The Large Hadron Collider (LHC) is a particle accelerator located at CERN, near Geneva, Switzerland. It lies in a tunnel under France and Switzerland.
The LHC is in the final stages of construction and commissioning, with some sections already being cooled down to their final operating temperature of approximately 2K. The first beams are due for injection mid June 2008 with the first collisions planned to take place 2 months later. The LHC will become the world's largest and highest-energy particle accelerator. The LHC is being funded and built in collaboration with over two thousand physicists from thirty-four countries as well as hundreds of universities and laboratories.
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 novel particles that might be produced, and for which searches are planned, include strangelets, micro black holes, magnetic monopoles and supersymmetric particles.
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 tunnel, constructed between 1983 and 1988, was formerly used to house the LEP, an electron-positron collider.
The 3.8 metre diameter, concrete-lined tunnel crosses the border between Switzerland and France at four points, although the majority of its length is inside France. The collider itself is underground, with surface buildings holding ancillary equipment such as compressors, ventilation equipment, control electronics and refrigeration plants.
The collider tunnel contains two pipes, each pipe containing a beam. The two beams travel in opposite directions around the ring. 1232 dipole magnets keep the beams on their circular path, while 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 1600 superconducting magnets are installed, with most weighing over 27 tonnes. 96 tonnes of liquid helium is needed to keep the magnets at the operating temperature.
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 an individual proton to travel once around the collider. 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 through a series of systems that successively increase the particle energy levels. The first system is the linear accelerator Linac 2 generating 50 MeV protons which feeds the Proton Synchrotron Booster (PSB). Protons are then injected at 1.4 GeV into the Proton Synchrotron (PS) at 26 GeV. Finally the Super Proton Synchrotron (SPS) is used to increase the energy of protons up to 450 GeV.
The LHC will also be used to collide lead (Pb) heavy ions with a collision energy of 1,150 TeV. The 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, ATLAS and CMS, are large, "general purpose" particle detectors. ALICE is a large detector designed to study the properties of quark-gluon plasma looking at the debris of heavy ion collisions. The other three (LHCb, TOTEM, and LHCf) are relatively smaller and more specialized. A seventh experiment, FP420 (Forward Physics at 420m), has been proposed which would add detectors to four available spaces located 420m on either side of the ATLAS and CMS detectors.
The size of the LHC constitutes an exceptional engineering challenge with unique safety issues. While running, the total energy stored in the magnets is 10 GJ, while each of the two beams carries an overall energy of 362 MJ. For comparison, 362 MJ is the kinetic energy of a TGV running at 157 km/h (98 mph), while 724 MJ, the total energy of the two beams, is equivalent to the detonation energy of approximately 157 kilograms (350 lb) of TNT, and 10 GJ is about 2.5 tons of TNT. Loss of only 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 kinetic energies become far more spectacular when you consider how little matter is carrying it. At its maximum energy rating (2.76TeV per particle with a total of 362MJ), there is just 1.15E-9 grams of hydrogen in the system (or 0.026 of one cubic millimeter).
When in operation, about seven thousand scientists from eighty countries will have access to the LHC, the largest national contingent of seven hundred being from the United States. 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 (Pb) ions. This will allow an advancement in the experimental programme currently in progress at the Relativistic Heavy Ion Collider (RHIC).
fter 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.
The construction of LHC was approved in 1995 with a budget of 2.6 billion Swiss francs, with another 210 million francs (140 M€) towards the cost of the experiments. However, cost over-runs, estimated in a major review in 2001 at around 480 million francs (300 M€) for the accelerator, and 50 million francs (30 M€) for the experiments, along with a reduction in CERN's budget, pushed the completion date from 2005 to April 2007. 180 million francs (120 M€) 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 (home to the world's largest particle accelerator until CERN finishes the Large Hadron Collider) or Fermilab. The total cost of the project is anticipated to be between $5 and $10 billion (US Dollars).
Main article: LHC@home
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.
