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Large Hadron Collider

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Template:Future scientific facility

Large Hadron Collider
(LHC)
Plan of the LHC experiments and the preaccelerators.
LHC experiments
ATLASA Toroidal LHC Apparatus
CMSCompact Muon Solenoid
LHCbLHC-beauty
ALICEA Large Ion Collider Experiment
TOTEMTotal Cross Section, Elastic Scattering and Diffraction Dissociation
LHCfLHC-forward
MoEDALMonopole and Exotics Detector At the LHC
FASERForwArd Search ExpeRiment
SNDScattering and Neutrino Detector
LHC preaccelerators
p and PbLinear accelerators for protons (Linac 4) and lead (Linac 3)
(not marked)Proton Synchrotron Booster
PSProton Synchrotron
SPSSuper Proton Synchrotron

The Large Hadron Collider (LHC) is a particle accelerator and collider located at CERN, near Geneva, Switzerland (46°14′N 6°03′E / 46.233°N 6.050°E / 46.233; 6.050). Currently under construction, the LHC is scheduled to begin operation in May 2008. The LHC is expected to 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, universities and laboratories.

When switched on, it is hoped that the collider will produce the elusive Higgs boson particle — often dubbed the God Particle — the observation of which could confirm the predictions and 'missing links' in the Standard Model of physics, and explain how other elementary particles acquire properties such as mass.

Technical Design

The collider is contained in a 27 kilometre (17 mi) circumference tunnel located underground at a depth ranging from 50 to 175 metres. The tunnel was formerly used to house the LEP, an electron-positron collider.

The three metre diameter, concrete-lined tunnel actually crosses the border between Switzerland and France at four points, although the majority of its length is inside France. The collider itself is located underground, with many surface buildings holding ancillary equipment such as compressors, ventilation equipment, control electronics and refrigeration plants.

The collider tunnel contains two pipes enclosed within superconducting magnets cooled by liquid helium, each pipe containing a proton beam. The two beams travel in opposite directions around the ring. Additional magnets are used to direct the beams to four intersection points where interactions between them will take place.

The protons will each have an energy of 7 TeV, giving a total collision energy of 14 TeV. It will take around ninety microseconds for an individual proton to travel once around the collider. Rather than continuous beams, the protons will be "bunched" together into approximately 2,800 bunches, so that interactions between the two beams will take place at discrete intervals never shorter than twenty-five nanoseconds apart. When the collider is first commissioned, it will be operated with fewer bunches, to give a bunch crossing interval of seventy-five nanoseconds. The number of bunches will later be increased to give a final bunch crossing interval of twenty-five nanoseconds.

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 Linac2 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. The Low-Energy Injector Ring (LEIR) will be used as an ion storage and cooler unit. The Antiproton Decelerator (AD) will produce a beam of anti-protons at 2 GeV, after cooling them down from 3.57 GeV. Finally the Super Proton Synchrotron (SPS) can be used to increase the energy of protons up to 450 GeV.

Six detectors are being constructed at the LHC. They are located underground, in large caverns excavated at the LHC's intersection points. Two of them, ATLAS and CMS are large, "general purpose" particle detectors. The other four (LHCb, ALICE, TOTEM, and LHCf) are smaller and more specialized.

The LHC can also be used to collide heavy ions such as lead (Pb) with a collision energy of 1,150 TeV.

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, and in the beam, 725 MJ. Loss of only 10 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. For comparison, 725 MJ is equivalent to the detonation energy of approximately 157 kg (347 pounds) of TNT, and 10 GJ is about 2.5 tons of TNT.

Research

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.
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, the largest national contingent (seven hundred) being from the United States. Physicists hope to use the collider to enhance their ability to answer the following questions:

LHC 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).

LHC proposed upgrade

CMS detector for LHC

After some years of running, any particle physics experiment typically begins to suffer from diminishing returns. 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 construction of LHC was originally approved in 1995 with a budget of 2600 million Swiss francs (currently about 1700 million euro), 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) in the accelerator, and 50 million francs (€30 m) for the experiments, along with a reduction in CERN's budget pushed the completion date out from 2005 to April 2007. 180 million francs (€120 m) of the cost increase has been the superconducting magnets. There were also engineering difficulties encountered while building the underground cavern for the Compact Muon Solenoid.

LHC@Home

Main article: LHC@home

LHC@Home, a distributed computing project, 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 concerns

While many have voiced concerns that the LHC will destroy the Universe, engineers close to the project admit that the possibility is infinitesimally small. As CERN has pointed out, if the Earth were in danger of any such fate, it would have happened billions of years ago from the bombardment of protons the planet receives that are millions of times more energetic than anything that could be produced by the LHC.

