like ATLAS, hunt for the Higgs boson and look for clues to the nature of dark matter. ALICE - will study a "liquid" form of matter called quark-gluon plasma that existed shortly after the Big Bang. LHCb - equal amounts of matter and anti-matter were created in the Big Bang. LHCb will try to investigate what happened to the "missing" anti-matter.PurposeWhen activated, it is theorized that the collider will produce the elusive Higgs boson. The verification of the existence of the Higgs boson would be a significant step in the search for a Grand Unified Theory, which seeks to unify three of the four known fundamental forces: electromagnetism, the strong nuclear force and the weak nuclear force, leaving out only gravity. The Higgs boson may also help to explain why gravitation is so weak compared with the other three forces. In addition to the Higgs boson, other theorized particles, models and states might be produced, and for some searches are planned, including supersymmetric particles,[8] compositeness (technicolor),[9]extra dimensions,[10] strangelets,[11]micro black holes[12] and magnetic monopoles.[13] [ ERROR: SPECIFIED ATTACHMENT MISSING ] A Feynman diagram of one way the Higgs boson may be produced at the LHC. Here, two quarks each emit a W or Z boson which combine to make a neutral Higgs.Research[ ERROR: SPECIFIED ATTACHMENT MISSING ] A simulated event in the CMS detector, featuring the appearance of the Higgs boson. When in operation, about seven thousand scientists from eighty countries will have access to the LHC. Physicists hope to use the collider to test various grand unified theories and enhance their ability to answer the following questions:Is the popular Higgs mechanism for generating elementary particle masses in the Standard Model realised in nature? If so, how many Higgs bosons are there, and what are their masses?[14] Will the more precise measurements of the masses of the quarks continue to be mutually consistent within the Standard Model? Do particles have supersymmetric ("SUSY") partners?[1] Why are there apparent violations of the symmetry between matter and antimatter?[1] See also CP-violation. Are there extra dimensions, as predicted by various models inspired by string theory, and can we "see" them? What is the nature of dark matter and dark energy?[1] Why is gravity so many orders of magnitude weaker than the other three fundamental forces? Renowned British astrophysicist Stephen Hawking has bet $100 the mega-experiment will not find the elusive particle seen as the holy grail of cosmic science. "I think it will be much more exciting if we don't find the Higgs. That will show something is wrong, and we need to think again. I have a bet of 100 dollars that we won't find the Higgs," said Prof Hawking.[15] Hawking said the experiment could discover superpartners, particles that would be "supersymmetric partners" to particles already known about. "Their existence would be a key confirmation of string theory, and they could make up the mysterious dark matter that holds galaxies together," he said on the BBC. "Whatever the LHC finds, or fails to find, the results will tell us a lot about the structure of the universe," he said.[15]As an ion colliderThe 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 programme. While lighter ions are considered as well, the baseline scheme deals with lead ions.[16] This will allow an advancement in the experimental programme currently in progress at the Relativistic Heavy Ion Collider (RHIC).Test timelineSeptember 2008The first beam was circulated through the collider on the morning of 10 September 2008.[17] CERN successfully fired the protons around the tunnel in stages, several kilometres at a time. The particles were fired in a clockwise direction into the accelerator and successfully steered around it at 10:28 am local time.[18] The LHC successfully completed its first major test: after a series of trial runs, two white dots flashed on a computer screen showing the protons traveled the full length of the Collider. CERN next successfully sent a beam of protons in a counterclockwise direction. Eventually two beams will be fired in opposite directions with the aim of smashing together protons to create particles from the energy of the proton collisions.[19][20][21] It took less than one hour to guide the stream of particles around its inaugural circuit.[22]
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