The Large Hadron Collider (LHC) is the world's largest particle accelerator complex, intended to collide opposing beams of 7 TeV protons. Its main purpose is to explore the validity and limitations of the Standard Model, the current theoretical picture for particle physics. The LHC was built by the European Organization for Nuclear Research (CERN), and lies under the Franco-Swiss border near Geneva, Switzerland.
The LHC is the world's largest and the highest-energy particle accelerator. It is funded and built in collaboration with over eight thousand physicists from over eighty-five countries as well as hundreds of universities and laboratories.
The collider is currently undergoing commissioning while being cooled down to its final operating temperature of approximately 1.9 K (−271.25 °C). Initial particle beam injections were successfully carried out on 8-11 August 2008, the first attempt to circulate a beam through the entire LHC is scheduled for 10 September 2008, and the first high-energy collisions are planned to take place after the LHC is officially unveiled, on 21 October 2008.
When activated, it is theorized that the collider will produce the elusive Higgs boson, the observation of which could confirm the predictions and missing links in the Standard Model of physics and could explain how other elementary particles acquire properties such as mass.
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 basis for any conceivable threat from the LHC particle collisions.
Development
The idea of the Large Hadron Collider (LHC), began in the early 1980s. The first approval of the project by the European Organization for Nuclear Research (CERN) Council occurred in December 1994 and the first civil engineering construction work began in April 1998. It is funded and built in collaboration with over eight thousand physicists from over eighty-five countries as well as hundreds of universities and laboratories.
Technical design
The LHC is the world's largest and the 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 underground.The 3.8 metre diameter, concrete-lined tunnel, constructed between 1983 and 1988, was formerly used to house the LEP, an electron-positron collider. It crosses the border between Switzerland and France at four points, although 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 bending magnets keep the beams on their circular path, while an additional 392 focusing 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 the operating temperature (1.9K), 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 bending 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. 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 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 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 .
The LHC will also be used to collide lead (Pb) nuclei 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 (LEIR) will be used as an ion storage and cooler unit. The ions then will be further accelerated by the Proton Synchrotron (PS) and Super Proton Synchrotron (SPS) before being injected into LHC ring, where they will reach an energy of 2.76 TeV per nucleon.
Six 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.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.
Purpose
When activated, it is theorized that the collider will produce the elusive Higgs boson, the observation of which could confirm the predictions and missing links in the Standard Model of physics and could explain how other elementary particles acquire properties such as mass. 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 to 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, compositeness (technicolor), extra dimensions, strangelets, micro black holes and magnetic monopoles[19]
Research
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?
- 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?
- Why are there apparent violations of the symmetry between matter and antimatter? See also CP-violation.
- Are there extra dimensions indicated by theoretical gravitons, as predicted by various models inspired by string theory, and can we "see" them?
- What is the nature of dark matter and dark energy?
- Why is gravity so many orders of magnitude weaker than the other three fundamental forces?
As an ion collider
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 programme. While lighter ions are considered as well, the baseline scheme deals with lead ions. will allow an advancement in the experimental programme currently in progress at the Relativistic Heavy Ion Collider (RHIC).
Proposed upgrade
After some years of running, any particle physics experiment typically begins to suffer from diminishing returns; each additional year of operation discovers less than the year before. The way around the diminishing returns is to upgrade the experiment, either in energy or in luminosity. A luminosity upgrade of the LHC, called the Super LHC, has been proposed, to be made after ten years of LHC operation. The optimal path for the LHC luminosity upgrade includes an increase in the beam current (i.e., the number of protons in the beams) and the modification of the two high luminosity interaction regions, ATLAS and CMS. To achieve these increases, the energy of the beams at the point that they are injected into the (Super) LHC should also be increased to 1 TeV. This will require an upgrade of the full pre-injector system, the needed changes in the Super Proton Synchrotron being the most expensive.