Concerns have been raised that performing collisions at previously unexplored energies might unleash new and disastrous phenomena. These include the production of micro black holes, and strangelets, potentially resulting in a doomsday scenario. Such issues were raised in connection with the RHIC accelerator, both in the media and in the scientific community; however, after detailed studies, scientists reached such conclusions as "beyond reasonable doubt, heavy-ion experiments at RHIC will not endanger our planet" and that there is "powerful empirical evidence against the possibility of dangerous strangelet production."
One argument against such fears is that collisions at these energies (and higher) have been happening in nature for billions of years apparently without hazardous effects, as ultra-high-energy cosmic rays impact Earth's atmosphere and other bodies in the universe. A concern against this cosmic-ray argument is that, if dangerous strangelets or micro black holes were created at LHC, a proportion would have less than the Earth's escape velocity (of 11.2 km/s), and therefore would be captured by the Earth's gravitational field, whereas those created by high-energy cosmic rays would leave the planet at high speed, due to the laws of conservation of momentum at relativistic speeds.
CERN's review concludes, after detailed analysis, that "there is no basis for any conceivable threat" from strangelets or black holes. However, the concern about the verity of Hawking radiation was not addressed, and another study was commissioned by CERN in 2007 for publication on CERN's web-site by the end of 2007.
The risk of a doomsday scenario was indicated by Sir Martin Rees, with respect to the RHIC, as being at least a 1 in 50,000,000 chance, and by Professor Frank Close, with regards to (dangerous) strangelets, that "the chance of this happening is like you winning the major prize on the lottery 3 weeks in succession; the problem is that people believe it is possible to win the lottery 3 weeks in succession." Accurate assessments of these risks are impossible due to the present incomplete, or even hypothetically flawed, standard model of particle physics (see also a list of unsolved problems in physics).
Micro black holes
Main article: Micro black hole
Although the Standard Model of particle physics predicts that LHC energies are far too low to create black holes, some extensions of the Standard Model posit the existence of extra spatial dimensions, in which it would be possible to create micro black holes at the LHC at a rate on the order of one per second. According to the standard calculations these are harmless because they would quickly decay by Hawking radiation. The concern is that among other disputed factors, Hawking radiation (whose existence is still debated) is not yet an experimentally-tested or naturally observed phenomenon. The opponents to the LHC consider that micro black holes produced in a terrestrial laboratory might not decay as rapidly as calculated, or might even not be prone to decay. According to CERN, physicists in general do not question the assumption that black holes are generally unstable and those few who have pointed out issues with Hawking's radiation were only attempting to achieve a more rigorous proof of it. CERN further argues that even if micro black holes were created and were stable, they would pose no threat to the Earth during its remaining 5 billion years of existence. However, Dr. Adam D. Helfer's thesis concludes "no compelling theoretical case for or against radiation by black holes", and Dr. Otto E. Rossler's thesis calculates that Earth accretion time could be as short as 50 months.
Main article: Strangelet
Strangelets are a hypothetical form of strange matter that contains roughly equal numbers of up, down, and strange quarks and are more stable than ordinary nuclei. If strangelets can actually exist, and if they were produced at LHC, they could conceivably initiate a runaway fusion process (reminiscent of the fictional ice-nine) in which all the nuclei in the planet were converted to strange matter, similar to a strange star.
On 21 March 2008 a complaint requesting an injunction against the LHC's startup was filed before the United States District Court for the District of Hawaii by a group of seven concerned individuals. This group includes Walter L. Wagner who notably was unable to obtain an injunction against the much lower energy RHIC for similar concerns. See: RHIC - Fears among the public
The restraining order is a demand for an injunction of 4 months time after issuance of the LHC Safety Assessment Group's (LSAG) Safety Review originally promised by January 1, 2008, to review the LHC's most recent safety documentation, after it has been issued, and a permanent injunction until the LHC can be demonstrated to be reasonably safe within industry standards.
Construction accidents and delays
On October 25, 2005, a technician, José Pereira Lages, was killed in the LHC tunnel when a crane load was accidentally dropped.
On March 27, 2007, there was an incident during a pressure test involving one of the LHC's inner triplet magnet assemblies provided by Fermilab and KEK. No people were injured, but a cryogenic magnet support broke. 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 copies used by LHC, in addition to a number of other small delays, caused a postponement of the planned November 26, 2007 startup date to May 2008.