As with the Relativistic Heavy Ion Collider (RHIC), people both inside and outside of the physics community have voiced concern that the LHC might trigger one of several theoretical disasters capable of destroying the Earth or even our entire Universe. Each advance in particle accelerator technology exposes the stability of the very fabric of the universe to more stringent tests. RHIC has been running since 2000 and has generated no major problems, however the Large Hadron Collider is set to create an environment significantly more alien to nature than the RHIC has ever created, and therefore the probability of catastrophe is greater.

Theoretical disasters include:

The Large Hadron Collider is expected to create tiny black holes within the Earth . On the face of it this is a spectacularly stupid thing to do, however there exists an entirely theoretical phenomenon known as Hawking Radiation, which some physicists expect to cause these black holes to dissipate. The primary cause for concern is the fact that Hawking Radiation - the only means by which these black holes could be dissipated, is entirely theoretical.

CERN performed a study to investigate whether such dangerous events as micro black holes, strangelets, or magnetic monopoles could occur. The report concluded, "We find no basis for any conceivable threat." If black holes are produced, they are expected to evaporate almost immediately via Hawking radiation and thus be harmless, although the existence of Hawking radiation is currently unconfirmed. It has been claimed that a strong argument for the safety of colliders such as the LHC comes from the simple fact that cosmic rays with energies up to twenty million times the LHC's 1.4×10¹³ eV capacity have been bombarding the Earth, Moon and other objects in the solar system for billions of years with no such effects. Yet CERN themselves claim that the Hadron Collider is to recreate conditions that haven't existed since a fraction of a second after the big bang, making it difficult to accept that the conditions within the collider are quite as everyday as R. A. Mewaldt makes them out to be.

And many people remain gravely concerned about the safety of the LHC such as the science watchdog group called the Lifeboat Foundation which has covered these dangers in detail. As with any new and untested experiment, it is not possible to say with utter certainty what will happen. John Nelson at the University of Birmingham stated of RHIC that "it is astonishingly unlikely that there is any risk—but I could not prove it." Furthermore, in academia there is some question, albeit among a minority of scientists, of whether the Hawking radiation theory is correct.

Construction accidents

On October 25, 2005, a technician 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. Analysis revealed that its design, made as thin as possible for better insulation, was not strong enough to withstand the forces generated by a sudden shutdown. 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 caused a postponement of the planned November 26, 2007 startup date to May 2008.

See also

Notes and references

  1. New start-up schedule for world's most powerful particle accelerator
  2. Symmetry magazine, April 2005
  3. "...in the public presentations of the aspiration of particle physics we hear too often that the goal of the LHC or a linear collider is to check off the last missing particle of the standard model, this year’s Holy Grail of particle physics, the Higgs boson. The truth is much less boring than that! What we’re trying to accomplish is much more exciting, and asking what the world would have been like without the Higgs mechanism is a way of getting at that excitement." -Chris Quigg, Nature's Greatest Puzzles
  4. Ions for LHC
  5. PDF presentation of proposed LHC upgrade
  6. Maiani, Luciano (16 October 2001). "LHC Cost Review to Completion". CERN. Retrieved 2001-01-15.
  7. Feder, Toni (2001). "CERN Grapples with LHC Cost Hike". Physics Today. 54 (12): 21. Retrieved 2007-01-15. {{cite journal}}: Unknown parameter |month= ignored (help)
  8. Tiny Black Holes - Physicist Dave Wark of Imperial College, London reporting for NOVA scienceNOW
  9. Dimopoulos, S. and Landsberg, G. Black Holes at the Large Hadron Collider. Phys. Rev. Lett. 87 (2001).
  10. American Institute of Physics Bulletin of Physics News, Number 558, September 26, 2001, by Phillip F. Schewe, Ben Stein, and James Riordon
  11. Blaizot, J.-P. et al. Study of Potentially Dangerous Events During Heavy-Ion Collisions at the LHC. (PDF)
  12. R. A. Mewaldt "Cosmic Rays" — an article accepted for publication in the Macmillan Encyclopedia of Physics in 1996
  13. Jonathan Leake:Big Bang machine could destroy Earth, Sunday Times
  14. Adam D. Helfer: General Relativity and Quantum Cosmology
  15. Hewett, JoAnne (25 October 2005). "Tragedy at CERN" (Blog). Cosmic Variance. Retrieved 2007-01-15. author and date indicate the beginning of the blog thread
  16. "Message from the Director-General" (Press release) (in English and French). CERN. 26 October 2005. Retrieved 2007-01-15.{{cite press release}}: CS1 maint: unrecognized language (link)
  17. LHC Magnet Test Failure
  18. Updates on LHC inner triplet failure
  19. "The God Particle". www.bbc.com. Retrieved 2007-05-22.
  20. "CERN announces new start-up schedule for world's most powerful particle accelerator" (Press release). CERN. 2007-06-22. Retrieved 2007-07-01.

External links

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