Cost
The total cost of the project is anticipated to be between €3.2 to €6.4 billion. The construction of LHC was approved in 1995 with a budget of 2.6 billion Swiss francs (€1.6 billion), with another 210 million francs (€140 million) towards the cost of the experiments. However, cost over-runs, estimated in a major review in 2001 at around 480 million francs (€300 million) for the accelerator, and 50 million francs (€30 million) for the experiments, along with a reduction in CERN's budget, pushed the completion date from 2005 to April 2007. 180 million francs (€120 million) of the cost increase have been due to the superconducting magnets. There were also engineering difficulties encountered while building the underground cavern for the Compact Muon Solenoid. In part this was due to faulty parts lent to CERN by fellow laboratories Argonne National Laboratory or Fermilab).[24]
Computing resources
The LHC Computing Grid is being constructed to handle the massive amounts of data produced by the Large Hadron Collider. It incorporates both private fibre optic cable links and existing high-speed portions of the public Internet, to get data from CERN to academic institutions around the world.
The distributed computing project LHC@home was started to support the construction and calibration of the LHC. The project uses the BOINC platform to simulate how particles will travel in the tunnel. With this information, the scientists will be able to determine how the magnets should be calibrated to gain the most stable "orbit" of the beams in the ring.
Safety issues
Safety of particle collisions
Although a few individuals have questioned the safety of the planned experiments at the LHC in the media and through the courts the consensus in the scientific community is that there is no basis for any conceivable threat.
Operational safety
The size of the LHC constitutes an exceptional engineering challenge with unique operational issues on account of the huge energy stored in the magnets and the beams.
While operating, the total energy stored in the magnets is 10 GJ and the total energy carried by the two beams reaches 724 MJ.
Loss of only one ten-millionth part (10−7) of the beam is sufficient to quench a superconducting magnet, while the beam dump must absorb an energy equivalent to a typical air-dropped bomb. These immense energies are even more impressive when one considers how little matter is carrying it. Under nominal operating conditions (2,808 bunches per beam, 1.15×1011 protons per bunch), the beam pipes contain 1.0×10-9 grams of hydrogen, which, in standard conditions for temperature and pressure, would fill the volume of one grain of fine sand.
Construction accidents and delays
On 25 October 2005, a technician was killed in the LHC tunnel when a crane load was accidentally dropped.
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, by several weeks.
In popular culture
The Large Hadron Collider has been featured in a number of novels, including Flashforward by Robert J. Sawyer, Black Hole, by Angelo Paratico, and Decipher by Stel Pavlou, which described it in some detail. One of the most visible examples is Angels & Demons by Dan Brown, which involves dangerous antimatter created at the LHC used as a weapon against the Vatican. CERN published a "Fact or Fiction?" page discussing the accuracy of the book's portrayal of the LHC, CERN, and particle physics in general. The movie version of the book had footage filmed on-site at one of the experiments at the LHC; the director, Ron Howard, also met with CERN experts in an effort to make the science in the story more accurate. BBC Radio 4 will be broadcasting "Big Bang Day" on 10 September 2008 to coincide with LHC being switched on. Included in this event will be a radio episode of the TV series Torchwood, with a plot surrounding the LHC, entitled Lost Souls. Katherine McAlpine, aka "alpinekat", a science writer working at CERN, wrote the lyrics for a personal rap video about the LHC called the "The Large Hadron Rap". The song was added to YouTube on July 28, 2008, and, as of September 6th, it had been viewed more than a million times.
| Intersecting Storage Rings | CERN, 1971–1984 |
| Super Proton Synchrotron | CERN, 1981–1984 |
| ISABELLE | BNL, cancelled in 1983 |
| Tevatron | Fermilab, 1987–2009 |
| Relativistic Heavy Ion Collider | BNL, operational since 2000 |
| Superconducting Super Collider | cancelled in 1993 |
| Large Hadron Collider | CERN, 2008– |
| Very Large Hadron Collider | theoretical |
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